Brendan Choat

Xylem embolism measured retrospectively is linked to canopy dieback in natural populations of Eucalyptus piperita following drought

Publisher: Oxford University Press (OUP)

Date: 11-05-2018

DOI: 10.1093/TREEPHYS/TPY052

Abstract: Manipulative experiments have suggested that embolism-induced hydraulic impairment underpins widespread tree mortality during extreme drought, yet in situ evidence is rare. One month after drought-induced leaf and branch dieback was observed in field populations of Eucalyptus piperita Sm. in the Blue Mountains (Australia), we measured the level of native stem embolism and characterized the extent of leaf death in co-occurring dieback and healthy (non-dieback) trees. We found that canopy dieback-affected trees showed significantly higher levels of native embolism (26%) in tertiary order branchlets than healthy trees (11%). Furthermore, there was a significant positive correlation (R2 = 0.51) between the level of leaf death and the level of native embolism recorded in branchlets from dieback-affected trees. This retrospective study suggests that hydraulic failure was the primary mechanism of leaf and branch dieback in response to a natural drought event in the field. It also suggests that post-drought embolism refilling is minimal or absent in this species of eucalypt.

Toward an index of desiccation time to tree mortality under drought

Publisher: Wiley

Date: 16-06-2016

DOI: 10.1111/PCE.12758

Abstract: Research in plant hydraulics has provided important insights into plant responses to drought and species absolute drought tolerance. However, our ability to predict when plants will die from hydraulic failure under extreme drought is limited by a lack of knowledge with regards to the dynamics of plant desiccation following stomatal closure. Thus, we develop a simple hydraulics model based on branch-level traits that incorporates key aspects of allometry, rates of water loss and resistance to embolism thresholds in order to define species differences in the time it takes plants to desiccate from stomatal closure to lethal levels of drought stress.

The Role of Hydraulic Failure in a Massive Mangrove Die-Off Event

Publisher: Frontiers Media SA

Date: 27-04-2022

DOI: 10.3389/FPLS.2022.822136

Abstract: Between late 2015 and early 2016, more than 7,000 ha of mangrove forest died along the coastline of the Gulf of Carpentaria, in northern Australia. This massive die-off was preceded by a strong 2015/2016 El Niño event, resulting in lower precipitation, a drop in sea level and higher than average temperatures in northern Australia. In this study, we investigated the role of hydraulic failure in the mortality and recovery of the dominant species, Avicennia marina , 2 years after the mortality event. We measured predawn water potential (Ψ pd ) and percent loss of stem hydraulic conductivity (PLC) in surviving in iduals across a gradient of impact. We also assessed the vulnerability to drought-induced embolism (Ψ 50 ) for the species. Areas with severe canopy dieback had higher native PLC (39%) than minimally impacted areas (6%), suggesting that hydraulic recovery was ongoing. The high resistance of A. marina to water-stress-induced embolism (Ψ 50 = −9.6 MPa), indicates that severe water stress (Ψ pd & −10 MPa) would have been required to cause mortality in this species. Our data indicate that the natural gradient of water-stress enhanced the impact of El Niño, leading to hydraulic failure and mortality in A. marina growing on severely impacted (SI) zones. It is likely that lowered sea levels and less frequent inundation by seawater, combined with lower inputs of fresh water, high evaporative demand and high temperatures, led to the development of hyper-salinity and extreme water stress during the 2015/16 summer.

The AusTraits Plant Dictionary

Publisher: Cold Spring Harbor Laboratory

Date: 18-06-2023

DOI: 10.1101/2023.06.16.545047

Abstract: Traits with intuitive names, a clear scope and explicit description are essential for all trait databases. Reanalysis of data from a single database, or analyses that integrate data across multiple databases, can only occur if researchers are confident the trait concepts are consistent within and across sources. The lack of a unified, comprehensive resource for plant trait definitions has previously limited the utility of trait databases. Here we describe the AusTraits Plant Dictionary (APD), which extends the trait definitions included in the new trait database AusTraits. The development process of the APD included three steps: review and formalisation of the scope of each trait and the accompanying trait description addition of trait meta-data and publication in both human and machine-readable forms. Trait definitions include keywords, references and links to related trait concepts in other databases, and the traits are grouped into a hierarchy for easy searching. As well as improving the usability of AusTraits, the Dictionary will foster the integration of trait data across global and regional plant trait databases.

The carbon cost of the 2019–20 Australian fires varies with fire severity and forest type

Publisher: Wiley

Date: 28-05-2022

DOI: 10.1111/GEB.13548

Abstract: To estimate loss of above‐ground carbon (AGC) and conversion of live carbon to dead carbon following understorey and canopy fire. South‐eastern Australia. 2019–2020. Four widespread resprouting eucalypt forests. Above‐ground carbon was measured in 15 plots in each of four forest types one‐year post‐fire. We also assessed topkill, that is, trees subject to canopy loss that failed to resprout epicormically. While canopy fire was associated with greater declines in AGC than understorey fire, this was only statistically significant for only one forest type, where AGC declined from 154 to 85 Mg C ha −1 following canopy fire. Significant post‐fire increases in dead AGC were observed in one forest type, where dead carbon increased from 22 to 60% after canopy fire. Topkill of trees following canopy fire (48–78% of stems) was higher than topkill after understorey fire (36–53% of stems) and in unburnt forest (12–55%). Topkill occurred primarily in small‐diameter stems. Consequently, there was no effect of fire on the proportion of dead AGC in trees, with the exception of the forest with lowest productivity (i.e., lowest biomass) and lowest annual rainfall, where dead tree carbon increased from 8% in unburnt forest to 13 and 53% after understorey and canopy fire, respectively. AGC in understorey vegetation and coarse woody debris was similar or lower in burnt compared with unburnt forest. Litter carbon was significantly lower and pyrogenic carbon significantly higher in burnt forest, with no difference between understorey and canopy fire. While increased fire severity was associated with increased changes to carbon stocks, there were differences among forest types. Specifically, the driest forest type had the highest rates of topkill following canopy fire. These results highlight the importance of spatial variability in fire severity and forest type in determining the effects of fire on carbon stocks.

In Vivo Visualizations of Drought-Induced Embolism Spread in Vitis vinifera      

Publisher: Oxford University Press (OUP)

Date: 05-03-2013

DOI: 10.1104/PP.112.212712

Abstract: Long-distance water transport through plant xylem is vulnerable to hydraulic dysfunction during periods of increased tension on the xylem sap, often coinciding with drought. While the effects of local and systemic embolism on plant water transport and physiology are well documented, the spatial patterns of embolism formation and spread are not well understood. Using a recently developed nondestructive diagnostic imaging tool, high-resolution x-ray computed tomography, we documented the dynamics of drought-induced embolism in grapevine (Vitis vinifera) plants in vivo, producing the first three-dimensional, high-resolution, time-lapse observations of embolism spread. Embolisms formed first in the vessels surrounding the pith at stem water potentials of approximately –1.2 megapascals in drought experiments. As stem water potential decreased, embolisms spread radially toward the epidermis within sectored vessel groupings via intervessel connections and conductive xylem relays, and infrequently (16 of 629 total connections) through lateral connections into adjacent vessel sectors. Theoretical loss of conductivity calculated from the high-resolution x-ray computed tomography images showed good agreement with previously published nuclear magnetic resonance imaging and hydraulic conductivity experiments also using grapevine. Overall, these data support a growing body of evidence that xylem organization is critically important to the isolation of drought-induced embolism spread and confirm that air seeding through the pit membranes is the principle mechanism of embolism spread.

The Relationships between Xylem Safety and Hydraulic Efficiency in the Cupressaceae: The Evolution of Pit Membrane Form and Function    

Publisher: Oxford University Press (OUP)

Date: 15-06-2010

DOI: 10.1104/PP.110.158824

Abstract: Water transport in conifers occurs through single-celled tracheids that are connected to one another via intertracheid pit membranes. These membranes have two components: the porous margo, which allows water to pass through the membrane, and the impermeable torus, which functions to isolate gas-filled tracheids. During drought, tracheids can become air filled and thus hydraulically dysfunctional, a result of air entering through the pit membrane and nucleating cavitation in the water column. What are the hydraulic tradeoffs associated with cavitation resistance at the pit level, and how do they vary within the structural components of the intertracheid pit? To address these questions, we examined pit structure in 15 species of Cupressaceae exhibiting a broad range of cavitation resistances. Across species, cavitation resistance was most closely correlated to the ratio of the torus to pit aperture diameter but did not vary systematically with margo porosity. Furthermore, our data indicate that constraints on pit hydraulic efficiency are shared: the pit aperture limits pit conductivity in more drought-resistant taxa, while increased margo resistance is more likely to control pit conductivity in species that are more vulnerable to cavitation. These results are coupled with additional data concerning pit membrane structure and function and are discussed in the context of the evolutionary biogeography of the Cupressaceae.

Variation in Xylem Hydraulic Structure and Function of Two Mangrove Species across a Latitudinal Gradient in Eastern Australia

Publisher: MDPI AG

Date: 19-03-2021

DOI: 10.3390/W13060850

Abstract: Mangroves growing in tropical and subtropical intertidal zones face challenges from warming and altered rainfall patterns associated with global climate change. Intraspecific variation in hydraulic traits may allow a mangrove species to acclimate to novel climatic conditions, yet little is known regarding the potential for adaptive plasticity in these traits. In this study, we aimed to quantify the variation in plant hydraulic traits of two widespread mangrove species growing across a latitudinal gradient. We investigated the xylem hydraulic structure and function of Avicennia marina and Aegiceras corniculatum, across three sites spanning a latitudinal gradient of 17.45° in eastern Australia. We found that both species were highly resistant to xylem embolism and that there was significant intraspecific variation in hydraulic traits between sites. The highest embolism resistance and sapwood-specific hydraulic conductivity (KS) were found at the lowest latitude site that had the highest mean annual temperature and precipitation. A. marina showed no differences in vessel size and density among sites. It has other special features such as successive cambia enhancing its ability to adapt to a large environmental gradient. In contrast, A. corniculatum showed higher vessel densities at lower latitudes. There was a significant and positive correlation (R2 = 0.72, p 0.05) between KS and embolism resistance across species and sites, suggesting the absence of a tradeoff between hydraulic efficiency and safety. Both embolism resistance and KS were negatively correlated with wood density but positively with vessel wall reinforcement. This study reveals that these two widespread mangrove species were adapted to warmer climates by enhancing both hydraulic efficiency and safety.

Maximum-likelihood estimation of xylem vessel length distributions

Publisher: Elsevier BV

Date: 10-2018

DOI: 10.1016/J.JTBI.2018.07.036

Abstract: Vessel length is an important functional trait for plant hydraulics, because it determines the ratio of flow resistances posed by lumen and pit membranes and hence controls xylem hydraulic efficiency. The most commonly applied methods to estimate vessel lengths are based on the injection of silicon or paint into cut-off stem segments. The number of stained vessels in a series of cross-sections in increasing distance from the injection point is then counted. The resulting infusion profiles are used to estimate the vessel length distribution using one of several statistical algorithms. However, the basis of these algorithms has not been systematically analysed using probability theory. We derive a general mathematical expression for the expected shape of the infusion profile for a given vessel length distribution, provide analytic solutions for five candidate distributions (exponential, Erlang(2), gamma, Weibull, and log-normal), and present maximum likelihood estimators for the parameters of these distributions including implementations in R based on two potential s ling schemes (counting all injected vessels or counting the injected and empty vessels in a random subset of each cross-section). We then explore the performance of these estimators relative to other methods with Monte Carlo experiments. Our analysis demonstrates that most published methods estimate the conditional length distribution of vessels that cross an injection point, which is a size-biased version of the overall length distribution in the stem. We show the mathematical relationship between these distributions and provide methods to estimate either of them. According to our simulation experiments, vessel length distribution was best described by the more flexible models, especially the Weibull distribution. In simulations, the estimators were able to recover the parameters of the vessel length distribution if its functional form was known, achieving an overlap of 90% or more between the true and predicted length distribution when counting no more than 500 injected vessels in 10 cross-sections. This s le size nowadays can easily be reached with the help of automated image analysis.

Testing the limits of plant drought stress and subsequent recovery in four provenances of a widely distributed subtropical tree species

Publisher: Wiley

Date: 14-01-2022

DOI: 10.1111/PCE.14254

Abstract: Drought‐induced tree mortality may increase with ongoing climate change. Unraveling the links between stem hydraulics and mortality thresholds, and the effects of intraspecific variation, remain important unresolved issues. We conducted a water manipulation experiment in a rain‐out shelter, using four provenances of Schima superba originating from a gradient of annual precipitation (1124–1796 mm) and temperature (16.4–22.4°C). Seedlings were droughted to three levels of percentage loss of hydraulic conductivity (i.e., P 50 , P 88 and P 99) and subsequently rewatered to field capacity for 30 days traits related to water and carbon relations were measured. The lethal water potential associated with incipient mortality was between P 50 and P 88 . Seedlings exhibited similar drought responses in xylem water potential, hydraulic conductivity and gas exchange. Upon rehydration, patterns of gas exchange differed among provenances but were not related to the climate at the origin. The four provenances exhibited a similar degree of stem hydraulic recovery, which was correlated with the magnitude of antecedent drought and stem soluble sugar at the end of the drought. Results suggest that there were intraspecific differences in the capacity of S. superba seedlings for carbon assimilation during recovery, indicating a decoupling between gas exchange recovery and stem hydraulics across provenances.

Conduit position and connectivity affect the likelihood of xylem embolism during natural drought in evergreen woodland species

Publisher: Oxford University Press (OUP)

Date: 14-04-2022

DOI: 10.1093/AOB/MCAC053

Abstract: Hydraulic failure is considered a main cause of drought-induced forest mortality. Yet, we have a limited understanding of how the varying intensities and long time scales of natural droughts induce and propagate embolism within the xylem. X-ray computed tomography (microCT) images were obtained from different aged branch xylem to study the number, size and spatial distribution of in situ embolized conduits among three dominant tree species growing in a woodland community. Among the three studied tree species, those with a higher xylem vulnerability to embolism (higher water potential at 50 % loss of hydraulic conductance P50) were more embolized than species with lower P50. Within in idual stems, the probability of embolism was independent of conduit diameter but associated with conduit position. Rather than the occurrence of random or radial embolism, we observed circumferential clustering of high and low embolism density, suggesting that embolism spreads preferentially among conduits of the same age. Older xylem also appeared more likely to accumulate embolisms than young xylem, but there was no pattern suggesting that branch tips were more vulnerable to cavitation than basal regions. The spatial analysis of embolism occurrence in field-grown trees suggests that embolism under natural drought probably propagates by air spreading from embolized into neighbouring conduits in a circumferential pattern. This pattern offers the possibility to understand the temporal aspects of embolism occurrence by examining stem cross-sections.

Hydraulic prediction of drought‐induced plant dieback and top‐kill depends on leaf habit and growth form

Publisher: Wiley

Date: 18-08-2021

DOI: 10.1111/ELE.13856

Abstract: Hydraulic failure caused by severe drought contributes to aboveground dieback and whole‐plant death. The extent to which dieback or whole‐plant death can be predicted by plant hydraulic traits has rarely been tested among species with different leaf habits and/or growth forms. We investigated 19 hydraulic traits in 40 woody species in a tropical savanna and their potential correlations with drought response during an extreme drought event during the El Niño–Southern Oscillation in 2015. Plant hydraulic trait variation was partitioned substantially by leaf habit but not growth form along a trade‐off axis between traits that support drought tolerance versus avoidance. Semi‐deciduous species and shrubs had the highest branch dieback and top‐kill (complete aboveground death) among the leaf habits or growth forms. Dieback and top‐kill were well explained by combining hydraulic traits with leaf habit and growth form, suggesting integrating life history traits with hydraulic traits will yield better predictions.

Pit membrane porosity and water stress-induced cavitation in four co-existing dry rainforest tree species

Publisher: Oxford University Press (OUP)

Date: 2003

DOI: 10.1104/PP.014100

Abstract: Aspects of xylem anatomy and vulnerability to water stress-induced embolism were examined in stems of two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret., growing in a seasonally dry rainforest. The deciduous species were more vulnerable to water stress-induced xylem embolism. B. australis andC. gillivraei reached a 50% loss of hydraulic conductivity at −3.17 MPa and −1.44 MPa, respectively a 50% loss of hydraulic conductivity occurred at −5.56 MPa in A. excelsa and −5.12 MPa in A. bidwillii. To determine whether pit membrane porosity was responsible for greater vulnerability to embolism (air seeding hypothesis), pit membrane structure was examined. Expected pore sizes were calculated from vulnerability curves however, the predicted inter-specific variation in pore sizes was not detected using scanning electron microscopy (pores were not visible to a resolution of 20 nm). Suspensions of colloidal gold particles were then perfused through branch sections. These experiments indicated that pit membrane pores were between 5 and 20 nm in diameter in all four species. The results may be explained by three possibilities: (a) the pores of the expected size range were not present, (b) larger pores, within the size range to cause air seeding, were present but were rare enough to avoid detection, or (c) pore sizes in the expected range only develop while the membrane is under mechanical stress (during air seeding) due to stretching/flexing.

Casting light on xylem vulnerability in an herbaceous species reveals a lack of segmentation

Publisher: Wiley

Date: 26-01-2017

DOI: 10.1111/NPH.14450

Abstract: Finding thresholds at which loss of plant functionality occurs during drought is critical for predicting future crop productivity and survival. Xylem resistance to embolism has been suggested as a key trait associated with water-stress tolerance. Although a substantial literature exists describing the vulnerability of woody stems to embolism, leaves and roots of herbaceous species remain under-represented. Also, little is known about vulnerability to embolism at a whole-plant scale or propagation of embolism within plants. New techniques to view the process of embolism formation provide opportunities to resolve long-standing questions. Here, we used multiple visual techniques, including X-ray micro-computed tomography and the optical vulnerability method, to investigate the spread of embolism within intact stems, leaves and roots of Solanum lycopersicum (common tomato). We found that roots, stems and leaves of tomato plants all exhibited similar vulnerability to embolism, suggesting that embolism rapidly propagates among tissues. Although we found scarce evidence for differentiation of xylem vulnerability among tissues at the scale of the whole plant, within a leaf the midrib embolized at higher water potentials than lower order veins. Substantial overlap between the onset of cavitation and incipient leaf damage suggests that cavitation represents a substantial damage to plants, but the point of lethal cavitation in this herbaceous species remains uncertain.

Non-invasive imaging reveals convergence in root and stem vulnerability to cavitation across five tree species

Publisher: Oxford University Press (OUP)

Date: 18-09-2020

DOI: 10.1093/JXB/ERAA381

Abstract: Root vulnerability to cavitation is challenging to measure and under-represented in current datasets. This gap limits the precision of models used to predict plant responses to drought because roots comprise the critical interface between plant and soil. In this study, we measured vulnerability to drought-induced cavitation in woody roots and stems of five tree species (Acacia aneura, Cedrus deodara, Eucalyptus crebra, Eucalytus saligna, and Quercus palustris) with a wide range of xylem anatomies. X-ray microtomography was used to visualize the accumulation of xylem embolism in stems and roots of intact plants that were naturally dehydrated to varying levels of water stress. Vulnerability to cavitation, defined as the water potential causing a 50% loss of hydraulic function (P50), varied broadly among the species (–4.51 MPa to –11.93 MPa in stems and –3.13 MPa to –9.64 MPa in roots). The P50 of roots and stems was significantly related across species, with species that had more vulnerable stems also having more vulnerable roots. While there was strong convergence in root and stem vulnerability to cavitation, the P50 of roots was significantly higher than the P50 of stems in three species. However, the difference in root and stem vulnerability for these species was small between 1% and 31% of stem P50. Thus, while some differences existed between organs, roots were not dramatically more vulnerable to embolism than stems, and the differences observed were less than those reported in previous studies. Further study is required to evaluate the vulnerability across root orders and to extend these conclusions to a greater number of species and xylem functional types.

In situ turgor stability in grape mesocarp cells and its relation to cell dimensions and microcapillary tip size and geometry

Publisher: Japanese Society of Agricultural, Biological and Environmental Engineers and Scientists

Date: 2011

DOI: 10.2525/ECB.49.61

Temperature alters the response of hydraulic architecture to CO2 in cotton plants (Gossypium hirsutum)

Publisher: Elsevier BV

Date: 04-2020

DOI: 10.1016/J.ENVEXPBOT.2020.104004

Predicting thresholds of drought-induced mortality in woody plant species

Publisher: Oxford University Press (OUP)

Date: 07-2013

DOI: 10.1093/TREEPHYS/TPT046

Weak tradeoff between xylem safety and xylem‐specific hydraulic efficiency across the world's woody plant species

Publisher: Wiley

Date: 17-09-2015

DOI: 10.1111/NPH.13646

Abstract: The evolution of lignified xylem allowed for the efficient transport of water under tension, but also exposed the vascular network to the risk of gas emboli and the spread of gas between xylem conduits, thus impeding sap transport to the leaves. A well‐known hypothesis proposes that the safety of xylem (its ability to resist embolism formation and spread) should trade off against xylem efficiency (its capacity to transport water). We tested this safety–efficiency hypothesis in branch xylem across 335 angiosperm and 89 gymnosperm species. Safety was considered at three levels: the xylem water potentials where 12%, 50% and 88% of maximal conductivity are lost. Although correlations between safety and efficiency were weak ( r 2 0.086), no species had high efficiency and high safety, supporting the idea for a safety–efficiency tradeoff. However, many species had low efficiency and low safety. Species with low efficiency and low safety were weakly associated ( r 2 0.02 in most cases) with higher wood density, lower leaf‐ to sapwood‐area and shorter stature. There appears to be no persuasive explanation for the considerable number of species with both low efficiency and low safety. These species represent a real challenge for understanding the evolution of xylem.

Linking hydraulic traits to tropical forest function in a size-structured and trait-driven model (TFS v.1-Hydro)

Publisher: Copernicus GmbH

Date: 24-11-2016

DOI: 10.5194/GMD-9-4227-2016

Abstract: Abstract. Forest ecosystem models based on heuristic water stress functions poorly predict tropical forest response to drought partly because they do not capture the ersity of hydraulic traits (including variation in tree size) observed in tropical forests. We developed a continuous porous media approach to modeling plant hydraulics in which all parameters of the constitutive equations are biologically interpretable and measurable plant hydraulic traits (e.g., turgor loss point πtlp, bulk elastic modulus ε, hydraulic capacitance Cft, xylem hydraulic conductivity ks,max, water potential at 50 % loss of conductivity for both xylem (P50,x) and stomata (P50,gs), and the leaf : sapwood area ratio Al : As). We embedded this plant hydraulics model within a trait forest simulator (TFS) that models light environments of in idual trees and their upper boundary conditions (transpiration), as well as providing a means for parameterizing variation in hydraulic traits among in iduals. We synthesized literature and existing databases to parameterize all hydraulic traits as a function of stem and leaf traits, including wood density (WD), leaf mass per area (LMA), and photosynthetic capacity (Amax), and evaluated the coupled model (called TFS v.1-Hydro) predictions, against observed diurnal and seasonal variability in stem and leaf water potential as well as stand-scaled sap flux. Our hydraulic trait synthesis revealed coordination among leaf and xylem hydraulic traits and statistically significant relationships of most hydraulic traits with more easily measured plant traits. Using the most informative empirical trait–trait relationships derived from this synthesis, TFS v.1-Hydro successfully captured in idual variation in leaf and stem water potential due to increasing tree size and light environment, with model representation of hydraulic architecture and plant traits exerting primary and secondary controls, respectively, on the fidelity of model predictions. The plant hydraulics model made substantial improvements to simulations of total ecosystem transpiration. Remaining uncertainties and limitations of the trait paradigm for plant hydraulics modeling are highlighted.

The role of freezing in setting the latitudinal limits of mangrove forests

Publisher: Wiley

Date: 29-11-2007

DOI: 10.1111/J.1469-8137.2006.01938.X

Abstract: Mangrove trees dominate coastal vegetation in tropical regions, but are completely replaced by herbaceous salt marshes at latitudes above 32 degrees N and 40 degrees S. Because water deficit can increase damage caused by freezing, we hypothesized that mangroves, which experience large deficits as a result of saline substrates, would suffer freeze-induced xylem failure. Vulnerability to freeze-induced xylem embolism was examined in the most poleward mangrove species in North America, in an area where freezing is rare but severe, and in Australia, in an area where freezing is frequent but mild. Percentage loss in hydraulic conductivity was measured following manipulations of xylem tension xylem sap ion concentration was determined using X-ray microanalysis. Species with wider vessels suffered 60-100% loss of hydraulic conductivity after freezing and thawing under tension, while species with narrower vessels lost as little as 13-40% of conductivity. These results indicate that freeze-induced embolism may play a role in setting the latitudinal limits of distribution in mangroves, either through massive embolism following freezing, or through constraints on water transport as a result of vessel size.

An empirical method that separates irreversible stem radial growth from bark water content changes in trees: theory and case studies

Publisher: Wiley

Date: 06-01-2017

DOI: 10.1111/PCE.12863

Xylem traits mediate a trade-off between resistance to freeze-thaw-induced embolism and photosynthetic capacity in overwintering evergreens

Publisher: Wiley

Date: 31-05-2011

DOI: 10.1111/J.1469-8137.2011.03772.X

Abstract: Hydraulic traits were studied in temperate, woody evergreens in a high-elevation heath community to test for trade-offs between the delivery of water to canopies at rates sufficient to sustain photosynthesis and protection against disruption to vascular transport caused by freeze-thaw-induced embolism. Freeze-thaw-induced loss in hydraulic conductivity was studied in relation to xylem anatomy, leaf- and sapwood-specific hydraulic conductivity and gas exchange characteristics of leaves. We found evidence that a trade-off between xylem transport capacity and safety from freeze-thaw-induced embolism affects photosynthetic activity in overwintering evergreens. The mean hydraulically weighted xylem vessel diameter and sapwood-specific conductivity correlated with susceptibility to freeze-thaw-induced embolism. There was also a strong correlation of hydraulic supply and demand across species interspecific differences in stomatal conductance and CO(2) assimilation rates were correlated linearly with sapwood- and leaf-specific hydraulic conductivity. Xylem vessel anatomy mediated an apparent trade-off between resistance to freeze-thaw-induced embolism and hydraulic and photosynthetic capacity during the winter. These results point to a new role for xylem functional traits in determining the degree to which species can maintain photosynthetic carbon gain despite freezing events and cold winter temperatures.

More than iso/anisohydry: Hydroscapes integrate plant water use and drought tolerance traits in 10 eucalypt species from contrasting climates

Publisher: Wiley

Date: 07-04-2019

DOI: 10.1111/1365-2435.13320

Drought response strategies and hydraulic traits contribute to mechanistic understanding of plant dry-down to hydraulic failure

Publisher: Oxford University Press (OUP)

Date: 16-03-2019

DOI: 10.1093/TREEPHYS/TPZ016

Abstract: Drought-induced tree mortality alters forest structure and function, yet our ability to predict when and how different species die during drought remains limited. Here, we explore how stomatal control and drought tolerance traits influence the duration of drought stress leading to critical levels of hydraulic failure. We examined the growth and physiological responses of four woody plant species (three angiosperms and one conifer) representing a range of water-use and drought tolerance traits over the course of two controlled drought–recovery cycles followed by an extended dry-down. At the end of the final dry-down phase, we measured changes in biomass ratios and leaf carbohydrates. During the first and second drought phases, plants of all species closed their stomata in response to decreasing water potential, but only the conifer species avoided water potentials associated with xylem embolism as a result of early stomatal closure relative to thresholds of hydraulic dysfunction. The time it took plants to reach critical levels of water stress during the final dry-down was similar among the angiosperms (ranging from 39 to 57 days to stemP88) and longer in the conifer (156 days to stemP50). Plant dry-down time was influenced by a number of factors including species stomatal-hydraulic safety margin (gsP90 – stemP50), as well as leaf succulence and minimum stomatal conductance. Leaf carbohydrate reserves (starch) were not depleted at the end of the final dry-down in any species, irrespective of the duration of drought. These findings highlight the need to consider multiple structural and functional traits when predicting the timing of hydraulic failure in plants.

Stem and leaf hydraulic properties are finely coordinated in three tropical rain forest tree species

Publisher: Wiley

Date: 14-07-2015

DOI: 10.1111/PCE.12581

Abstract: Coordination of stem and leaf hydraulic traits allows terrestrial plants to maintain safe water status under limited water supply. Tropical rain forests, one of the world's most productive biomes, are vulnerable to drought and potentially threatened by increased aridity due to global climate change. However, the relationship of stem and leaf traits within the plant hydraulic continuum remains understudied, particularly in tropical species. We studied within-plant hydraulic coordination between stems and leaves in three tropical lowland rain forest tree species by analyses of hydraulic vulnerability [hydraulic methods and ultrasonic emission (UE) analysis], pressure-volume relations and in situ pre-dawn and midday water potentials (Ψ). We found finely coordinated stem and leaf hydraulic features, with a strategy of sacrificing leaves in favour of stems. Fifty percent of hydraulic conductivity (P50 ) was lost at -2.1 to -3.1 MPa in stems and at -1.7 to -2.2 MPa in leaves. UE analysis corresponded to hydraulic measurements. Safety margins (leaf P50 - stem P50 ) were very narrow at -0.4 to -1.4 MPa. Pressure-volume analysis and in situ Ψ indicated safe water status in stems but risk of hydraulic failure in leaves. Our study shows that stem and leaf hydraulics were finely tuned to avoid embolism formation in the xylem.

Mechanisms of woody-plant mortality under rising drought, CO2 and vapour pressure deficit

Publisher: Springer Science and Business Media LLC

Date: 29-03-2022

DOI: 10.1038/S43017-022-00272-1

TRY plant trait database – enhanced coverage and open access

Publisher: Wiley

Date: 31-12-2019

DOI: 10.1111/GCB.14904

Abstract: Plant traits—the morphological, anatomical, physiological, biochemical and phenological characteristics of plants—determine how plants respond to environmental factors, affect other trophic levels, and influence ecosystem properties and their benefits and detriments to people. Plant trait data thus represent the basis for a vast area of research spanning from evolutionary biology, community and functional ecology, to bio ersity conservation, ecosystem and landscape management, restoration, biogeography and earth system modelling. Since its foundation in 2007, the TRY database of plant traits has grown continuously. It now provides unprecedented data coverage under an open access data policy and is the main plant trait database used by the research community worldwide. Increasingly, the TRY database also supports new frontiers of trait‐based plant research, including the identification of data gaps and the subsequent mobilization or measurement of new data. To support this development, in this article we evaluate the extent of the trait data compiled in TRY and analyse emerging patterns of data coverage and representativeness. Best species coverage is achieved for categorical traits—almost complete coverage for ‘plant growth form’. However, most traits relevant for ecology and vegetation modelling are characterized by continuous intraspecific variation and trait–environmental relationships. These traits have to be measured on in idual plants in their respective environment. Despite unprecedented data coverage, we observe a humbling lack of completeness and representativeness of these continuous traits in many aspects. We, therefore, conclude that reducing data gaps and biases in the TRY database remains a key challenge and requires a coordinated approach to data mobilization and trait measurements. This can only be achieved in collaboration with other initiatives.

Visualization of xylem embolism by X-ray microtomography: a direct test against hydraulic measurements

Publisher: Wiley

Date: 14-02-2017

DOI: 10.1111/NPH.14462

Abstract: X-ray microtomography (microCT) is becoming a valuable noninvasive tool for advancing our understanding of plant-water relations. Laboratory-based microCT systems are becoming more affordable and provide better access than synchrotron facilities. However, some systems come at the cost of comparably lower signal quality and spatial resolution than synchrotron facilities. In this study, we evaluated laboratory-based X-ray microCT imaging as a tool to nondestructively analyse hydraulic vulnerability to drought-induced embolism in a woody plant species. We analysed the vulnerability to drought-induced embolism of benchtop-dehydrated Eucalyptus camaldulensis plants using microCT and hydraulic flow measurements on the same s le material, allowing us to directly compare the two methods. Additionally, we developed a quantitative procedure to improve microCT image analysis at limited resolution and accurately measure vessel lumens. Hydraulic measurements matched closely with microCT imaging of the current-year growth ring, with similar hydraulic conductivity and loss of conductivity due to xylem embolism. Optimized thresholding of vessel lumens during image analysis, based on a physiologically meaningful parameter (theoretical conductivity), allowed us to overcome common potential constraints of some lab-based systems. Our results indicate that estimates of vulnerability to embolism provided by microCT visualization agree well with those obtained from hydraulic measurements on the same s le material.

Rapid hydraulic recovery in Eucalyptus pauciflora after drought: linkages between stem hydraulics and leaf gas exchange

Publisher: Wiley

Date: 09-09-2014

DOI: 10.1111/PCE.12182

Abstract: In woody plants, photosynthetic capacity is closely linked to rates at which the plant hydraulic system can supply water to the leaf surface. Drought-induced embolism can cause sharp declines in xylem hydraulic conductivity that coincide with stomatal closure and reduced photosynthesis. Recovery of photosynthetic capacity after drought is dependent on restored xylem function, although few data exist to elucidate this coordination. We examined the dynamics of leaf gas exchange and xylem function in Eucalyptus pauciflora seedlings exposed to a cycle of severe water stress and recovery after re-watering. Stomatal closure and leaf turgor loss occurred at water potentials that delayed the extensive spread of embolism through the stem xylem. Stem hydraulic conductance recovered to control levels within 6 h after re-watering despite a severe drought treatment, suggesting an active mechanism embolism repair. However, stomatal conductance did not recover after 10 d of re-watering, effecting tighter control of transpiration post drought. The dynamics of recovery suggest that a combination of hydraulic and non-hydraulic factors influenced stomatal behaviour post drought.

Using High Resolution Computed Tomography to Visualize the Three Dimensional Structure and Function of Plant Vasculature

Publisher: MyJove Corporation

Date: 05-04-2013

DOI: 10.3791/50162

Canopy dieback and recovery in Australian native forests following extreme drought

Publisher: Springer Science and Business Media LLC

Date: 14-12-2022

DOI: 10.1038/S41598-022-24833-Y

Abstract: In 2019, south-eastern Australia experienced its driest and hottest year on record, resulting in massive canopy dieback events in eucalypt dominated forests. A subsequent period of high precipitation in 2020 provided a rare opportunity to quantify the impacts of extreme drought and consequent recovery. We quantified canopy health and hydraulic impairment (native percent loss of hydraulic conductivity, PLC) of 18 native tree species growing at 15 sites that were heavily impacted by the drought both during and 8–10 months after the drought. Most species exhibited high PLC during drought (PLC:65.1 ± 3.3%), with no clear patterns across sites or species. Heavily impaired trees (PLC 70%) showed extensive canopy browning. In the post-drought period, most surviving trees exhibited hydraulic recovery (PLC:26.1 ± 5.1%), although PLC remained high in some trees (50–70%). Regained hydraulic function (PLC 50%) corresponded to decreased canopy browning indicating improved tree health. Similar drought (37.1 ± 4.2%) and post-drought (35.1 ± 4.4%) percentages of basal area with dead canopy suggested that trees with severely compromised canopies immediately after drought were not able to recover. This dataset provides insights into the impacts of severe natural drought on the health of mature trees, where hydraulic failure is a major contributor in canopy dieback and tree mortality during extreme drought events.

The correlations and sequence of plant stomatal, hydraulic, and wilting responses to drought

Publisher: Proceedings of the National Academy of Sciences

Date: 02-11-2016

DOI: 10.1073/PNAS.1604088113

Abstract: Many plant species face increasing drought under climate change, making plant drought tolerance integral to predicting species and ecosystem responses. Many physiology traits interact to determine overall drought tolerance, but trait relationships have not been assessed for general patterns across global plant ersity. We analyzed stomatal, hydraulic, and mesophyll drought tolerance traits for 310 species from ecosystems worldwide. We evaluated the sequence of drought responses for plants under increasing water stress, and showed that coselection with environmental water stress drives most trait correlations across species, with functional coordination additionally important for some relationships. These results provide insight into how variation in multiple traits should be represented within plants and across species in models of plant responses to drought.

The spatial pattern of air seeding thresholds in mature sugar maple trees

Publisher: Wiley

Date: 12-04-2005

DOI: 10.1111/J.1365-3040.2005.01336.X

Hydraulic failure and tree size linked with canopy die‐back in eucalypt forest during extreme drought

Publisher: Wiley

Date: 03-04-2021

DOI: 10.1111/NPH.17298

Abstract: Eastern Australia was subject to its hottest and driest year on record in 2019. This extreme drought resulted in massive canopy die‐back in eucalypt forests. The role of hydraulic failure and tree size on canopy die‐back in three eucalypt tree species during this drought was examined. We measured pre‐dawn and midday leaf water potential (Ψ leaf ), per cent loss of stem hydraulic conductivity and quantified hydraulic vulnerability to drought‐induced xylem embolism. Tree size and tree health was also surveyed. Trees with most, or all, of their foliage dead exhibited high rates of native embolism (78–100%). This is in contrast to trees with partial canopy die‐back (30–70% canopy die‐back: 72–78% native embolism), or relatively healthy trees (little evidence of canopy die‐back: 25–31% native embolism). Midday Ψ leaf was significantly more negative in trees exhibiting partial canopy die‐back (−2.7 to −6.3 MPa), compared with relatively healthy trees (−2.1 to −4.5 MPa). In two of the species the majority of in iduals showing complete canopy die‐back were in the small size classes. Our results indicate that hydraulic failure is strongly associated with canopy die‐back during drought in eucalypt forests. Our study provides valuable field data to help constrain models predicting mortality risk.

Xylem resistance to embolism: presenting a simple diagnostic test for the open vessel artefact

Publisher: Wiley

Date: 03-05-2017

DOI: 10.1111/NPH.14589

Abstract: Xylem vulnerability to embolism represents an essential trait for the evaluation of the impact of hydraulics in plant function and ecology. The standard centrifuge technique is widely used for the construction of vulnerability curves, although its accuracy when applied to species with long vessels remains under debate. We developed a simple diagnostic test to determine whether the open‐vessel artefact influences centrifuge estimates of embolism resistance. Xylem s les from three species with differing vessel lengths were exposed to less negative xylem pressures via centrifugation than the minimum pressure the s le had previously experienced. Additional calibration was obtained from non‐invasive measurement of embolism on intact olive plants by X‐ray microtomography. Results showed artefactual decreases in hydraulic conductance ( k ) for s les with open vessels when exposed to a less negative xylem pressure than the minimum pressure they had previously experienced. X‐Ray microtomography indicated that most of the embolism formation in olive occurs at xylem pressures below −4.0 MP a, reaching 50% loss of hydraulic conductivity at −5.3 MP a. The artefactual reductions in k induced by centrifugation underestimate embolism resistance data of species with long vessels. A simple test is suggested to avoid this open vessel artefact and to ensure the reliability of this technique in future studies.

Methods for measuring plant vulnerability to cavitation: a critical review

Publisher: Oxford University Press (OUP)

Date: 25-07-2013

DOI: 10.1093/JXB/ERT193

Abstract: Xylem cavitation resistance has profound implications for plant physiology and ecology. This process is characterized by a 'vulnerability curve' (VC) showing the variation of the percentage of cavitation as a function of xylem pressure potential. The shape of this VC varies from 'sigmoidal' to 'exponential'. This review provides a panorama of the techniques that have been used to generate such a curve. The techniques differ by (i) the way cavitation is induced (e.g. bench dehydration, centrifugation, or air injection), and (ii) the way cavitation is measured (e.g. percentage loss of conductivity (PLC) or acoustic emission), and a nomenclature is proposed based on these two methods. A survey of the literature of more than 1200 VCs was used to draw statistics on the usage of these methods and on their reliability and validity. Four methods accounted for more than 96% of all curves produced so far: bench dehydration-PLC, centrifugation-PLC, pressure sleeve-PLC, and Cavitron. How the shape of VCs varies across techniques and species xylem anatomy was also analysed. Strikingly, it was found that the vast majority of curves obtained with the reference bench dehydration-PLC method are 'sigmoidal'. 'Exponential' curves were more typical of the three other methods and were remarkably frequent for species having large xylem conduits (ring-porous), leading to a substantial overestimation of the vulnerability of cavitation for this functional group. We suspect that 'exponential' curves may reflect an open-vessel artefact and call for more precautions with the usage of the pressure sleeve and centrifugation techniques.

High safety margins to drought‐induced hydraulic failure found in five pasture grasses

Publisher: Wiley

Date: 31-03-2022

DOI: 10.1111/PCE.14318

Abstract: Determining the relationship between reductions in stomatal conductance ( g s ) and leaf water transport during dehydration is key to understanding plant drought responses. While numerous studies have analysed the hydraulic function of woody species, minimal research has been conducted on grasses. Here, we sought to characterize hydraulic vulnerability in five widely‐occurring pasture grasses (including both C3 and C4 grasses) and determine whether reductions in g s and leaf hydraulic conductance ( K leaf ) during dehydration could be attributed to xylem embolism. Using the optical vulnerability (OV) technique, we found that all species were highly resistant to xylem embolism when compared to other herbaceous angiosperms, with 50% xylem embolism ( P X50 ) occurring at xylem pressures ranging from −4.4 to −6.1 MPa. We observed similar reductions in g s and K leaf under mild water stress for all species, occurring well before P X50 . The onset of xylem embolism ( P X12 ) occurred consistently after stomatal closure and 90% reduction of K leaf . Our results suggest that factors other than xylem embolism are responsible for the majority of reductions in g s and K leaf during drought and reductions in the productivity of pasture species under moderate drought may not be driven by embolism.

Linking Forest Flammability and Plant Vulnerability to Drought

Publisher: MDPI AG

Date: 20-07-2020

DOI: 10.3390/F11070779

Abstract: Globally, fire regimes are being altered by changing climatic conditions. New fire regimes have the potential to drive species extinctions and cause ecosystem state changes, with a range of consequences for ecosystem services. Despite the co-occurrence of forest fires with drought, current approaches to modelling flammability largely overlook the large body of research into plant vulnerability to drought. Here, we outline the mechanisms through which plant responses to drought may affect forest flammability, specifically fuel moisture and the ratio of dead to live fuels. We present a framework for modelling live fuel moisture content (moisture content of foliage and twigs) from soil water content and plant traits, including rooting patterns and leaf traits such as the turgor loss point, osmotic potential, elasticity and leaf mass per area. We also present evidence that physiological drought stress may contribute to previously observed fuel moisture thresholds in south-eastern Australia. Of particular relevance is leaf cavitation and subsequent shedding, which transforms live fuels into dead fuels, which are drier, and thus easier to ignite. We suggest that capitalising on drought research to inform wildfire research presents a major opportunity to develop new insights into wildfires, and new predictive models of seasonal fuel dynamics.

Stability of tropical forest tree carbon‐water relations in a rainfall exclusion treatment through shifts in effective water uptake depth

Publisher: Wiley

Date: 21-09-2021

DOI: 10.1111/GCB.15869

Abstract: Increasing severity and frequency of drought is predicted for large portions of the terrestrial biosphere, with major impacts already documented in wet tropical forests. Using a 4‐year rainfall exclusion experiment in the Daintree Rainforest in northeast Australia, we examined canopy tree responses to reduced precipitation and soil water availability by quantifying seasonal changes in plant hydraulic and carbon traits for 11 tree species between control and drought treatments. Even with reduced soil volumetric water content in the upper 1 m of soil in the drought treatment, we found no significant difference between treatments for predawn and midday leaf water potential, photosynthesis, stomatal conductance, foliar stable carbon isotope composition, leaf mass per area, turgor loss point, xylem vessel anatomy, or leaf and stem nonstructural carbohydrates. While empirical measurements of aboveground traits revealed homeostatic maintenance of plant water status and traits in response to reduced soil moisture, modeled belowground dynamics revealed that trees in the drought treatment shifted the depth from which water was acquired to deeper soil layers. These findings reveal that belowground acclimation of tree water uptake depth may buffer tropical rainforests from more severe droughts that may arise in future with climate change.

Pit membranes in tracheary elements of Rosaceae and related families: new records of tori and pseudotori

Publisher: Wiley

Date: 04-2007

DOI: 10.3732/AJB.94.4.503

Abstract: The micromorphology of pits in tracheary elements was examined in 35 species representing 29 genera of Rosaceae and related families to evaluate the assumption that angiosperm pits are largely invariant. In most Rosaceae, pit membranes between fibers and tracheids frequently appear to have amorphous thickenings with an irregular distribution. Although these structures are torus-like under the light microscope, observations by electron microscopy illustrate that they represent "pseudotori" or plasmodesmata-associated thickenings. These thickenings frequently extend from the periphery of the pit membrane and form a cap-like, hollow structure. Pseudotori are occasionally found in few Elaeagnaceae and Rhamnaceae and appear to be related to species with fiber-tracheids and/or tracheids. True tori are strongly associated with round to oval pit apertures and are consistently present in narrow tracheary elements of Cercocarpus (Rosaceae), Planera (Ulmaceae), and ring-porous species of Ulmus and Zelkova (Ulmaceae). Vestured pits with homogenous pit membranes are reported for Hemiptelea (Ulmaceae). The homoplastic nature of pit membrane characteristics may be related to functional adaptations in terms of safety and efficiency of water transport or may reflect different developmental processes of xylem elements. These observations illustrate that there is more variation in angiosperm pits than previously thought.

Drought resistance of cotton (Gossypium hirsutum) is promoted by early stomatal closure and leaf shedding

Publisher: CSIRO Publishing

Date: 2020

DOI: 10.1071/FP19093

Abstract: Water relations have been well documented in tree species, but relatively little is known about the hydraulic characteristics of crops. Here, we report on the hydraulic strategy of cotton (Gossypium hirsutum L.). Leaf gas exchange and in vivo embolism formation were monitored simultaneously on plants that were dried down in situ under controlled environment conditions, and xylem vulnerability to embolism of leaves, stems and roots was measured using intact plants. Water potential inducing 50% embolised vessels (P50) in leaves was significantly higher (less negative) than P50 of stems and roots, suggesting that leaves were the most vulnerable organ to embolism. Furthermore, the water potential generating stomatal closure (Pgs) was higher than required to generate embolism formation, and complete stomatal closure always preceded the onset of embolism with declining soil water content. Although protracted drought resulted in massive leaf shedding, stem embolism remained minimal even after ~90% leaf area was lost. Overall, cotton maintained hydraulic integrity during long-term drought stress through early stomatal closure and leaf shedding, thus exhibiting a drought avoidance strategy. Given that water potentials triggering xylem embolism are uncommon under field conditions, cotton is unlikely to experience hydraulic dysfunction except under extreme climates. Results of this study provide physiological evidence for drought resistance in cotton with regard to hydraulics, and may provide guidance in developing irrigation schedules during periods of water shortage.

Vulnerability to xylem cavitation of Hakea species (Proteaceae) from a range of biomes and life histories predicted by climatic niche

Publisher: Oxford University Press (OUP)

Date: 19-02-2021

DOI: 10.1093/AOB/MCAB020

Abstract: Extreme drought conditions across the globe are impacting bio ersity, with serious implications for the persistence of native species. However, quantitative data on physiological tolerance are not available for erse flora to inform conservation management. We quantified physiological resistance to cavitation in the erse Hakea genus (Proteaceae) to test predictions based on climatic origin, life history and functional traits. We s led terminal branches of replicate plants of 16 species in a common garden. Xylem cavitation was induced in branches under varying water potentials (tension) in a centrifuge, and the tension generating 50 % loss of conductivity (stem P50) was characterized as a metric for cavitation resistance. The same branches were used to estimate plant functional traits, including wood density, specific leaf area and Huber value (sap flow area to leaf area ratio). There was significant variation in stem P50 among species, which was negatively associated with the species climate origin (rainfall and aridity). Cavitation resistance did not differ among life histories however, a drought avoidance strategy with terete leaf form and greater Huber value may be important for species to colonize and persist in the arid biome. This study highlights climate (rainfall and aridity), rather than life history and functional traits, as the key predictor of variation in cavitation resistance (stem P50). Rainfall for species origin was the best predictor of cavitation resistance, explaining variation in stem P50, which appears to be a major determinant of species distribution. This study also indicates that stem P50 is an adaptive trait, genetically determined, and hence reliable and robust for predicting species vulnerability to climate change. Our findings will contribute to future prediction of species vulnerability to drought and adaptive management under climate change.

Diversity of hydraulic traits in nine Cordia species growing in tropical forests with contrasting precipitation

Publisher: Wiley

Date: 21-06-2007

DOI: 10.1111/J.1469-8137.2007.02137.X

Abstract: Inter- and intraspecific variation in hydraulic traits was investigated in nine Cordia (Boraginaceae) species growing in three tropical rainforests differing in mean annual precipitation (MAP). Interspecific variation was examined for the different Cordia species found at each site, and intraspecific variation was studied in populations of the widespread species Cordia alliodora across the three sites. Strong intra- and interspecific variation were observed in vulnerability to drought-induced embolism. Species growing at drier sites were more resistant to embolism than those growing at moister sites the same pattern was observed for populations of C. alliodora. By contrast, traits related to hydraulic capacity, including stem xylem vessel diameter, sapwood specific conductivity (K(s)) and leaf specific conductivity (K(L)), varied strongly but independently of MAP. For C. alliodora, xylem anatomy, K(s), K(L) and Huber value varied little across sites, with K(s) and K(L) being consistently high relative to other Cordia species. A constitutively high hydraulic capacity coupled with plastic or genotypic adjustment in vulnerability to embolism and leaf water relations would contribute to the ability of C. alliodora to establish and compete across a wide precipitation gradient.

Drought responses of two gymnosperm species with contrasting stomatal regulation strategies under elevated [CO₂] and temperature

Publisher: Oxford University Press (OUP)

Date: 09-06-2015

DOI: 10.1093/TREEPHYS/TPV047

Abstract: Future climate regimes characterized by rising [CO2], rising temperatures and associated droughts may differentially affect tree growth and physiology. However, the interactive effects of these three factors are complex because elevated [CO2] and elevated temperature may generate differential physiological responses during drought. To date, the interactive effects of elevated [CO2] and elevated temperature on drought-induced tree mortality remain poorly understood in gymnosperm species that differ in stomatal regulation strategies. Water relations and carbon dynamics were examined in two species with contrasting stomatal regulation strategies: Pinus radiata D. Don (relatively isohydric gymnosperm regulating stomata to maintain leaf water potential above critical thresholds) and Callitris rhomboidea R. Br (relatively anisohydric gymnosperm allowing leaf water potential to decline as the soil dries), to assess response to drought as a function of [CO2] and temperature. Both species were grown in two [CO2] (C(a) (ambient, 400 μl l(-1)) and C(e) (elevated, 640 μl l(-1))) and two temperature (T(a) (ambient) and T(e) (ambient +4 °C)) treatments in a sun-lit glasshouse under well-watered conditions. Drought plants were then exposed to a progressive drought until mortality. Prior to mortality, extensive xylem cavitation occurred in both species, but significant depletion of non-structural carbohydrates was not observed in either species. Te resulted in faster mortality in P. radiata, but it did not modify the time-to-mortality in C. rhomboidea. C(e) did not delay the time-to-mortality in either species under drought or T(e) treatments. In summary, elevated temperature (+4 °C) had greater influence than elevated [CO2] (+240 μl l(-1)) on drought responses of the two studied gymnosperm species, while stomatal regulation strategies did not generally affect the relative contributions of hydraulic failure and carbohydrate depletion to mortality under severe drought.

Non-invasive measurement of leaf water content and pressure–volume curves using terahertz radiation

Publisher: Springer Science and Business Media LLC

Date: 03-12-2020

DOI: 10.1038/S41598-020-78154-Z

Abstract: In this paper we describe a non-invasive method of measuring leaf water content using THz radiation and combine this with psychrometry for determination of leaf pressure–volume relationships. In contrast to prior investigations using THz radiation to measure plant water status, the reported method exploits the differential absorption characteristic of THz radiation at multiple frequencies within plant leaves to determine absolute water content in real-time. By combining the THz system with a psychrometer, pressure–volume curves were generated in a completely automated fashion for the determination of leaf tissue water relations parameters including water potential at turgor loss, osmotic potential at full turgor and the relative water content at the turgor loss point. This novel methodology provides for repeated, non-destructive measurement of leaf water content and greatly increased efficiency in generation of leaf PV curves by reducing user handling time.

Stomatal and non-stomatal limitations of photosynthesis for four tree species under drought: A comparison of model formulations

Publisher: Elsevier BV

Date: 12-2017

DOI: 10.1016/J.AGRFORMET.2017.08.026

Current controversies and challenges in applying plant hydraulic techniques

Publisher: Wiley

Date: 12-01-2015

DOI: 10.1111/NPH.13229

Direct X-Ray Microtomography Observation Confirms the Induction of Embolism upon Xylem Cutting under Tension

Publisher: Oxford University Press (OUP)

Date: 06-11-2014

DOI: 10.1104/PP.114.249706

CO2 availability influences hydraulic function of C3 and C4 grass leaves

Publisher: Oxford University Press (OUP)

Date: 10-03-2018

DOI: 10.1093/JXB/ERY095

Changes in pit membrane porosity due to deflection and stretching: the role of vestured pits

Publisher: Oxford University Press (OUP)

Date: 04-06-2004

DOI: 10.1093/JXB/ERH173

New insights into bordered pit structure and cavitation resistance in angiosperms and conifers

Publisher: Wiley

Date: 16-04-2009

DOI: 10.1111/J.1469-8137.2009.02847.X

Identifying areas at risk of drought‐induced tree mortality across South‐Eastern Australia

Publisher: Wiley

Date: 19-08-2020

DOI: 10.1111/GCB.15215

Xylem vessel relays contribute to radial connectivity in grapevine stems (Vitis vinifera and V. arizonica; Vitaceae)

Publisher: Wiley

Date: 02-2013

DOI: 10.3732/AJB.1100606

Abstract: Xylem network connections play an important role in water and nutrient transport in plants, but also facilitate the spread of air embolisms and xylem-dwelling pathogens. This study describes the structure and function of vessel relays found in grapevine xylem that form radial and tangential connections between spatially discrete vessels. We used high-resolution computed tomography, light microscopy, scanning electron microscopy, and single-vessel dye injections to characterize vessel relays in stems and compare their distributions and structure in two Vitis species. Vessel relays were composed of 1-8 narrow diameter (~25 µm) vessel elements and were oriented radially, connecting vessels via scalariform pitting within a xylem sector delineated by rays. The functional connectedness of vessels linked by vessel relays was confirmed with single-vessel dye injections. In 4.5-cm sections of stem tissue, there were 26% more vessel relays in V. vinifera compared with V. arizonica. • Because of their spatial distribution within Vitis xylem, vessel relays increase the connectivity between vessels that would otherwise remain isolated. Differences in vessel relays between Vitis species suggest these anatomical features could contribute to disease and embolism resistance in some species.

Mechanisms of xylem hydraulic recovery after drought in Eucalyptus saligna

Publisher: Wiley

Date: 10-02-2022

DOI: 10.1111/PCE.14265

Abstract: The mechanisms by which woody plants recover xylem hydraulic capacity after drought stress are not well understood, particularly with regard to the role of embolism refilling. We evaluated the recovery of xylem hydraulic capacity in young Eucalyptus saligna plants exposed to cycles of drought stress and rewatering. Plants were exposed to moderate and severe drought stress treatments, with recovery monitored at time intervals from 24 h to 6 months after rewatering. The percentage loss of xylem vessels due to embolism (PLV) was quantified at each time point using microcomputed tomography with stem water potential (Ψ x ) and canopy transpiration ( E c ) measured before scans. Plants exposed to severe drought stress suffered high levels of embolism (47.38% ± 10.97% PLV) and almost complete canopy loss. No evidence of embolism refilling was observed at 24 h, 1 week, or 3 weeks after rewatering despite rapid recovery in Ψ x . Recovery of hydraulic capacity was achieved over a 6‐month period by growth of new xylem tissue, with canopy leaf area and E c recovering over the same period. These findings indicate that E. saligna recovers slowly from severe drought stress, with potential for embolism to persist in the xylem for many months after rainfall events.

Synchrotron X‐ray microtomography of xylem embolism in Sequoia sempervirens saplings during cycles of drought and recovery

Publisher: Wiley

Date: 10-11-2014

DOI: 10.1111/NPH.13110

Abstract: The formation of emboli in xylem conduits can dramatically reduce hydraulic capacity and represents one of the principal mechanisms of drought‐induced mortality in woody plants. However, our understanding of embolism formation and repair is constrained by a lack of tools to directly and nondestructively measure these processes at high spatial resolution. Using synchrotron‐based microcomputed tomography (micro CT ), we examined embolism in the xylem of coast redwood ( Sequoia sempervirens ) saplings that were subjected to cycles of drought and rewatering. Embolism formation was observed occurring by three different mechanisms: as tracheids embolizing in wide tangential bands as isolated tracheids in seemingly random events and as functional groups connected to photosynthetic organs. Upon rewatering, stem water potential recovered to predrought stress levels within 24 h however, no evidence of embolism repair was observed even after a further 2 wk under well‐watered conditions. The results indicate that intertracheid air seeding is the primary mechanism by which embolism spreads in the xylem of S. sempervirens , but also show that a small number of tracheids initially become gas‐filled via another mechanism. The inability of S. sempervirens saplings to reverse drought‐induced embolism is likely to have important ecological impacts on this species.

The global spectrum of plant form and function: enhanced species-level trait dataset

Publisher: Springer Science and Business Media LLC

Date: 07-12-2022

DOI: 10.1038/S41597-022-01774-9

Abstract: Here we provide the ‘Global Spectrum of Plant Form and Function Dataset’, containing species mean values for six vascular plant traits. Together, these traits –plant height, stem specific density, leaf area, leaf mass per area, leaf nitrogen content per dry mass, and diaspore (seed or spore) mass – define the primary axes of variation in plant form and function. The dataset is based on ca. 1 million trait records received via the TRY database (representing ca. 2,500 original publications) and additional unpublished data. It provides 92,159 species mean values for the six traits, covering 46,047 species. The data are complemented by higher-level taxonomic classification and six categorical traits (woodiness, growth form, succulence, adaptation to terrestrial or aquatic habitats, nutrition type and leaf type). Data quality management is based on a probabilistic approach combined with comprehensive validation against expert knowledge and external information. Intense data acquisition and thorough quality control produced the largest and, to our knowledge, most accurate compilation of empirically observed vascular plant species mean traits to date.

Towards species‐level forecasts of drought‐induced tree mortality risk

Publisher: Wiley

Date: 22-04-2022

DOI: 10.1111/NPH.18129

Abstract: Predicting species‐level responses to drought at the landscape scale is critical to reducing uncertainty in future terrestrial carbon and water cycle projections. We embedded a stomatal optimisation model in the Community Atmosphere Biosphere Land Exchange (CABLE) land surface model and parameterised the model for 15 canopy dominant eucalypt tree species across South‐Eastern Australia (mean annual precipitation range: 344–1424 mm yr −1 ). We conducted three experiments: applying CABLE to the 2017–2019 drought a 20% drier drought and a 20% drier drought with a doubling of atmospheric carbon dioxide (CO 2 ). The severity of the drought was highlighted as for at least 25% of their distribution ranges, 60% of species experienced leaf water potentials beyond the water potential at which 50% of hydraulic conductivity is lost due to embolism. We identified areas of severe hydraulic stress within‐species’ ranges, but we also pinpointed resilience in species found in predominantly semiarid areas. The importance of the role of CO 2 in ameliorating drought stress was consistent across species. Our results represent an important advance in our capacity to forecast the resilience of in idual tree species, providing an evidence base for decision‐making around the resilience of restoration plantings or net‐zero emission strategies.

On research priorities to advance understanding of the safety–efficiency tradeoff in xylem

Publisher: Wiley

Date: 27-06-2016

DOI: 10.1111/NPH.14043

Automated analysis of three‐dimensional xylem networks using high‐resolution computed tomography

Publisher: Wiley

Date: 13-05-2011

DOI: 10.1111/J.1469-8137.2011.03754.X

Abstract: Connections between xylem vessels represent important links in the vascular network, but the complexity of three-dimensional (3D) organization has been difficult to access. This study describes the development of a custom software package called TANAX (Tomography-derived Automated Network Analysis of Xylem) that automatically extracts vessel dimensions and the distribution of intervessel connections from high-resolution computed tomography scans of grapevine (Vitis vinifera) stems, although the method could be applied to other species. Manual and automated analyses of vessel networks yielded similar results, with the automated method generating orders of magnitude more data in a fraction of the time. In 4.5-mm-long internode sections, all vessels and all intervessel connections among 115 vessels were located, and the connections were analyzed for their radial distribution, orientation, and predicted shared wall area. Intervessel connections were more frequent in lateral than in dorsal/ventral zones. The TANAX-reconstructed network, in combination with commercial software, was used to visualize vessel networks in 3D. The 3D volume renderings of vessel networks were freely rotated for observation from any angle, and the 4.5 μm virtual serial sections were capable of being viewed in any plane, revealing aspects of vessel organization not possible with traditional serial sections.

Visual and hydraulic techniques produce similar estimates of cavitation resistance in woody species

Publisher: Wiley

Date: 26-07-2020

DOI: 10.1111/NPH.16746

Abstract: Hydraulic failure of the plant vascular system is a principal cause of forest die‐off under drought. Accurate quantification of this process is essential to our understanding of the physiological mechanisms underpinning plant mortality. Imaging techniques increasingly are applied to estimate xylem cavitation resistance. These techniques allow for in situ measurement of embolism formation in real time, although the benefits and trade‐offs associated with different techniques have not been evaluated in detail. Here we compare two imaging methods, microcomputed tomography (microCT) and optical vulnerability (OV), to standard hydraulic methods for measurement of cavitation resistance in seven woody species representing a ersity of major phylogenetic and xylem anatomical groups. Across the seven species, there was strong agreement between cavitation resistance values (P 50 ) estimated from visualization techniques (microCT and OV) and between visual techniques and hydraulic techniques. The results indicate that visual techniques provide accurate estimates of cavitation resistance and the degree to which xylem hydraulic function is impacted by embolism. Results are discussed in the context of trade‐offs associated with each technique and possible causes of discrepancy between estimates of cavitation resistance provided by visual and hydraulic techniques.

Specific leaf area and vapour pressure deficit control live fuel moisture content

Publisher: Wiley

Date: 06-02-2023

DOI: 10.1111/1365-2435.14271

Abstract: The live fuel moisture content (LFMC) is an important precondition for wildfire activity, yet it remains challenging to predict LFMC due to the dynamic interplay between atmospheric and hydrological conditions that determine the plant's access to, and loss of water. We monitored LFMC and a range of plant water‐use traits (predawn and midday leaf water potentials [Ψ leaf ]), leaf traits (specific leaf area [SLA]), hydrological status (soil water content [SWC] in the shallow layer and full profile) and atmospheric variables (air temperature, vapour pressure deficit [VPD], CO 2 concentrations) in a mature eucalypt woodland at the Eucalyptus Free‐Air CO 2 Enrichment (EucFACE) facility during a drought. We combined plant traits, hydrological status and atmospheric variables into a biophysical model to predict LFMC dynamics, and compared these with predictions of LFMC based on a satellite model and established relationships between Ψ leaf and LFMC from pressure–volume curves. Predawn Ψ leaf could be well predicted from changes in SWC, but variation in midday Ψ leaf and LFMC were more responsive to atmospheric than hydrological variables. The biophysical model explained up to 89% of variability in LFMC and outperformed established approaches to predict LFMC. SLA was the single most important variable to predict LFMC, followed by VPD, which explained 33% of the remaining variability in LFMC. Our study demonstrates that the co‐variation of plant traits and atmospheric and hydrological conditions affect LFMC during drought, suggesting a new way forward for predicting LFMC by combining biophysical and satellite‐based models of LFMC with seasonal forecasts of meteorological and hydrological variables. Read the free Plain Language Summary for this article on the Journal blog.

Cavitation Resistance in Seedless Vascular Plants: The Structure and Function of Interconduit Pit Membranes    

Publisher: Oxford University Press (OUP)

Date: 28-04-2014

DOI: 10.1104/PP.113.226522

Abstract: Plant water transport occurs through interconnected xylem conduits that are separated by partially digested regions in the cell wall known as pit membranes. These structures have a dual function. Their porous construction facilitates water movement between conduits while limiting the spread of air that may enter the conduits and render them dysfunctional during a drought. Pit membranes have been well studied in woody plants, but very little is known about their function in more ancient lineages such as seedless vascular plants. Here, we examine the relationships between conduit air seeding, pit hydraulic resistance, and pit anatomy in 10 species of ferns (pteridophytes) and two lycophytes. Air seeding pressures ranged from 0.8 ± 0.15 MPa (mean ± sd) in the hydric fern Athyrium filix-femina to 4.9 ± 0.94 MPa in Psilotum nudum, an epiphytic species. Notably, a positive correlation was found between conduit pit area and vulnerability to air seeding, suggesting that the rare-pit hypothesis explains air seeding in early- erging lineages much as it does in many angiosperms. Pit area resistance was variable but averaged 54.6 MPa s m−1 across all surveyed pteridophytes. End walls contributed 52% to the overall transport resistance, similar to the 56% in angiosperm vessels and 64% in conifer tracheids. Taken together, our data imply that, irrespective of phylogenetic placement, selection acted on transport efficiency in seedless vascular plants and woody plants in equal measure by compensating for shorter conduits in tracheid-bearing plants with more permeable pit membranes.

The effects of Pierce's disease on leaf and petiole hydraulic conductance in Vitis vinifera cv. Chardonnay

Publisher: Wiley

Date: 10-07-2009

DOI: 10.1111/J.1399-3054.2009.01231.X

Abstract: In this study, we test the hypothesis that the symptoms of Pierce's Disease (PD) result from the occlusion of xylem conduits by the bacteria Xylella fastidiosa (Xf ). Four treatments were imposed on greenhouse-grown Vitis vinifera cv. Chardonnay: well-watered and deficit-irrigated plants with and without petiole inoculation with Xf. The hydraulic conductance of the stem-petiole junction (k(jun)) and leaves (k(leaf)) were measured, and Xf concentrations were established by quantitative polymerase chain reaction (qPCR). Leaf hydraulic conductance decreased with increasing leaf scorch symptoms in both irrigation treatments. The positive relationship between Xf concentration and symptom formation in deficit-irrigated plants suggests that water-stress increases susceptibility to PD. In field-grown vines, water relations of symptomatic leaves were similar to naturally senescing leaves but differed from green control leaves. Overall, these results suggest that the development of PD symptoms represents a form of accelerated senescence as part of a systemic response of the plant to Xf infection.

Seasonal variation in the xylem sap composition of six Australian trees and shrubs

Publisher: Oxford University Press (OUP)

Date: 12-09-2023

DOI: 10.1093/AOBPLA/PLAD064

Living on the edge

Publisher: Wiley

Date: 20-05-2021

DOI: 10.1111/GCB.15641

Abstract: Globally, forests are facing an increasing risk of mass tree mortality events associated with extreme droughts and higher temperatures. Hydraulic dysfunction is considered a key mechanism of drought‐triggered dieback. By leveraging the climate breadth of the Australian landscape and a national network of research sites (Terrestrial Ecosystem Research Network), we conducted a continental‐scale study of physiological and hydraulic traits of 33 native tree species from contrasting environments to disentangle the complexities of plant response to drought across communities. We found strong relationships between key plant hydraulic traits and site aridity. Leaf turgor loss point and xylem embolism resistance were correlated with minimum water potential experienced by each species. Across the data set, there was a strong coordination between hydraulic traits, including those linked to hydraulic safety, stomatal regulation and the cost of carbon investment into woody tissue. These results illustrate that aridity has acted as a strong selective pressure, shaping hydraulic traits of tree species across the Australian landscape. Hydraulic safety margins were constrained across sites, with species from wetter sites tending to have smaller safety margin compared with species at drier sites, suggesting trees are operating close to their hydraulic thresholds and forest biomes across the spectrum may be susceptible to shifts in climate that result in the intensification of drought.

Incorporating non-stomatal limitation improves the performance of leaf and canopy models at high vapour pressure deficit

Publisher: Oxford University Press (OUP)

Date: 14-10-2019

DOI: 10.1093/TREEPHYS/TPZ103

Abstract: Vapour pressure deficit (D) is projected to increase in the future as temperatures rise. In response to increased D, stomatal conductance (gs) and photosynthesis (A) are reduced, which may result in significant reductions in terrestrial carbon, water, and energy fluxes. It is thus important for gas exchange models to capture the observed responses of gs and A with increasing D. We tested a series of coupled A-gs models against leaf gas exchange measurements from the Cumberland Plain Woodland (Australia), where D regularly exceeds 2 kPa and can reach 8 kPa in summer. Two commonly used A-gs models (Leuning 1995 and Medlyn et al. 2011) were not able to capture the observed decrease in A and gs with increasing D at the leaf scale. To explain this decrease in A and gs, two alternative hypotheses were tested: hydraulic limitation (i.e., plants reduce gs and/or A due to insufficient water supply) and non-stomatal limitation (i.e., downregulation of photosynthetic capacity). We found that the model that incorporated a non-stomatal limitation captured the observations with high fidelity and required the fewest number of parameters. While the model incorporating hydraulic limitation captured the observed A and gs, it did so via a physical mechanism that is incorrect. We then incorporated a non-stomatal limitation into the stand model, MAESPA, to examine its impact on canopy transpiration and gross primary production. Accounting for a non-stomatal limitation reduced the predicted transpiration by ~19%, improving the correspondence with sap flow measurements, and gross primary production by ~14%. Given the projected global increases in D associated with future warming, these findings suggest that models may need to incorporate non-stomatal limitation to accurately simulate A and gs in the future with high D. Further data on non-stomatal limitation at high D should be a priority, in order to determine the generality of our results and develop a widely applicable model.

Dynamics of freeze–thaw embolism in Smilax rotundifolia (Smilacaceae)

Publisher: Wiley

Date: 04-2007

DOI: 10.3732/AJB.94.4.640

Abstract: Freeze-thaw cycles pose a major physiological challenge for all temperate perennial plants, but monocotyledonous vines face a still greater risk because their few large vessels are especially susceptible to embolism and are not replaced by secondary growth. The genus Smilax is particularly remarkable because it is widespread in the tropics but includes species that survive the hard frosts of New England winters. Smilax rotundifolia was monitored for a year for evidence of stem xylem freeze-thaw cavitation and refilling. Embolism of metaxylem was complete by late November and was completely reversed by late April, when root pressures rose as high as 100 kPa. Protoxylem remained full of sap throughout the year in cryogenic scanning electron micrographs. Three methods were used to quantify embolism: percent loss conductivity (PLC), gravimetric air fraction (GAF: mass of water in stem xylem relative to capacity), and cryogenic scanning electron microscopy (cryo-SEM). The three methods corroborated one another well and gave quantitatively similar results. Osmolality of xylem sap extracted from exuding stems was 64 mol/kg (±7.0, N = 8), consistent with the root pressures observed. Strong root pressure can account for Smilax's survival in temperate regions with severe frosts, where few monocots with persistent aboveground organs are found.

Tree hydraulic traits are coordinated and strongly linked to climate-of-origin across a rainfall gradient

Publisher: Wiley

Date: 29-01-2018

DOI: 10.1111/PCE.13129

Abstract: Plant hydraulic traits capture the impacts of drought stress on plant function, yet vegetation models lack sufficient information regarding trait coordination and variation with climate-of-origin across species. Here, we investigated key hydraulic and carbon economy traits of 12 woody species in Australia from a broad climatic gradient, with the aim of identifying the coordination among these traits and the role of climate in shaping cross-species trait variation. The influence of environmental variation was minimized by a common garden approach, allowing us to factor out the influence of environment on phenotypic variation across species. We found that hydraulic traits (leaf turgor loss point, stomatal sensitivity to drought [P

Measurement of vulnerability to water stress-induced cavitation in grapevine: a comparison of four techniques applied to a long-vesseled species

Publisher: Wiley

Date: 22-04-2010

DOI: 10.1111/J.1365-3040.2010.02160.X

Abstract: Among woody plants, grapevines are often described as highly vulnerable to water-stress induced cavitation with emboli forming at slight tensions. However, we found native embolism never exceeded 30% despite low xylem water potentials (Psi(x)) for stems of field grown vines. The discrepancy between native embolism measurements and those of previous reports led us to assess vulnerability curve generation using four separate methods and alterations (i.e. segment length and with/without flushing to remove embolism prior to measurement) of each. Centrifuge, dehydration and air-injection methods, which rely on measurement of percentage loss of hydraulic conductivity (PLC) in detached stems, were compared against non-invasive monitoring of xylem cavitation with nuclear magnetic resonance (NMR) imaging. Short segment air-injection and flushed centrifuge stems reached >90 PLC at Psi(x) of-0.5 and -1.5 MPa, respectively, whereas dehydration and long-segment air-injection measurements indicated no significant embolism at Psi(x) > -2.0 MPa. Observations from NMR agreed with the dehydration and long segment air-injection methods, showing the majority of vessels were still water-filled at Psi(x) > -1.5 MPa. Our findings show V. vinifera stems are far less vulnerable to water stress-induced cavitation than previously reported, and dehydration and long segment air-injection techniques are more appropriate for long-vesseled species and organs.

Unlocking Drought-Induced Tree Mortality: Physiological Mechanisms to Modeling

Publisher: Frontiers Media SA

Date: 04-04-2022

DOI: 10.3389/FPLS.2022.835921

Abstract: Drought-related tree mortality has become a major concern worldwide due to its pronounced negative impacts on the functioning and sustainability of forest ecosystems. However, our ability to identify the species that are most vulnerable to drought, and to pinpoint the spatial and temporal patterns of mortality events, is still limited. Model is useful tools to capture the dynamics of vegetation at spatiotemporal scales, yet contemporary land surface models (LSMs) are often incapable of predicting the response of vegetation to environmental perturbations with sufficient accuracy, especially under stressful conditions such as drought. Significant progress has been made regarding the physiological mechanisms underpinning plant drought response in the past decade, and plant hydraulic dysfunction has emerged as a key determinant for tree death due to water shortage. The identification of pivotal physiological events and relevant plant traits may facilitate forecasting tree mortality through a mechanistic approach, with improved precision. In this review, we (1) summarize current understanding of physiological mechanisms leading to tree death, (2) describe the functionality of key hydraulic traits that are involved in the process of hydraulic dysfunction, and (3) outline their roles in improving the representation of hydraulic function in LSMs. We urge potential future research on detailed hydraulic processes under drought, pinpointing corresponding functional traits, as well as understanding traits variation across and within species, for a better representation of drought-induced tree mortality in models.

Drought-related leaf functional traits control spatial and temporal dynamics of live fuel moisture content

Publisher: Elsevier BV

Date: 05-2022

DOI: 10.1016/J.AGRFORMET.2022.108941

Trees tolerate an extreme heatwave via sustained transpirational cooling and increased leaf thermal tolerance

Publisher: Wiley

Date: 02-02-2018

DOI: 10.1111/GCB.14037

Abstract: Heatwaves are likely to increase in frequency and intensity with climate change, which may impair tree function and forest C uptake. However, we have little information regarding the impact of extreme heatwaves on the physiological performance of large trees in the field. Here, we grew Eucalyptus parramattensis trees for 1 year with experimental warming (+3°C) in a field setting, until they were greater than 6 m tall. We withheld irrigation for 1 month to dry the surface soils and then implemented an extreme heatwave treatment of 4 consecutive days with air temperatures exceeding 43°C, while monitoring whole-canopy exchange of CO

Measurements of stem xylem hydraulic conductivity in the laboratory and field

Publisher: Wiley

Date: 25-04-2012

DOI: 10.1111/J.2041-210X.2012.00204.X

Leaf water potential measurements using the pressure chamber: Synthetic testing of assumptions towards best practices for precision and accuracy.

Publisher: Wiley

Date: 23-04-2022

DOI: 10.1111/PCE.14330

Abstract: Leaf water potential (ψ

Non-invasive imaging shows no evidence of embolism repair after drought in tree species of two genera.

Publisher: Oxford University Press (OUP)

Date: 22-08-2018

DOI: 10.1093/TREEPHYS/TPY093

Abstract: Drought stress can result in significant impairment of the plant hydraulic system via blockage of xylem conduits by gas emboli. Recovery after drought stress is an essential component of plant survival but is still a poorly understood process. In this study, we examined the capacity of woody species from two genera (Eucalyptus and Quercus) to refill embolized xylem vessels during a cycle of drought and recovery. Observations were made on intact plants of Eucalyptus calmudulensis, E. grandis, E. saligna and Quercus palustris using X-ray microtomography. We found no evidence of an effective xylem refilling mechanism in any of the plant species. Despite rehydration and recovery of plant water potential to near pre-drought levels, embolized vessels were not refilled up to 72 h after rewatering. In E. saligna, water droplets accumulated in previously air-filled vessels for a very small percentage of vessels. However, no instances of complete refilling that would restore embolized vessels to hydraulic function were observed. Our observations suggest that rapid refilling of embolized vessels after drought may not be a wide spread mechanism in woody plants and that embolism formed during drought represents long term cost to the plant hydraulic system.

Plant hydraulic modelling of leaf and canopy fuel moisture content reveals increasing vulnerability of a Mediterranean forest to wildfires under extreme drought

Publisher: Wiley

Date: 09-12-2022

DOI: 10.1111/NPH.18614

Abstract: Fuel moisture content (FMC) is a crucial driver of forest fires in many regions world‐wide. Yet, the dynamics of FMC in forest canopies as well as their physiological and environmental determinants remain poorly understood, especially under extreme drought. We embedded a FMC module in the trait‐based, plant‐hydraulic SurEau‐Ecos model to provide innovative process‐based predictions of leaf live fuel moisture content (LFMC) and canopy fuel moisture content (CFMC) based on leaf water potential (). SurEau‐Ecos‐FMC relies on pressure–volume ( p‐v ) curves to simulate LFMC and vulnerability curves to cavitation to simulate foliage mortality. SurEau‐Ecos‐FMC accurately reproduced and LFMC dynamics as well as the occurrence of foliage mortality in a Mediterranean Quercus ilex forest. Several traits related to water use (leaf area index, available soil water, and transpiration regulation), vulnerability to cavitation, and p‐v curves (full turgor osmotic potential) had the greatest influence on LFMC and CFMC dynamics. As the climate gets drier, our results showed that drought‐induced foliage mortality is expected to increase, thereby significantly decreasing CFMC. Our results represent an important advance in our capacity to understand and predict the sensitivity of forests to wildfires.

Woody plants optimise stomatal behaviour relative to hydraulic risk

Publisher: Wiley

Date: 23-04-2018

DOI: 10.1111/ELE.12962

Abstract: Stomatal response to environmental conditions forms the backbone of all ecosystem and carbon cycle models, but is largely based on empirical relationships. Evolutionary theories of stomatal behaviour are critical for guarding against prediction errors of empirical models under future climates. Longstanding theory holds that stomata maximise fitness by acting to maintain constant marginal water use efficiency over a given time horizon, but a recent evolutionary theory proposes that stomata instead maximise carbon gain minus carbon costs/risk of hydraulic damage. Using data from 34 species that span global forest biomes, we find that the recent carbon-maximisation optimisation theory is widely supported, revealing that the evolution of stomatal regulation has not been primarily driven by attainment of constant marginal water use efficiency. Optimal control of stomata to manage hydraulic risk is likely to have significant consequences for ecosystem fluxes during drought, which is critical given projected intensification of the global hydrological cycle.

Warming Reduces Net Carbon Gain and Productivity in Medicago sativa L. and Festuca arundinacea

Publisher: MDPI AG

Date: 19-10-2020

DOI: 10.3390/AGRONOMY10101601

Abstract: High temperature stress imposes constraints on the productivity of agricultural systems, such as pastures, and predicted increases in global temperatures are set to exacerbate these limitations. Here, we sought to understand the impact of warmer growth temperature on gas exchange and net primary productivity for two widely cultivated pasture species. We grew a C3 legume, Medicago sativa (lucerne), and a C3 grass, Festuca arundinacea Schreb. (tall fescue), in a climate-controlled facility exposed to two temperature treatments (ambient: 26 °C, aT elevated: 30 °C, eT). Soil water was maintained at non-limiting conditions in both temperature treatments to control for the confounding effects of warming on soil moisture. We found that warming reduced photosynthetic capacity and increased leaf dark respiration (Rdark) in lucerne, while tall fescue showed little physiological change at the leaf level, but increased ecosystem respiration (Reco). Growth temperature had no significant impact on the thermal optimum of photosynthesis (Topt) or water use efficiency in either species. Both species exhibited significant reductions in productivity with warming lucerne had greater reductions in shoot biomass, while tall fescue had greater reductions in root biomass. Our results highlight the potential for significant declines in pasture productivity associated with even modest increases in average temperature and highlights the need for suitable management strategies and implementation of more heat-resistant cultivars. Improvements in photosynthetic performance for greater heat tolerance in lucerne, and traits associated with biomass allocation and root performance at higher temperatures in tall fescue, should be the focus for improving high temperature resistance in these plant species.

Triggers of tree mortality under drought

Publisher: Springer Science and Business Media LLC

Date: 06-2018

DOI: 10.1038/S41586-018-0240-X

Abstract: Severe droughts have caused widespread tree mortality across many forest biomes with profound effects on the function of ecosystems and carbon balance. Climate change is expected to intensify regional-scale droughts, focusing attention on the physiological basis of drought-induced tree mortality. Recent work has shown that catastrophic failure of the plant hydraulic system is a principal mechanism involved in extensive crown death and tree mortality during drought, but the multi-dimensional response of trees to desiccation is complex. Here we focus on the current understanding of tree hydraulic performance under drought, the identification of physiological thresholds that precipitate mortality and the mechanisms of recovery after drought. Building on this, we discuss the potential application of hydraulic thresholds to process-based models that predict mortality.

Embracing 3D Complexity in Leaf Carbon–Water Exchange

Publisher: Elsevier BV

Date: 2019

DOI: 10.1016/J.TPLANTS.2018.09.005

Abstract: Leaves are a nexus for the exchange of water, carbon, and energy between terrestrial plants and the atmosphere. Research in recent decades has highlighted the critical importance of the underlying biophysical and anatomical determinants of CO

A unique web resource for physiology, ecology and the environmental sciences: PrometheusWiki

Publisher: CSIRO Publishing

Date: 2010

DOI: 10.1071/FP10097

Abstract: PROtocols, METHods, Explanations and Updated Standards Wiki (PrometheusWiki, www.publish.csiro.au rometheuswiki/) is a new open access, fully searchable web resource that contains protocols and methods for plant physiology, ecology and environmental sciences. Contributions can be uploaded by anyone in the community, with attributed authorship, and are open for wiki-style comment. This resource allows the gathering in one place of methods, links to published methods and detailed protocols used by leading laboratories around the world, with annotation. As a web resource, PrometheusWiki is continually evolving and updatable, easily and rapidly searchable and highly accessible. It will also enhance communication, allowing multimedia description of protocols and techniques, with spreadsheet tools, slide shows and video files easily integrated into the text. This resource is anticipated to lead to strong benefits in standardising methods, improving access to training for students and professionals, promoting collaborations and expanding the cutting edge of research.

Global convergence in the vulnerability of forests to drought

Publisher: Springer Science and Business Media LLC

Date: 11-2012

DOI: 10.1038/NATURE11688

Abstract: Shifts in rainfall patterns and increasing temperatures associated with climate change are likely to cause widespread forest decline in regions where droughts are predicted to increase in duration and severity. One primary cause of productivity loss and plant mortality during drought is hydraulic failure. Drought stress creates trapped gas emboli in the water transport system, which reduces the ability of plants to supply water to leaves for photosynthetic gas exchange and can ultimately result in desiccation and mortality. At present we lack a clear picture of how thresholds to hydraulic failure vary across a broad range of species and environments, despite many in idual experiments. Here we draw together published and unpublished data on the vulnerability of the transport system to drought-induced embolism for a large number of woody species, with a view to examining the likely consequences of climate change for forest biomes. We show that 70% of 226 forest species from 81 sites worldwide operate with narrow (<1 megapascal) hydraulic safety margins against injurious levels of drought stress and therefore potentially face long-term reductions in productivity and survival if temperature and aridity increase as predicted for many regions across the globe. Safety margins are largely independent of mean annual precipitation, showing that there is global convergence in the vulnerability of forests to drought, with all forest biomes equally vulnerable to hydraulic failure regardless of their current rainfall environment. These findings provide insight into why drought-induced forest decline is occurring not only in arid regions but also in wet forests not normally considered at drought risk.

Linking physiological processes with mangrove forest structure: phosphorus deficiency limits canopy development, hydraulic conductivity and photosynthetic carbon gain in dwarf Rhizophora mangle

Publisher: Wiley

Date: 21-02-2006

DOI: 10.1111/J.1365-3040.2005.01446.X

Abstract: Spatial gradients in mangrove tree height in barrier islands of Belize are associated with nutrient deficiency and sustained flooding in the absence of a salinity gradient. While nutrient deficiency is likely to affect many parameters, here we show that addition of phosphorus (P) to dwarf mangroves stimulated increases in diameters of xylem vessels, area of conductive xylem tissue and leaf area index (LAI) of the canopy. These changes in structure were consistent with related changes in function, as addition of P also increased hydraulic conductivity (Ks), stomatal conductance and photosynthetic assimilation rates to the same levels measured in taller trees fringing the seaward margin of the mangrove. Increased xylem vessel size and corresponding enhancements in stem hydraulic conductivity in P fertilized dwarf trees came at the cost of enhanced mid-day loss of hydraulic conductivity and was associated with decreased assimilation rates in the afternoon. Analysis of trait plasticity identifies hydraulic properties of trees as more plastic than those of leaf structural and physiological characteristics, implying that hydraulic properties are key in controlling growth in mangroves. Alleviation of P deficiency, which released trees from hydraulic limitations, reduced the structural and functional distinctions between dwarf and taller fringing tree forms of Rhizophora mangle.

Is embolism resistance in plant xylem associated with quantity and characteristics of lignin?

Publisher: Springer Science and Business Media LLC

Date: 24-06-2017

DOI: 10.1007/S00468-017-1574-Y

Testing hypotheses that link wood anatomy to cavitation resistance and hydraulic conductivity in the genus Acer

Publisher: Wiley

Date: 04-11-2018

DOI: 10.1111/J.1469-8137.2010.03518.X

Abstract: Tansley medal winner p 509

Direct measurements of intervessel pit membrane hydraulic resistance in two angiosperm tree species

Publisher: Wiley

Date: 07-2006

DOI: 10.3732/AJB.93.7.993

Abstract: The hydraulic resistance of pit membranes was measured directly in earlywood vessels of Fraxinus americana and Ulmus americana. The area-specific resistance of pit membranes (r(mem)) was higher than modeled or measured values obtained previously for hardwood species, with r(mem) of 5.24 × 10(3) MPa·s·m(-1) for Fraxinus and 2.56 × 10(3) MPa·s·m(-1) for Ulmus. The calculated resistance of pit canals was three orders of magnitude below total pit resistance indicating that pit membranes contributed the majority of resistance. Scanning electron microscopy indicated that pit membranes of Ulmus were thinner and more porous than those of Fraxinus, consistent with the difference in r(mem) between the species. Measurements of average vessel diameter and length and area of wall overlap with neighboring vessels were used to partition the vascular resistance between vessel lumen and pit membrane components. Pit membrane resistance accounted for 80% of the total resistance in Fraxinus and 87% in Ulmus in 2-yr-old branch sections. However, measurements of vessel dimensions in the trunk suggest that the ision of resistance between pit membrane and lumen components would be closer to co-limiting in older regions of the tree. Thus, pit membrane resistance may be of greater relative importance in small branches than in older regions of mature trees.

Mitigating the open vessel artefact in centrifuge-based measurement of embolism resistance

Publisher: Oxford University Press (OUP)

Date: 08-2018

DOI: 10.1093/TREEPHYS/TPY083

Abstract: Centrifuge-based techniques to assess xylem vulnerability to embolism are increasingly being used, although we are yet to reach a consensus on the nature and extent of artefactual embolism observed in some angiosperm species. In particular, there is disagreement over whether these artefacts influence both the spin (Cavitron) and static versions of the centrifuge technique equally. We tested two methods for inducing embolism: bench dehydration and centrifugation. We used three methods to measure the resulting loss of conductivity: gravimetric flow measured in bench-dehydrated and centrifuged s les (static centrifuge), in situ flow measured under tension during spinning in the centrifuge (Cavitron) and direct imaging using X-ray computed microtomography (microCT) observations in stems of two species of Hakea that differ in vessel length. Both centrifuge techniques were prone to artefactual embolism in s les with maximum vessel length longer than, or similar to, the centrifuge rotor diameter. Observations with microCT indicated that this artefactual embolism occurred in the outermost portions of s les. The artefact was largely eliminated if flow was measured in an excised central part of the segment in the static centrifuge or starting measurements with the Cavitron at pressures lower than the threshold of embolism formation in open vessels. The simulations of loss of conductivity in centrifuged s les with a new model, CAVITOPEN, confirmed that the impact of open vessels on the vulnerability to embolism curve was higher when vessels were long, s les short and when embolism is formed in open vessels at less negative pressures. This model also offers a robust and quantitative tool to test and correct for artefactual embolism at low xylem tensions.

Elevated [CO₂] does not ameliorate the negative effects of elevated temperature on drought‐induced mortality in Eucalyptus radiata seedlings

Publisher: Wiley

Date: 13-02-2014

DOI: 10.1111/PCE.12260

Abstract: It has been reported that elevated temperature accelerates the time-to-mortality in plants exposed to prolonged drought, while elevated [CO(2)] acts as a mitigating factor because it can reduce stomatal conductance and thereby reduce water loss. We examined the interactive effects of elevated [CO(2)] and temperature on the inter-dependent carbon and hydraulic characteristics associated with drought-induced mortality in Eucalyptus radiata seedlings grown in two [CO(2)] (400 and 640 μL L(-1)) and two temperature (ambient and ambient +4 °C) treatments. Seedlings were exposed to two controlled drying and rewatering cycles, and then water was withheld until plants died. The extent of xylem cavitation was assessed as loss of stem hydraulic conductivity. Elevated temperature triggered more rapid mortality than ambient temperature through hydraulic failure, and was associated with larger water use, increased drought sensitivities of gas exchange traits and earlier occurrence of xylem cavitation. Elevated [CO(2)] had a negligible effect on seedling response to drought, and did not ameliorate the negative effects of elevated temperature on drought. Our findings suggest that elevated temperature and consequent higher vapour pressure deficit, but not elevated [CO(2)], may be the primary contributors to drought-induced seedling mortality under future climates.

Is xylem of angiosperm leaves less resistant to embolism than branches? Insights from microCT, hydraulics, and anatomy

Publisher: Oxford University Press (OUP)

Date: 09-2018

DOI: 10.1093/JXB/ERY321

Desiccation time during drought is highly predictable across species of Eucalyptus from contrasting climates

Publisher: Wiley

Date: 08-08-2019

DOI: 10.1111/NPH.16042

Abstract: Catastrophic failure of the water transport pathway in trees is a principal mechanism of mortality during extreme drought. To be able to predict the probability of mortality at an in idual and landscape scale we need knowledge of the time for plants to reach critical levels of hydraulic failure. We grew plants of eight species of Eucalyptus originating from contrasting climates before allowing a subset to dehydrate. We tested whether a trait-based model of time to plant desiccation t

Xylem Embolism Spreads by Single-Conduit Events in Three Dry Forest Angiosperm Stems

Publisher: Oxford University Press (OUP)

Date: 24-06-2020

DOI: 10.1104/PP.20.00464

Leaf economics and plant hydraulics drive leaf : wood area ratios

Publisher: Wiley

Date: 15-07-2019

DOI: 10.1111/NPH.15998

Abstract: Biomass and area ratios between leaves, stems and roots regulate many physiological and ecological processes. The Huber value H

How drought and deciduousness shape xylem plasticity in three Costa Rican woody plant species

Publisher: Brill

Date: 06-12-2014

DOI: 10.1163/22941932-00000070

Abstract: This paper explores the phenotypic plasticity of xylem in tropical trees in order to test the hypothesis that different leaf phenological patterns influence levels of xylem plasticity. Wood anatomy was studied in the wet-deciduous species Cordia alliodora (Ruiz & Pav.) Oken, the dry-deciduous species Tabebuia rosea (Bertol.) DC., and the evergreen species Ocotea veraguensis (Meisn.) Mez., collected from seasonally dry forest and tropical cloud forest in Costa Rica. Xylem plasticity and trait conservatism were examined by analysing the coefficient of variation ( CV ) and the relative distance plasticity index ( RDPI ) of xylem anatomical traits. The two deciduous species exhibited wider vessels, lower wood density, and higher Huber values than the evergreen species. Furthermore, intervessel connectivity was highest for the two deciduous species in seasonally dry forest compared to cloud forest, whereas the opposite was found for the evergreen species. Overall highest trait variability was found for all plants at the seasonally dry site. The evergreen species O. veraguensis had the highest plasticity values compared to the deciduous species. Highest plasticity was found for vessel composition index (vessel area/vessel number), whilestrongest trait conservatism was found for vessel diameter. In conclusion, our data indicate that evergreen O. veraguensis does not show less xylem plasticity than the two deciduous tree species studied.

Species climate range influences hydraulic and stomatal traits in Eucalyptus species

Publisher: Oxford University Press (OUP)

Date: 27-03-2017

DOI: 10.1093/AOB/MCX020

Increasing leaf hydraulic conductance with transpiration rate minimizes the water potential drawdown from stem to leaf

Publisher: Oxford University Press (OUP)

Date: 29-12-2015

DOI: 10.1093/JXB/ERU481

Drought tolerance of Hakea species (Proteaceae) from a range of biomes and life-histories predicted by climatic niche

Publisher: Cold Spring Harbor Laboratory

Date: 02-09-2020

DOI: 10.1101/2020.09.01.276931

Abstract: Extreme drought conditions across the globe are impacting bio ersity with serious implications for the persistence of native species. However, quantitative data on drought tolerance is not available for erse flora to inform conservation management. We quantified physiological drought tolerance in the erse Hakea genus (Proteaceae) to test predictions based on climatic-origin, life history and functional traits. We s led terminal branches of replicate plants of 16 species in a common garden. Xylem cavitation was induced in branches under varying water potential (tension) in a centrifuge and the tension generating 50% loss of conductivity (stem P50) was characterized as a metric for drought tolerance. The same branches were used to estimate plant functional traits, including wood density, specific leaf area, and Huber value (sap flow area to leaf area ratio). There was significant variation in stem P50 among species, which was negatively associated with the species climate-origin (rainfall and aridity). Drought tolerance did not differ among life histories however, a drought avoidance strategy with terete leaf form and greater Huber value may be important for species to colonize and persist in the arid biome. Our findings will contribute to future prediction of species vulnerability to drought and adaptive management under climate change.

Corrigendum: Unlocking drought-induced tree mortality: Physiological mechanisms to modelling(Front. Plant Sci., (2022), 13, (835921), 10.3389/fpls.2022.835921)

Publisher: Frontiers Media SA

Date: 09-01-2023

DOI: 10.3389/FPLS.2022.1126049

One Stomatal Model to Rule Them All? Toward Improved Representation of Carbon and Water Exchange in Global Models

Publisher: American Geophysical Union (AGU)

Date: 04-2022

DOI: 10.1029/2021MS002761

Abstract: Stomatal conductance schemes that optimize with respect to photosynthetic and hydraulic functions have been proposed to address biases in land‐surface model (LSM) simulations during drought. However, systematic evaluations of both optimality‐based and alternative empirical formulations for coupling carbon and water fluxes are lacking. Here, we embed 12 empirical and optimization approaches within a LSM framework. We use theoretical model experiments to explore parameter identifiability and understand how model behaviors differ in response to abiotic changes. We also evaluate the models against leaf‐level observations of gas‐exchange and hydraulic variables, from xeric to wet forest/woody species spanning a mean annual precipitation range of 361–3,286 mm yr −1 . We find that models differ in how easily parameterized they are, due to: (a) poorly constrained optimality criteria (i.e., resulting in multiple solutions), (b) low influence parameters, (c) sensitivities to environmental drivers. In both the idealized experiments and compared to observations, sensitivities to variability in environmental drivers do not agree among models. Marked differences arise in sensitivities to soil moisture (soil water potential) and vapor pressure deficit. For ex le, stomatal closure rates at high vapor pressure deficit range between −45% and +70% of those observed. Although over half the new generation of stomatal schemes perform to a similar standard compared to observations of leaf‐gas exchange, two models do so through large biases in simulated leaf water potential (up to 11 MPa). Our results provide guidance for LSM development, by highlighting key areas in need for additional experimentation and theory, and by constraining currently viable stomatal hypotheses.

AusTraits, a curated plant trait database for the Australian flora

Publisher: Springer Science and Business Media LLC

Date: 30-09-2021

DOI: 10.1038/S41597-021-01006-6

Abstract: We introduce the AusTraits database - a compilation of values of plant traits for taxa in the Australian flora (hereafter AusTraits). AusTraits synthesises data on 448 traits across 28,640 taxa from field c aigns, published literature, taxonomic monographs, and in idual taxon descriptions. Traits vary in scope from physiological measures of performance (e.g. photosynthetic gas exchange, water-use efficiency) to morphological attributes (e.g. leaf area, seed mass, plant height) which link to aspects of ecological variation. AusTraits contains curated and harmonised in idual- and species-level measurements coupled to, where available, contextual information on site properties and experimental conditions. This article provides information on version 3.0.2 of AusTraits which contains data for 997,808 trait-by-taxon combinations. We envision AusTraits as an ongoing collaborative initiative for easily archiving and sharing trait data, which also provides a template for other national or regional initiatives globally to fill persistent gaps in trait knowledge.

Evidence for Hydraulic Vulnerability Segmentation and Lack of Xylem Refilling under Tension

Publisher: Oxford University Press (OUP)

Date: 09-09-2016

DOI: 10.1104/PP.16.01079

Cover Image

Publisher: Wiley

Date: 07-10-2021

DOI: 10.1111/ELE.13560

Water, nitrogen and phosphorus use efficiencies of four tree species in response to variable water and nutrient supply

Publisher: Springer Science and Business Media LLC

Date: 31-03-2016

DOI: 10.1007/S11104-016-2873-6

Carbon dynamics of eucalypt seedlings exposed to progressive drought in elevated [CO2] and elevated temperature

Publisher: Oxford University Press (OUP)

Date: 08-2013

DOI: 10.1093/TREEPHYS/TPT061

Abstract: Elevated [CO2] and temperature may alter the drought responses of tree seedling growth, photosynthesis, respiration and total non-structural carbohydrate (TNC) status depending on drought intensity and duration. Few studies have addressed these important climatic interactions or their consequences. We grew Eucalyptus globulus Labill. seedlings in two [CO2] concentrations (400 and 640 μl l(-1)) and two temperatures (28/17 and 32/21 °C) (day/night) in a sun-lit glasshouse, and grew them in well-watered conditions or exposed them to two drought treatments having undergone different previous water conditions (i.e., rewatered drought and sustained drought). Progressive drought in both drought treatments led to similar limitations in growth, photosynthesis and respiration, but reductions in TNC concentration were not observed. Elevated [CO2] ameliorated the impact of the drought during the moderate drought phase (i.e., Day 63 to Day 79) by increasing photosynthesis and enhancing leaf and whole-plant TNC content. In contrast, elevated temperature exacerbated the impact of the drought during the moderate drought phase by reducing photosynthesis, increasing leaf respiration and decreasing whole-plant TNC content. Extreme drought (i.e., Day 79 to Day 103) eliminated [CO2] and temperature effects on plant growth, photosynthesis and respiration. The combined effects of elevated [CO2] and elevated temperature on moderate drought stressed seedlings were reduced with progressive drought, with no sustained effects on growth despite greater whole-plant TNC content.

Intervascular pit membranes with a torus in the wood of Ulmus (Ulmaceae) and related genera

Publisher: Wiley

Date: 13-05-2004

DOI: 10.1111/J.1469-8137.2004.01097.X

Noninvasive Measurement of Vulnerability to Drought-Induced Embolism by X-Ray Microtomography

Publisher: Oxford University Press (OUP)

Date: 02-11-2015

DOI: 10.1104/PP.15.00732

Vascular Function in Grape Berries across Development and Its Relevance to Apparent Hydraulic Isolation

Publisher: Oxford University Press (OUP)

Date: 09-09-2009

DOI: 10.1104/PP.109.143172

Coordination of stem and leaf traits define different strategies to regulate water loss and tolerance ranges to aridity

Publisher: Wiley

Date: 18-02-2021

DOI: 10.1111/NPH.17185

Abstract: Adaptation to drought involves complex interactions of traits that vary within and among species. To date, few data are available to quantify within‐species variation in functional traits and they are rarely integrated into mechanistic models to improve predictions of species response to climate change. We quantified intraspecific variation in functional traits of two Hakea species growing along an aridity gradient in southeastern Australia. Measured traits were later used to parameterise the model SurEau to simulate a transplantation experiment to identify the limits of drought tolerance. Embolism resistance varied between species but not across populations. Instead, populations adjusted to drier conditions via contrasting sets of trait trade‐offs that facilitated homeostasis of plant water status. The species from relatively mesic climate, Hakea dactyloides , relied on tight stomatal control whereas the species from xeric climate, Hakea leucoptera dramatically increased Huber value and leaf mass per area, while leaf area index (LAI) and epidermal conductance ( g min ) decreased. With trait variability, SurEau predicts the plasticity of LAI and g min buffers the impact of increasing aridity on population persistence. Knowledge of within‐species variability in multiple drought tolerance traits will be crucial to accurately predict species distributional limits.

Plasticity in Vulnerability to Cavitation of Pinus canariensis Occurs Only at the Driest End of an Aridity Gradient

Publisher: Frontiers Media SA

Date: 03-06-2016

DOI: 10.3389/FPLS.2016.00769

The Dynamics of Embolism Repair in Xylem: In Vivo Visualizations Using High-Resolution Computed Tomography      

Publisher: Oxford University Press (OUP)

Date: 14-09-2010

DOI: 10.1104/PP.110.162396

Abstract: Water moves through plants under tension and in a thermodynamically metastable state, leaving the nonliving vessels that transport this water vulnerable to blockage by gas embolisms. Failure to reestablish flow in embolized vessels can lead to systemic loss of hydraulic conductivity and ultimately death. Most plants have developed a mechanism to restore vessel functionality by refilling embolized vessels, but the details of this process in vessel networks under tension have remained unclear for decades. Here we present, to our knowledge, the first in vivo visualization and quantification of the refilling process for any species using high-resolution x-ray computed tomography. Successful vessel refilling in grapevine (Vitis vinifera) was dependent on water influx from surrounding living tissue at a rate of 6 × 10−4 μm s−1, with in idual droplets expanding over time, filling vessels, and forcing the dissolution of entrapped gas. Both filling and draining processes could be observed in the same vessel, indicating that successful refilling requires hydraulic isolation from tensions that would otherwise prevent embolism repair. Our study demonstrates that despite the presence of tensions in the bulk xylem, plants are able to restore hydraulic conductivity in the xylem.

Lack of vulnerability segmentation in four angiosperm tree species: evidence from direct X-ray microtomography observation

Publisher: Springer Science and Business Media LLC

Date: 31-03-2020

DOI: 10.1007/S13595-020-00944-2

Hanging by a thread? Forests and drought

Publisher: American Association for the Advancement of Science (AAAS)

Date: 17-04-2020

DOI: 10.1126/SCIENCE.AAT7631

Abstract: Trees are the living foundations on which most terrestrial bio ersity is built. Central to the success of trees are their woody bodies, which connect their elevated photosynthetic canopies with the essential belowground activities of water and nutrient acquisition. The slow construction of these carbon-dense, woody skeletons leads to a slow generation time, leaving trees and forests highly susceptible to rapid changes in climate. Other long-lived, sessile organisms such as corals appear to be poorly equipped to survive rapid changes, which raises questions about the vulnerability of contemporary forests to future climate change. The emerging view that, similar to corals, tree species have rather inflexible damage thresholds, particularly in terms of water stress, is especially concerning. This Review examines recent progress in our understanding of how the future looks for forests growing in a hotter and drier atmosphere.

Coordination between leaf, stem, and root hydraulics and gas exchange in three arid‐zone angiosperms during severe drought and recovery

Publisher: Wiley

Date: 14-09-2018

DOI: 10.1111/PCE.13418

Abstract: The ability to resist hydraulic dysfunction in leaves, stems, and roots strongly influences whether plants survive and recover from drought. However, the coordination of hydraulic function among different organs within species and their links to gas exchange during drought and recovery remains understudied. Here, we examine the interaction between gas exchange and hydraulic function in the leaves, stems, and roots of three semiarid evergreen species exposed to a cycle of severe water stress (associated with substantial cavitation) and recovery. In all species, stomatal closure occurred at water potentials well before 50% loss of stem hydraulic conductance, while in two species, leaves and/or roots were more vulnerable than stems. Following soil rewetting, leaf-level photosynthesis (A

Drought tolerance traits do not vary across sites differing in water availability in Banksia serrata (Proteaceae)

Publisher: CSIRO Publishing

Date: 2019

DOI: 10.1071/FP18238

Abstract: Interspecific variation in plant hydraulic traits plays a major role in shaping species distributions across climates, yet variation within species is poorly understood. Here we report on intraspecific variation of hydraulic traits in Banksia serrata (L.f.) s led from three sites characterised by contrasting climates (warm-wet, warm-dry and cool-wet). Hydraulic characteristics including vulnerability to embolism, hydraulic conductance, pressure-volume traits and key morphological traits were measured. Vulnerability to embolism in leaf and stem, defined by the water potential inducing 50 and 88% loss of hydraulic conductivity (P50 and P88 respectively), did not differ across sites. However, plants from the warm-dry environment exhibited higher stem conductivity (Ks) than the cool-wet environment. Leaf turgor loss point (TLP) did not vary among sites, but warm-dry site plants showed lower leaf capacitance (C*FT) and higher modulus of elasticity (ε) than the other two sites. Plants from the cool-wet site had lower specific leaf area (SLA) and plants from the warm-dry site had lower sapwood density (WD). Overall, key hydraulic traits were generally conserved across populations despite differences in mean site water availability, and the safety-efficiency trade-off was absent in this species. These results suggest that B. serrata has limited ability to adjust hydraulic architecture in response to environmental change and thus may be susceptible to climate change-type drought stress.

Morphological variation of intervessel pit membranes and implications to xylem function in angiosperms

Publisher: Wiley

Date: 02-2009

DOI: 10.3732/AJB.0800248

Abstract: Pit membranes between xylem vessels have been suggested to have functional adaptive traits because of their influence on hydraulic resistance and vulnerability to embolism in plants. Observations of intervessel pit membranes in 26 hardwood species using electron microscopy showed significant variation in their structure, with a more than 25-fold difference in thickness (70-1892 nm) and observed maximum pore diameter (10-225 nm). In some SEM images, pit membrane porosity was affected by s le preparation, although pores were resolvable in intact pit membranes of many species. A significant relationship (r(2) = 0.7, P = 0.002) was found between pit membrane thickness and maximum pore diameter, indicating that the thinner membranes are usually more porous. In a subset of nine species, maximum pore diameter determined from SEM was correlated with pore diameter calculated from air-seeding thresholds (r(2) = 0.8, P < 0.001). Our data suggest that SEM images of intact pit membranes underestimate the porosity of pit membranes in situ. Pit membrane porosity based on SEM offers a relative estimate of air-seeding thresholds, but absolute pore diameters must be treated with caution. The implications of variation in pit membrane thickness and porosity to plant function are discussed.

Are needles of Pinus pinaster more vulnerable to xylem embolism than branches? New insights from X‐ray computed tomography

Publisher: Wiley

Date: 21-12-2015

DOI: 10.1111/PCE.12680

Abstract: Plants can be highly segmented organisms with an independently redundant design of organs. In the context of plant hydraulics, leaves may be less embolism resistant than stems, allowing hydraulic failure to be restricted to distal organs that can be readily replaced. We quantified drought-induced embolism in needles and stems of Pinus pinaster using high-resolution computed tomography (HRCT). HRCT observations of needles were compared with the rehydration kinetics method to estimate the contribution of extra-xylary pathways to declining hydraulic conductance. High-resolution computed tomography images indicated that the pressure inducing 50% of embolized tracheids was similar between needle and stem xylem (P50 needle xylem = -3.62 MPa, P50 stem xylem = -3.88 MPa). Tracheids in both organs showed no difference in torus overlap of bordered pits. However, estimations of the pressure inducing 50% loss of hydraulic conductance at the whole needle level by the rehydration kinetics method were significantly higher (P50 needle = -1.71 MPa) than P50 needle xylem derived from HRCT. The vulnerability segmentation hypothesis appears to be valid only when considering hydraulic failure at the entire needle level, including extra-xylary pathways. Our findings suggest that native embolism in needles is limited and highlight the importance of imaging techniques for vulnerability curves.

Hydraulic architecture of deciduous and evergreen dry rainforest tree species from north-eastern Australia

Publisher: Springer Science and Business Media LLC

Date: 09-12-2005

DOI: 10.1007/S00468-004-0392-1

Plant water potential improves prediction of empirical stomatal models

Publisher: Public Library of Science (PLoS)

Date: 12-10-2017

DOI: 10.1371/JOURNAL.PONE.0185481

AusTraits – a curated plant trait database for the Australian flora

Publisher: Cold Spring Harbor Laboratory

Date: 07-01-2021

DOI: 10.1101/2021.01.04.425314

Abstract: We introduce the AusTraits database - a compilation of measurements of plant traits for taxa in the Australian flora (hereafter AusTraits). AusTraits synthesises data on 375 traits across 29230 taxa from field c aigns, published literature, taxonomic monographs, and in idual taxa descriptions. Traits vary in scope from physiological measures of performance (e.g. photosynthetic gas exchange, water-use efficiency) to morphological parameters (e.g. leaf area, seed mass, plant height) which link to aspects of ecological variation. AusTraits contains curated and harmonised in idual-, species- and genus-level observations coupled to, where available, contextual information on site properties. This data descriptor provides information on version 2.1.0 of AusTraits which contains data for 937243 trait-by-taxa combinations. We envision AusTraits as an ongoing collaborative initiative for easily archiving and sharing trait data to increase our collective understanding of the Australian flora.

Meta-analysis reveals that hydraulic traits explain cross-species patterns of drought-induced tree mortality across the globe

Publisher: Proceedings of the National Academy of Sciences

Date: 18-04-2016

DOI: 10.1073/PNAS.1525678113

Abstract: Predicting the impacts of climate extremes on plant communities is a central challenge in ecology. Physiological traits may improve prediction of drought impacts on forests globally. We perform a meta-analysis across 33 studies that span all forested biomes and find that, among the examined traits, hydraulic traits explain cross-species patterns in mortality from drought. Gymnosperm and angiosperm mortality was associated with different hydraulic traits, giving insight into the relative weights of different traits and mechanisms in mortality prediction. Our results provide a foundation for more mechanistic predictions of drought-induced tree mortality across Earth’s erse forests.

Leaf gas exchange performance and the lethal water potential of five European species during drought

Publisher: Oxford University Press (OUP)

Date: 27-11-2015

DOI: 10.1093/TREEPHYS/TPV117

Abstract: Establishing physiological thresholds to drought-induced mortality in a range of plant species is crucial in understanding how plants respond to severe drought. Here, five common European tree species were selected (Acer c estre L., Acer pseudoplatanus L., Carpinus betulus L., Corylus avellana L. and Fraxinus excelsior L.) to study their hydraulic thresholds to mortality. Photosynthetic parameters during desiccation and the recovery of leaf gas exchange after rewatering were measured. Stem vulnerability curves and leaf pressure-volume curves were investigated to understand the hydraulic coordination of stem and leaf tissue traits. Stem and root s les from well-watered and severely drought-stressed plants of two species were observed using transmission electron microscopy to visualize mortality of cambial cells. The lethal water potential (ψlethal) correlated with stem P99 (i.e., the xylem water potential at 99% loss of hydraulic conductivity, PLC). However, several plants that were stressed beyond the water potential at 100% PLC showed complete recovery during the next spring, which suggests that the ψlethal values were underestimated. Moreover, we observed a 1 : 1 relationship between the xylem water potential at the onset of embolism and stomatal closure, confirming hydraulic coordination between leaf and stem tissues. Finally, ultrastructural changes in the cytoplasm of cambium tissue and mortality of cambial cells are proposed to provide an alternative approach to investigate the point of no return associated with plant death.

Centrifuge technique consistently overestimates vulnerability to water stress‐induced cavitation in grapevines as confirmed with high‐resolution computed tomography

Publisher: Wiley

Date: 16-07-2012

DOI: 10.1111/J.1469-8137.2012.04244.X

Structure and function of bordered pits: new discoveries and impacts on whole‐plant hydraulic function

Publisher: Wiley

Date: 13-12-2007

DOI: 10.1111/J.1469-8137.2007.02317.X

Abstract: Bordered pits are cavities in the lignified cell walls of xylem conduits (vessels and tracheids) that are essential components in the water‐transport system of higher plants. The pit membrane, which lies in the center of each pit, allows water to pass between xylem conduits but limits the spread of embolism and vascular pathogens in the xylem. Averaged across a wide range of species, pits account for 50% of total xylem hydraulic resistance, indicating that they are an important factor in the overall hydraulic efficiency of plants. The structure of pits varies dramatically across species, with large differences evident in the porosity and thickness of pit membranes. Because greater porosity reduces hydraulic resistance but increases vulnerability to embolism, differences in pit structure are expected to correlate with trade‐offs between efficiency and safety of water transport. However, trade‐offs in hydraulic function are influenced both by pit‐level differences in structure (e.g. average porosity of pit membranes) and by tissue‐level changes in conduit allometry (average length, diameter) and the total surface area of pit membranes that connects vessels. In this review we address the impact of variation in pit structure on water transport in plants from the level of in idual pits to the whole plant. Contents Summary 1 I. Introduction 2 II. The basic structures 2 III. Safety of transport ‐ pits as safety valves 5 IV. Hydraulic resistance of pits and pit membranes ‐ the cost of safety 9 V. Impacts of pit function on whole‐plant hydraulics 14 VI. Future directions 15 Acknowledgements 15 References 15

Using dense Sentinel-2 time series to explore combined fire and drought impacts in eucalypt forests

Publisher: Frontiers Media SA

Date: 02-02-2023

DOI: 10.3389/FFGC.2023.1018936

Abstract: Following one of the driest years on record, millions of hectares of forests in southeast Australia were burned in the 2019–2020 “Black Summer” wildfires. In addition to the areas burned, drought related canopy collapse, dieback and tree mortality was widely observed. In this paper, we present a method to map canopy damage due to drought and fire across a large area. Sentinel-2 satellite imagery was used in a monthly time series to highlight areas of forest where the Normalized Burn Ratio index was significantly below a pre-disturbance “stable” period. The stable period was defined as the 3 years prior to 2019 and the disturbance thresholds are based on bioregion specific standard deviations below pre-disturbance means. The novel methods enabled drought impacted forests to be identified, including those which were subsequently burned by wildfire. Across the 20 Mha of forests studied, 9.9 Mha (49%) fell below the disturbance threshold. Of that, 5.8 Mha was disturbed by fire and a further 4.1 Mha by drought outside of the fire extent. Within the fire extent, almost 0.9 Mha was identified as being significantly drought affected prior to being burned. An analysis of spectral recovery following substantial rainfall from February 2020 onward indicates that most of the areas impacted by both drought and fire have similar rates of recovery to those impacted only by fire. There are some areas, however, where the combined effects of the “double disturbance” appears to be hindering recovery. The methods presented here are easily transferrable and demonstrate an approach for monitoring forest disturbance at higher temporal and spatial scales than those typically used.

An inconvenient truth about xylem resistance to embolism in the model species for refilling Laurus nobilis L.

Publisher: Springer Science and Business Media LLC

Date: 09-2018

DOI: 10.1007/S13595-018-0768-9

Jame Cook University

Location: Australia

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James Cook University

Location: Australia

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Western Sydney University

Location: Australia

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Discovery Projects - Grant ID: DP220103440

Start Date: 2023

End Date: 12-2025

Amount: $431,137.00

Funder: Australian Research Council

Linkage Projects - Grant ID: LP190100436

Start Date: 10-2020

End Date: 10-2024

Amount: $524,027.00

Funder: Australian Research Council

ARC Future Fellowships - Grant ID: FT130101115

Start Date: 06-2014

End Date: 12-2019

Amount: $722,214.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP1096749

Start Date: 2010

End Date: 12-2012

Amount: $326,000.00

Funder: Australian Research Council

Linkage Projects - Grant ID: LP170100103

Start Date: 07-2018

End Date: 07-2021

Amount: $263,000.00

Funder: Australian Research Council

Discovery Projects - Grant ID: DP170100761

Start Date: 2017

End Date: 12-2020

Amount: $375,500.00

Funder: Australian Research Council