ORCID Profile
0000-0003-3627-5260
Current Organisation
University of Wyoming
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Publisher: Springer Science and Business Media LLC
Date: 10-12-2014
Publisher: Springer Science and Business Media LLC
Date: 28-08-2007
DOI: 10.1007/S00442-007-0832-X
Abstract: The stable isotope composition of spines produced serially from the apex of columnar cacti has the potential to be used as a record of changes in climate and physiology. To investigate this potential, we measured the delta(18)O, delta(13)C and F(14)C values of spines from a long-lived columnar cactus, saguaro (Carnegiea gigantea). To determine plant age, we collected spines at 11 different heights along one rib from the stem apex (3.77 m height) to the base of a naturally occurring saguaro. Fractions of modern carbon (F(14)C) ranged from 0.9679 to 1.5537, which is consistent with ages between 1950 and 2004. We observed a very strong positive correlation (r = 0.997) between the F(14)C age of spines and the age of spines determined from direct and repeated height measurements taken on this in idual over the past 37 years. A series of 96 spines collected from this in idual had delta(18)O values ranging from 38 per thousand to 50 per thousand [Vienna standard mean ocean water (VSMOW)] and delta(13)C values from -11.5 per thousand to -8.5 per thousand [Vienna Peedee belemnite (VPDB)]. The delta(18)O and delta(13)C values of spines were positively correlated (r = 0.45, P < 0.0001) and showed near-annual oscillations over the approximately 15-year record. This pattern suggests that seasonal periods of reduced evaporative demand or greater precipitation input may correspond to increased daytime CO(2) uptake. The lowest delta(18)O and delta(13)C values of spines observed occurred during the 1983 and 1993 El Niño years, suggesting that the stable isotope composition recorded in spine tissue may serve as a proxy for these climate events. We compared empirical models and data from potted experimental cacti to validate these observations and test our hypotheses. The isotopic records presented here are the first ever reported from a chronosequence of cactus spines and demonstrate that tissues of columnar cacti, and potentially other long-lived succulents, may contain a record of past physiological and climatic variation.
Publisher: Springer Science and Business Media LLC
Date: 06-2004
DOI: 10.1038/NATURE02561
Publisher: Oxford University Press (OUP)
Date: 04-2003
DOI: 10.1093/TREEPHYS/23.5.353
Abstract: Downward redistribution of soil water through plant roots has important consequences for water and nutrient balance of arid and semi-arid ecosystems. Nevertheless, information on the seasonal patterns and magnitudes of redistribution is lacking for all but a few plant species. We measured sap flow in the taproot and three main lateral roots of a 10-year-old Juglans major Torr. tree, on an ephemeral catchment in southeastern Arizona, to determine how patterns of redistribution respond to pulses of summer precipitation. Groundwater was beyond rooting depth and a hardpan prevented recharge of surface water to deep soil layers. Reverse flow (hydraulic descent) commenced in the taproot and deep lateral roots in early August after a series of moderate precipitation events, and abruptly ceased after all shallow roots were experimentally severed in mid-August. On some days, hydraulic descent continued in the deep lateral roots during periods of daytime transpiration, and the daily volume of hydraulic descent (deep lateral roots plus taproot) ranged from 10 to nearly 60% of daily transpiration. The persistent pattern of reverse flow demonstrates that, in some plants, water potential gradients from soil to leaf during transpiration are often smaller than those between soil layers within the rooting zone. Hydraulic descent may be an important component of the water balance of phreatophytic trees by facilitating root growth in deep soil layers and by transferring water away from shallow-rooted competitors.
Publisher: Springer Science and Business Media LLC
Date: 06-08-2011
DOI: 10.1007/S00442-011-2021-1
Abstract: Crassulacean acid metabolism (CAM) and the capacity to store large quantities of water are thought to confer high water use efficiency (WUE) and survival of succulent plants in warm desert environments. Yet the highly variable precipitation, temperature and humidity conditions in these environments likely have unique impacts on underlying processes regulating photosynthetic gas exchange and WUE, limiting our ability to predict growth and survival responses of desert CAM plants to climate change. We monitored net CO(2) assimilation (A(net)), stomatal conductance (g(s)), and transpiration (E) rates periodically over 2 years in a natural population of the giant columnar cactus Carnegiea gigantea (saguaro) near Tucson, Arizona USA to investigate environmental and physiological controls over carbon gain and water loss in this ecologically important plant. We hypothesized that seasonal changes in daily integrated water use efficiency (WUE(day)) in this constitutive CAM species would be driven largely by stomatal regulation of nighttime transpiration and CO(2) uptake responding to shifts in nighttime air temperature and humidity. The lowest WUE(day) occurred during time periods with extreme high and low air vapor pressure deficit (D(a)). The diurnal with the highest D(a) had low WUE(day) due to minimal net carbon gain across the 24 h period. Low WUE(day) was also observed under conditions of low D(a) however, it was due to significant transpiration losses. Gas exchange measurements on potted saguaro plants exposed to experimental changes in D(a) confirmed the relationship between D(a) and g(s). Our results suggest that climatic changes involving shifts in air temperature and humidity will have large impacts on the water and carbon economy of the giant saguaro and potentially other succulent CAM plants of warm desert environments.
Publisher: Wiley
Date: 30-04-2015
DOI: 10.1111/GCB.12910
Abstract: Terrestrial plant and soil respiration, or ecosystem respiration (R
Publisher: Wiley
Date: 16-06-2020
DOI: 10.1111/PCE.13781
Publisher: Wiley
Date: 11-10-2013
DOI: 10.1111/GCB.12314
Abstract: Eucalyptus species are grown widely outside of their native ranges in plantations on all vegetated continents of the world. We predicted that such a plantation species would show high potential for acclimation of photosynthetic traits across a wide range of growth conditions, including elevated [CO2] and climate warming. To test this prediction, we planted temperate Eucalyptus globulus Labill. seedlings in climate-controlled chambers in the field located >700 km closer to the equator than the nearest natural occurrence of this species. Trees were grown in a complete factorial combination of elevated CO2 concentration (eC ambient [CO2] +240 ppm) and air warming treatments (eT ambient +3 °C) for 15 months until they reached ca. 10 m height. There was little acclimation of photosynthetic capacity to eC and hence the CO2-induced photosynthetic enhancement was large (ca. 50%) in this treatment during summer. The warming treatment significantly increased rates of both carboxylation capacity (V(cmax)) and electron transport (Jmax) (measured at a common temperature of 25 °C) during winter, but decreased them significantly by 20-30% in summer. The photosynthetic CO2 compensation point in the absence of dark respiration (Γ*) was relatively less sensitive to temperature in this temperate eucalypt species than for warm-season tobacco. The temperature optima for photosynthesis and Jmax significantly changed by about 6 °C between winter and summer, but without further adjustment from early to late summer. These results suggest that there is an upper limit for the photosynthetic capacity of E. globulus ssp. globulus outside its native range to acclimate to growth temperatures above 25 °C. Limitations to temperature acclimation of photosynthesis in summer may be one factor that defines climate zones where E. globulus plantation productivity can be sustained under anticipated global environmental change.
Publisher: Wiley
Date: 25-04-2006
DOI: 10.1111/J.1469-8137.2006.01732.X
Abstract: The influences of prior monsoon-season drought (PMSD) and the seasonal timing of episodic rainfall ('pulses') on carbon and water exchange in water-limited ecosystems are poorly quantified. *In the present study, we estimated net ecosystem exchange of CO(2) (NEE) and evapotranspiration (ET) before, and for 15 d following, experimental irrigation in a semi-arid grassland during June and August 2003. Rainout shelters near Tucson, Arizona, USA, were positioned on contrasting soils (clay and sand) and planted with native (Heteropogon contortus) or non-native invasive (Eragrostis lehmanniana) C4 bunchgrasses. Plots received increased ('wet') or decreased ('dry') monsoon-season (July-September) rainfall during 2002 and 2003. Following a June 2003 39-mm pulse, species treatments had similar NEE and ET dynamics including 15-d integrated NEE (NEE(pulse)). Contrary to predictions, PMSD increased net C uptake during June in plots of both species. Greater flux rates after an August 2003 39-mm pulse reflected biotic activity associated with the North American Monsoon. Furthermore, August NEE(pulse) and ecosystem pulse-use efficiency (PUE(e) = NEE(pulse)/ET(pulse)) was greatest in Heteropogon plots. PMSD and rainfall seasonal timing may interact with bunchgrass invasions to alter NEE and ET dynamics with consequences for PUE(e) in water-limited ecosystems.
Publisher: Oxford University Press (OUP)
Date: 2003
Publisher: American Geophysical Union (AGU)
Date: 06-05-2010
DOI: 10.1029/2009JG001008
Publisher: Springer Science and Business Media LLC
Date: 14-10-2003
DOI: 10.1007/S00442-003-1389-Y
Abstract: Physiological activity and structural dynamics in arid and semi-arid ecosystems are driven by discrete inputs or "pulses" of growing season precipitation. Here we describe the short-term dynamics of ecosystem physiology in experimental stands of native (Heteropogon contortus) and invasive (Eragrostis lehmanniana) grasses to an irrigation pulse across two geomorphic surfaces with distinctly different soils: a Pleistocene-aged surface with high clay content in a strongly horizonated soil, and a Holocene-aged surface with low clay content in homogenously structured soils. We evaluated whole-ecosystem and leaf-level CO2 and H2O exchange, soil CO2 efflux, along with plant and soil water status to understand potential constraints on whole-ecosystem carbon exchange during the initiation of the summer monsoon season. Prior to the irrigation pulse, both invasive and native grasses had less negative pre-dawn water potentials (Psipd), greater leaf photosynthetic rates (Anet) and stomatal conductance (gs), and greater rates of net ecosystem carbon exchange (NEE) on the Pleistocene surface than on the Holocene. Twenty-four hours following the experimental application of a 39 mm irrigation pulse, soil CO2 efflux increased leading to all plots losing CO2 to the atmosphere over the course of a day. Invasive species stands had greater evapotranspiration rates (ET) immediately following the precipitation pulse than did native stands, while maximum instantaneous NEE increased for both species and surfaces at roughly the same rate. The differential ET patterns through time were correlated with an earlier decline in NEE in the invasive species as compared to the native species plots. Plots with invasive species accumulated between 5% and 33% of the carbon that plots with the native species accumulated over the 15-day pulse period. Taken together, these results indicate that system CO2 efflux (both the physical displacement of soil CO2 by water along with plant and microbial respiration) strongly controls whole-ecosystem carbon exchange during precipitation pulses. Since CO2 and H2O loss to the atmosphere was partially driven by species effects on soil microclimate, understanding the mechanistic relationships between the soil characteristics, plant ecophysiological responses, and canopy structural dynamics will be important for understanding the effects of shifting precipitation and vegetation patterns in semi-arid environments.
Publisher: Springer Science and Business Media LLC
Date: 13-04-2005
DOI: 10.1007/S00442-005-0078-4
Abstract: Plant metabolic activity in arid and semi-arid environments is largely tied to episodic precipitation events or "pulses". The ability of plants to take up and utilize rain pulses during the growing season in these water-limited ecosystems is determined in part by pulse timing, intensity and amount, and by hydrological properties of the soil that translate precipitation into plant-available soil moisture. We assessed the sensitivity of an invasive woody plant, velvet mesquite (Prosopis velutina Woot.), to large (35 mm) and small (10 mm) isotopically labeled irrigation pulses on two contrasting soil textures (sandy-loam vs. loamy-clay) in semi-desert grassland in southeastern Arizona, USA. Predawn leaf water potential (psi(pd)), the isotopic abundance of deuterium in stem water (deltaD), the abundance of 13C in soluble leaf sugar (delta13C), and percent volumetric soil water content (theta(v)) were measured prior to irrigation and repeatedly for 2 weeks following irrigation. Plant water potential and the percent of pulse water present in the stem xylem indicated that although mesquite trees on both coarse- and fine-textured soils quickly responded to the large irrigation pulse, the magnitude and duration of this response substantially differed between soil textures. After reaching a maximum 4 days after the irrigation, the fraction of pulse water in stem xylem decreased more rapidly on the loamy-clay soil than the sandy-loam soil. Similarly, on both soil textures mesquite significantly responded to the 10-mm pulse. However, the magnitude of this response was substantially greater for mesquite on the sandy-loam soil compared to loamy-clay soil. The relationship between psi(pd) and delta13C of leaf-soluble carbohydrates over the pulse period did not differ between plants at the two sites, indicating that differences in photosynthetic response of mesquite trees to the moisture pulses was a function of soil water availability within the rooting zone rather than differences in plant biochemical or physiological constraints. Patterns of resource acquisition by mesquite during the dynamic wetting-drying cycle following rainfall pulses is controlled by a complex interaction between pulse size and soil hydraulic properties. A better understanding of how this interaction affects plant water availability and photosynthetic response is needed to predict how grassland structure and function will respond to climate change.
Publisher: Wiley
Date: 23-08-2013
DOI: 10.1111/NPH.12459
Abstract: As global changes reorganize plant communities, invasive plants may benefit. We hypothesized that elevated CO 2 and warming would strongly influence invasive species success in a semi‐arid grassland, as a result of both direct and water‐mediated indirect effects. To test this hypothesis, we transplanted the invasive forb L inaria dalmatica into mixed‐grass prairie treated with free‐air CO 2 enrichment and infrared warming, and followed survival, growth, and reproduction over 4 yr. We also measured leaf gas exchange and carbon isotopic composition in L . dalmatica and the dominant native C 3 grass P ascopyrum smithii . CO 2 enrichment increased L . dalmatica biomass 13‐fold, seed production 32‐fold, and clonal expansion seven‐fold, while warming had little effect on L . dalmatica biomass or reproduction. Elevated CO 2 decreased stomatal conductance in P . smithii , contributing to higher soil water, but not in L . dalmatica . Elevated CO 2 also strongly increased L . dalmatica photosynthesis (87% versus 23% in P . smithii ), as a result of both enhanced carbon supply and increased soil water. More broadly, rapid growth and less conservative water use may allow invasive species to take advantage of both carbon fertilization and water savings under elevated CO 2 . Water‐limited ecosystems may therefore be particularly vulnerable to invasion as CO 2 increases.
Publisher: Springer Science and Business Media LLC
Date: 14-02-2003
DOI: 10.1007/S00442-002-1165-4
Abstract: We measured sap flow in taproots, lateral roots and stems within a single in idual in each of three co-occurring tree species in a Chihuahuan desert arroyo to assess the seasonality and magnitude of hydraulic redistribution. Nocturnal reverse flow (hydraulic redistribution) was detected in shallow lateral roots of Fraxinus velutina and Juglans major during periods when surface soils were dry. Reverse flow in the Fraxinus lateral root ranged from near zero to 120 g h(-1), and was inversely correlated with nighttime vapor pressure deficit (D), suggesting that nighttime transpiration may have inhibited hydraulic redistribution. Reverse flow in the Juglans lateral root ranged from near zero to 18 g h(-1). There was no relationship between reverse flow and nighttime D in the Juglans lateral root, despite a weak positive relationship between nighttime D and rates of basipetal flow (flow towards the stem) in the taproot. Reverse flow in Fraxinus and Juglans ceased when surface soils were wetted by monsoon rains and flooding. We found no reverse flow or seasonal variation in root sap flow in Celtis reticulata. However, basipetal sap flow in Celtis roots continued throughout most of the evening, even during periods when D was near zero, and commenced in the morning more than two hours after the onset of sap flow in the main stem. Patterns of nocturnal root sap flow in Celtis may have been facilitated by the diurnal withdrawal from, and refilling of above ground storage compartments (i.e. above ground diurnal storage capacity), which may have prevented hydraulic redistribution. Species differences in nocturnal root function may have significant impacts on ecosystem hydrological fluxes, and should be considered when scaling fluxes to catchment, landscape, and regional levels.
Publisher: Elsevier BV
Date: 10-2005
Publisher: Springer Science and Business Media LLC
Date: 27-11-2009
DOI: 10.1007/S00442-009-1511-X
Abstract: Predicting net C balance under future global change scenarios requires a comprehensive understanding of how ecosystem photosynthesis (gross primary production GPP) and respiration (Re) respond to elevated atmospheric [CO(2)] and altered water availability. We measured net ecosystem exchange of CO(2) (NEE), GPP and Re under ambient and elevated [CO(2)] in a northern mixed-grass prairie (Wyoming, USA) during dry intervals and in response to simulated precipitation pulse events. Elevated [CO(2)] resulted in higher rates of both GPP and Re across the 2006 growing season, and the balance of these two fluxes (NEE) accounted for cumulative growing season C uptake (-14.4 +/- 8.3 g C m(-2)). Despite lower GPP and Re, experimental plots under ambient [CO(2)] had greater cumulative uptake (-36.2 +/- 8.2 g C m(-2)) than plots under elevated [CO(2)]. Non-irrigated control plots received 50% of average precipitation during the drought of 2006, and had near-zero NEE (1.9 +/- 6.4 g C m(-2)) for the growing season. Elevated [CO(2)] extended the magnitude and duration of pulse-related increases in GPP, resulting in a significant [CO(2)] treatment by pulse day interaction, demonstrating the potential for elevated [CO(2)] to increase the capacity of this ecosystem to respond to late-season precipitation. However, stimulation of Re throughout the growing season under elevated [CO(2)] reduced net C uptake compared to plots under ambient [CO(2)]. These results indicate that although elevated [CO(2)] stimulates gross rates of ecosystem C fluxes, it does not necessarily enhance net C uptake, and that C cycle responses in semi-arid grasslands are likely to be more sensitive to changes in precipitation than atmospheric [CO(2)].
Publisher: Proceedings of the National Academy of Sciences
Date: 13-10-2014
Abstract: Evaluating ecological responses to climate change is essential to predict ecosystem function under future climate scenarios. In a mixed-grass prairie, we use a multifactor field experiment to show that the effects of elevated CO 2 and warming on plant community structure and productivity depend on interannual variation in precipitation. We also show that shifts in plant dominance patterns driven by elevated CO 2 in a mixed-grass prairie ecosystem promoted biomass and compositional stability and resistance to interannual variation in precipitation. The economic value of grasslands is largely dependent on the relative abundance of key forage species. Thus, our results have implications for how we manage native grasslands in the face of changing climate.
Publisher: Elsevier BV
Date: 10-2005
Publisher: Wiley
Date: 25-09-2012
DOI: 10.1111/J.1469-8137.2012.04349.X
Abstract: Nitrogen ( N ) and phosphorus ( P ) are essential nutrients for primary producers and decomposers in terrestrial ecosystems. Although climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, the impacts of climate change on the relative availability of N with respect to P remain highly uncertain. In a semiarid grassland in Wyoming, USA , we studied the effects of atmospheric CO 2 enrichment (to 600 ppmv) and warming (1.5/3.0°C above ambient temperature during the day/night) on plant, microbial and available soil pools of N and P. Elevated CO 2 increased P availability to plants and microbes relative to that of N , whereas warming reduced P availability relative to N . Across years and treatments, plant N : P ratios varied between 5 and 18 and were inversely related to soil moisture. Our results indicate that soil moisture is important in controlling P supply from inorganic sources, causing reduced P relative to N availability during dry periods. Both wetter soil conditions under elevated CO 2 and drier conditions with warming can further alter N : P . Although warming may alleviate N constraints under elevated CO 2 , warming and drought can exacerbate P constraints on plant growth and microbial activity in this semiarid grassland.
Publisher: Springer Science and Business Media LLC
Date: 13-07-2005
DOI: 10.1007/S00442-005-0164-7
Abstract: Stable hydrogen and oxygen isotopic composition of bulk leaf water (deltaD(lw) and delta(18)O(lw)) in piñon pine (Pinus edulis and P. monophylla) and gas exchange parameters were measured under field conditions to examine the effects of seasonal moisture stress on leaf water isotopic enrichment. Study sites were located near the lower elevation limit for piñon in the southwestern USA. Leaf-level transpiration measurements were made four times daily in spring, summer and early autumn simultaneously, leaf s les were collected for water extraction and stable isotope analysis. Diurnal variations in deltaD(lw) and delta(18)O(lw) values were small, especially when leaf water residence times (molar leaf water content ided by transpiration rate) were high. Stomatal conductance explained most of the variance (60%) in leaf water enrichment across the dataset. Observed leaf water enrichment was compared with predictions of steady-state and nonsteady-state models. Nonsteady-state predictions fit observations the best, although D enrichment was often lower than predicted by any model. Hydrogen isotope ratios of leaf water and cellulose nitrate were strongly correlated, demonstrating preservation of a leaf water signal in wood and leaf cellulose.
Publisher: CSIRO Publishing
Date: 2009
DOI: 10.1071/FP08216
Abstract: Non-photosynthetic, or heterotrophic, tissues in C3 plants tend to be enriched in 13C compared with the leaves that supply them with photosynthate. This isotopic pattern has been observed for woody stems, roots, seeds and fruits, emerging leaves, and parasitic plants incapable of net CO2 fixation. Unlike in C3 plants, roots of herbaceous C4 plants are generally not 13C-enriched compared with leaves. We review six hypotheses aimed at explaining this isotopic pattern in C3 plants: (1) variation in biochemical composition of heterotrophic tissues compared with leaves (2) seasonal separation of growth of leaves and heterotrophic tissues, with corresponding variation in photosynthetic discrimination against 13C (3) differential use of day v. night sucrose between leaves and sink tissues, with day sucrose being relatively 13C-depleted and night sucrose 13C-enriched (4) isotopic fractionation during dark respiration (5) carbon fixation by PEP carboxylase and (6) developmental variation in photosynthetic discrimination against 13C during leaf expansion. Although hypotheses (1) and (2) may contribute to the general pattern, they cannot explain all observations. Some evidence exists in support of hypotheses (3) through to (6), although for hypothesis (6) it is largely circumstantial. Hypothesis (3) provides a promising avenue for future research. Direct tests of these hypotheses should be carried out to provide insight into the mechanisms causing within-plant variation in carbon isotope composition.
Publisher: Wiley
Date: 09-2020
DOI: 10.1002/AJB2.1532
Publisher: Public Library of Science (PLoS)
Date: 19-08-2013
Publisher: Elsevier BV
Date: 06-2005
Publisher: Springer Science and Business Media LLC
Date: 08-2011
DOI: 10.1038/NATURE10274
Abstract: Global warming is predicted to induce desiccation in many world regions through increases in evaporative demand. Rising CO(2) may counter that trend by improving plant water-use efficiency. However, it is not clear how important this CO(2)-enhanced water use efficiency might be in offsetting warming-induced desiccation because higher CO(2) also leads to higher plant biomass, and therefore greater transpirational surface. Furthermore, although warming is predicted to favour warm-season, C(4) grasses, rising CO(2) should favour C(3), or cool-season plants. Here we show in a semi-arid grassland that elevated CO(2) can completely reverse the desiccating effects of moderate warming. Although enrichment of air to 600 p.p.m.v. CO(2) increased soil water content (SWC), 1.5/3.0 °C day/night warming resulted in desiccation, such that combined CO(2) enrichment and warming had no effect on SWC relative to control plots. As predicted, elevated CO(2) favoured C(3) grasses and enhanced stand productivity, whereas warming favoured C(4) grasses. Combined warming and CO(2) enrichment stimulated above-ground growth of C(4) grasses in 2 of 3 years when soil moisture most limited plant productivity. The results indicate that in a warmer, CO(2)-enriched world, both SWC and productivity in semi-arid grasslands may be higher than previously expected.
Publisher: Elsevier BV
Date: 07-2010
No related grants have been discovered for David Williams.