Stable isotope analysis of water is a proven tool for gaining insight into the processes that gov... more Stable isotope analysis of water is a proven tool for gaining insight into the processes that govern the hydrologic cycle. One of the most challenging aspects of characterizing the movement of water between the biosphere, hydrosphere, and the atmosphere has been capturing shifts in the isotope value of ambient water vapor. Traditional methods of cryogenically trapping ambient vapor for subsequent
This study examines the effect of four tree species on nitrogen (N) retention within forested cat... more This study examines the effect of four tree species on nitrogen (N) retention within forested catch-ments of the Catskill Mountains, New York (NY). We conducted a 300-day 15N field tracer experi-ment to determine how N moves through soil, microbial, and plant pools under ...
Many recent studies on drought-induced vegetation mortality have explored how plant functional tr... more Many recent studies on drought-induced vegetation mortality have explored how plant functional traits, and classifications of such traits along axes of, for example, isohydry-anisohydry, might contribute to predicting drought survival and recovery. As these studies proliferate, the consistency and predictive value of such classifications need to be carefully examined. Here, we outline the basis for a systematic classification of plant drought responses that accounts for both environmental conditions and functional traits. We use non-dimensional analysis to integrate plant traits and metrics of environmental variation into groups that can be associated with alternative drought stress pathways (hydraulic failure and carbon limitation), and demonstrate that these groupings predict physiological drought outcomes using both synthetic and measured data. In doing so, we aim to untangle some confounding effects of environment and trait variations that undermine current classification schemes, advocate for more careful treatment of the environmental context within which plants experience and respond to drought, and outline a pathway towards a general classification of drought vulnerability.
Fog water deposition is thought to influence the ecological function of many coastal ecosystems, ... more Fog water deposition is thought to influence the ecological function of many coastal ecosystems, including coast redwood forests. We examined cation and anion inputs from fog and rain, as well as the fate of these inputs, within a Sonoma County, California, coast redwood forest to elucidate the availability of these ions and some of the biotic and abiotic processes that may influence their relative abundance. At this site, the patterns of water and chemical inputs via fog and rain and their movement through the soil-plant ecosystem differed between the summer fog and winter rain seasons. Most (98 percent) of the annual water and more than three quarters of the total ionic load was delivered to the forest during the rain season. Soil water patterns followed those of throughfall. Water for plant use was most available in the rain season; however, after large fog events (fog season) plant-available soil water was also present at the forest edge. Differences between soil water and throughfall chemistry were a function of the mobility of each ion, whether or not an ion was a soil weathering product, and the likely biological demand for the ion. The impact of redwoods as fog catchers, transformers, and redistributors of both water and nutrients may extend all the way into the soil profile: in our plots, organic-rich soil horizons were thicker at the forest edge than in the forest interior. Our data show that, although total fog water inputs were small compared to inputs from rain, fog carried a large proportion of the total aqueous ionic inputs-inputs that, presumably, continued to be biologically available until their loss during the rain season. Cross-seasonal, functional coupling of aboveground (canopy) and belowground (soil) processes are likely to be prevalent in this redwood and other foginundated forests.
PremiseEpiphytes are abundant in ecosystems such as tropical montane cloud forests where low‐lyin... more PremiseEpiphytes are abundant in ecosystems such as tropical montane cloud forests where low‐lying clouds are often in contact with vegetation. Climate projections for these regions include more variability in rainfall and an increase in cloud base heights, which would lead to drier conditions in the soil and atmosphere. While recent studies have examined the effects of drought on epiphytic water relations, the influence that atmospheric moisture has, either alone or in combination with drought, on the health and performance of epiphyte communities remains unclear.MethodsWe conducted a 10‐week drought experiment on seven vascular epiphyte species in two shadehouses, one with warmer and drier conditions and another that was cooler and more humid. We measured water relations across control and drought‐treatment groups and assessed functional traits of leaves produced during drought conditions to evaluate trait plasticity.ResultsEpiphytes exposed to drought and drier atmospheric conditions had a significant reduction in stomatal conductance and leaf water potential and an increase in leaf dry matter. Nonsucculent epiphytes from the drier shadehouse had the greatest shifts in functional traits, whereas succulent epiphytes released stored leaf water to maintain water status.ConclusionsIndividuals in the drier shadehouse had a substantial reduction in performance, whereas drought‐treated individuals that experienced cloud immersion displayed minimal changes in water status. Our results indicate that projected increases in the cloud base height will reduce growth and performance of epiphytic communities and that nonsucculent epiphytes may be particularly vulnerable.
In tall trees, the mechanisms by which foliage maintains sufficient turgor pressure and water con... more In tall trees, the mechanisms by which foliage maintains sufficient turgor pressure and water content against height-related constraints remain poorly understood. Pressure-volume curves generated from leafy shoots collected crown-wide from 12 large Sequoiadendron giganteum (Lindley) J. Buchholz (giant sequoia) trees provided mechanistic insights into how the components of water potential vary with height in tree and over time. The turgor loss point (TLP) decreased with height at a rate indistinguishable from the gravitational potential gradient and was controlled by changes in tissue osmotica. For all measured shoots, total relative water content at the TLP remained above 75%. This high value has been suggested to help leaves avoid precipitous declines in leaf-level physiological function, and in giant sequoia was controlled by both tissue elasticity and the balance of water between apoplasm and symplasm. Hydraulic capacitance decreased only slightly with height, but importantly this parameter was nearly double in value to that reported for other tree species. Total water storage capacity also decreased with height, but this trend essentially disappeared when considering only water available within the typical range of water potentials experienced by giant sequoia. From summer to fall measurement periods we did not observe osmotic adjustment that would depress the TLP. Instead we observed a proportional shift of water into less mobile apoplastic compartments leading to a reduction in hydraulic capacitance. This collection of foliar traits allows giant sequoia to routinely, but safely, operate close to its TLP, and suggests that gravity plays a major role in the water relations of Earth's largest tree species.
Optimality theory states that whole-tree carbon gain is maximized when leaf N and photosynthetic ... more Optimality theory states that whole-tree carbon gain is maximized when leaf N and photosynthetic capacity profiles are distributed along vertical light gradients such that the marginal gain of nitrogen investment is identical among leaves. However, observed photosynthetic N gradients in trees do not follow this prediction, and the causes for this apparent discrepancy remain uncertain. Our objective was to evaluate how hydraulic limitations potentially modify crown-level optimization in Sequoiadendron giganteum (giant sequoia) trees up to 90 m tall. Leaf water potential (Ψ l ) and branch sap flow closely followed diurnal patterns of solar radiation throughout each tree crown. Minimum leaf water potential correlated negatively with height above ground, while leaf mass per area (LMA), shoot mass per area (SMA), leaf nitrogen content (%N), and bulk leaf stable carbon isotope ratios (δ(13)C) correlated positively with height. We found no significant vertical trends in maximum leaf photosynthesis (A), stomatal conductance (g s), and intrinsic water-use efficiency (A/g s), nor in branch-averaged transpiration (E L), stomatal conductance (G S), and hydraulic conductance (K L). Adjustments in hydraulic architecture appear to partially compensate for increasing hydraulic limitations with height in giant sequoia, allowing them to sustain global maximum summer water use rates exceeding 2000 kg day(-1). However, we found that leaf N and photosynthetic capacity do not follow the vertical light gradient, supporting the hypothesis that increasing limitations on water transport capacity with height modify photosynthetic optimization in tall trees.
This is the author manuscript accepted for publication and has undergone full peer review but has... more This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as
Water stored in tree stems (i.e., trunks and branches) is an important contributor to transpirati... more Water stored in tree stems (i.e., trunks and branches) is an important contributor to transpiration that can improve photosynthetic carbon gain and reduce the probability of cavitation. However, in tall trees, the capacity to store water may decline with height because of chronically low water potentials associated with the gravitational potential gradient. We quantified the importance of elastic stem water storage in the top 5–6 m of large (4.2–5.0 m diameter at breast height, 82.1–86.3 m tall) Sequoiadendron giganteum (Lindley) J. Buchholz (giant sequoia) trees using a combination of architectural measurements and automated sensors that monitored summertime diel rhythms in sap flow, stem diameter and water potential. Stem water storage contributed 1.5–1.8% of water transpired at the tree tops, and hydraulic capacitance ranged from 2.6 to 4.1 l MPa−1 m−3. These values, which are considerably smaller than reported for shorter trees, may be associated with persistently low water pote...
Epiphyte communities comprise important components of many forest ecosystems in terms of biomass ... more Epiphyte communities comprise important components of many forest ecosystems in terms of biomass and diversity, but little is known regarding trade-offs that underlie diversity and structure in these communities or the impact that microclimate has on epiphyte trait allocation. We measured 22 functional traits in vascular epiphyte communities across six sites that span a microclimatic gradient in a tropical montane cloud forest region in Costa Rica. We quantified traits that relate to carbon and nitrogen allocation, gas exchange, water storage, and drought tolerance. Functional diversity was high in all but the lowest elevation site where drought likely limits the success of certain species with particular trait combinations. For most traits, variation was explained by relationships with other traits (trait co-variance), rather than differences in microclimate across sites. Although there were significant differences in microclimate, epiphyte abundance, and diversity, we found substa...
Proceedings of the National Academy of Sciences, 2021
Significance A fundamental association between sustained water transport and downstream tissue su... more Significance A fundamental association between sustained water transport and downstream tissue survival should select for xylem that avoids embolism in long-lived woody plants. Previous studies suggest that long-vessel species, such as oaks and vines, are more susceptible to drought-induced loss of function than other species. We show that western North American oaks—even those occurring in wet temperate forest—possess xylem capable of tolerating substantial water stress. Evolutionary relationships between drought tolerance traits combined with plant–climate interactions yield positive hydraulic safety margins in oaks from diverse habitats, demonstrating that these key species are not yet on the verge of hydraulically mediated loss of function. Quantifying physical tolerance limits to desiccation is imperative for predicting ecological consequences of future droughts.
Summary Vulnerability to embolism varies between con‐generic species distributed along aridity gr... more Summary Vulnerability to embolism varies between con‐generic species distributed along aridity gradients, yet little is known about intraspecific variation and its drivers. Even less is known about intraspecific variation in tissues other than stems, despite results suggesting that roots, stems and leaves can differ in vulnerability. We hypothesized that intraspecific variation in vulnerability in leaves and stems is adaptive and driven by aridity. We quantified leaf and stem vulnerability of Quercus douglasii using the optical technique. To assess contributions of genetic variation and phenotypic plasticity to within‐species variation, we quantified the vulnerability of individuals growing in a common garden, but originating from populations along an aridity gradient, as well as individuals from the same wild populations. Intraspecific variation in water potential at which 50% of total embolism in a tissue is observed (P50) was explained mostly by differences between individuals (&...
AimWithin C3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stoma... more AimWithin C3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stomata and demand by enzymes within chloroplasts. This process is dynamic and a complex but crucial aspect of photosynthesis. We sought to understand the spatial pattern in CO2 supply–demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (Δ13C), which provide an integrative record of CO2 drawdown during photosynthesis.LocationGlobalTime period1951–2011.Major taxa studiedVascular plants.MethodsWe assembled a database of leaf carbon isotope ratios containing 3,979 species–site combinations from across the globe, including 3,645 for C3 species. We examined a wide array of potential climate and soil drivers of variation in Δ13C.ResultsThe strongest drivers of carbon isotope discrimination at the global scale included atmospheric pressure, potential evapotranspiration and soil pH, which explained 44% of the variation in Δ13C. Addition of eight more climate and ...
Summary A defining feature of the redwood forest in coastal California is the presence of fog in ... more Summary A defining feature of the redwood forest in coastal California is the presence of fog in the summer months, a time when there is typically little rainfall. Our goal was to determine the role of summer fog in canopy transformation of nitrogen, nitrogen uptake by trees and photosynthesis within a coastal redwood forest ecosystem. We measured horizontal and vertical inputs of nitrogen, the isotopic composition of nitrogen in a variety of atmospheric sources (summer fog, winter rain and throughfall throughout the year), nitrogen pools (soil solution) and plant tissue (roots and foliage), as well as rates of photosynthesis and nitrogen uptake by trees. Throughfall nitrogen fluxes were greater at the forest edge compared to the interior both within the canopy (sampled 10 m above‐ground) and onto the forest floor (sampled 1 m above‐ground; P < 0.05). Similarly, soil solution and total inorganic nitrogen were greater at the forest edge compared to the interior (P = 0.0014 and 0.0...
ABSTRACT The oxygen and hydrogen isotope ratios of water in plants and soils are powerful tools f... more ABSTRACT The oxygen and hydrogen isotope ratios of water in plants and soils are powerful tools for identifying the water sources in terrestrial ecosystems, partitioning evapotranspiration fluxes between evaporation and transpiration, and validating global climate models. To date, water isotope samples have only been collected at very few sites for any particular region or continent and often these collections are not made in coordination with important complementary observations such as eddy covariance measurements of latent and sensible heat fluxes and carbon dioxide exchange. We present data from 2005 and 2006 on the seasonal and interannual variation in the oxygen and hydrogen isotope ratios of leaf, stem and soil water across 12 eddy covariance flux sites comprising the MIBA (Moisture Isotopes in the Biosphere and Atmosphere) network within the continental United States. Values of delta18O in leaf, stem and soil water ranged from -10 to +30°, -16 to -2°, and -16 to 0°, respectively, reflecting the large variation across the major climatic and vegetation zones in the U.S. Stems were often more depleted than soils, but both approached the isotope ratios of local precipitation. As expected leaves were always more enriched than soil or stems and exhibited the greatest seasonal variation. Within each site, variation in deltaD and delta18O were strongly related, yet there were different &quot;evaporation lines&quot; for leaves, stems and soil. Differences in the slopes and intercepts of these evaporation lines appeared to be most strongly related to climatic differences among the sites. We demonstrate the utility of the MIBA data for constraining process-based models by comparing against response functions and spatial and temporal patters predicted by ISOLSM for the continental U.S. We also compare measurements against predictions by the widely used Craig-Gordon model of fractionation during evaporation.
Stable isotope analysis of water is a proven tool for gaining insight into the processes that gov... more Stable isotope analysis of water is a proven tool for gaining insight into the processes that govern the hydrologic cycle. One of the most challenging aspects of characterizing the movement of water between the biosphere, hydrosphere, and the atmosphere has been capturing shifts in the isotope value of ambient water vapor. Traditional methods of cryogenically trapping ambient vapor for subsequent
This study examines the effect of four tree species on nitrogen (N) retention within forested cat... more This study examines the effect of four tree species on nitrogen (N) retention within forested catch-ments of the Catskill Mountains, New York (NY). We conducted a 300-day 15N field tracer experi-ment to determine how N moves through soil, microbial, and plant pools under ...
Many recent studies on drought-induced vegetation mortality have explored how plant functional tr... more Many recent studies on drought-induced vegetation mortality have explored how plant functional traits, and classifications of such traits along axes of, for example, isohydry-anisohydry, might contribute to predicting drought survival and recovery. As these studies proliferate, the consistency and predictive value of such classifications need to be carefully examined. Here, we outline the basis for a systematic classification of plant drought responses that accounts for both environmental conditions and functional traits. We use non-dimensional analysis to integrate plant traits and metrics of environmental variation into groups that can be associated with alternative drought stress pathways (hydraulic failure and carbon limitation), and demonstrate that these groupings predict physiological drought outcomes using both synthetic and measured data. In doing so, we aim to untangle some confounding effects of environment and trait variations that undermine current classification schemes, advocate for more careful treatment of the environmental context within which plants experience and respond to drought, and outline a pathway towards a general classification of drought vulnerability.
Fog water deposition is thought to influence the ecological function of many coastal ecosystems, ... more Fog water deposition is thought to influence the ecological function of many coastal ecosystems, including coast redwood forests. We examined cation and anion inputs from fog and rain, as well as the fate of these inputs, within a Sonoma County, California, coast redwood forest to elucidate the availability of these ions and some of the biotic and abiotic processes that may influence their relative abundance. At this site, the patterns of water and chemical inputs via fog and rain and their movement through the soil-plant ecosystem differed between the summer fog and winter rain seasons. Most (98 percent) of the annual water and more than three quarters of the total ionic load was delivered to the forest during the rain season. Soil water patterns followed those of throughfall. Water for plant use was most available in the rain season; however, after large fog events (fog season) plant-available soil water was also present at the forest edge. Differences between soil water and throughfall chemistry were a function of the mobility of each ion, whether or not an ion was a soil weathering product, and the likely biological demand for the ion. The impact of redwoods as fog catchers, transformers, and redistributors of both water and nutrients may extend all the way into the soil profile: in our plots, organic-rich soil horizons were thicker at the forest edge than in the forest interior. Our data show that, although total fog water inputs were small compared to inputs from rain, fog carried a large proportion of the total aqueous ionic inputs-inputs that, presumably, continued to be biologically available until their loss during the rain season. Cross-seasonal, functional coupling of aboveground (canopy) and belowground (soil) processes are likely to be prevalent in this redwood and other foginundated forests.
PremiseEpiphytes are abundant in ecosystems such as tropical montane cloud forests where low‐lyin... more PremiseEpiphytes are abundant in ecosystems such as tropical montane cloud forests where low‐lying clouds are often in contact with vegetation. Climate projections for these regions include more variability in rainfall and an increase in cloud base heights, which would lead to drier conditions in the soil and atmosphere. While recent studies have examined the effects of drought on epiphytic water relations, the influence that atmospheric moisture has, either alone or in combination with drought, on the health and performance of epiphyte communities remains unclear.MethodsWe conducted a 10‐week drought experiment on seven vascular epiphyte species in two shadehouses, one with warmer and drier conditions and another that was cooler and more humid. We measured water relations across control and drought‐treatment groups and assessed functional traits of leaves produced during drought conditions to evaluate trait plasticity.ResultsEpiphytes exposed to drought and drier atmospheric conditions had a significant reduction in stomatal conductance and leaf water potential and an increase in leaf dry matter. Nonsucculent epiphytes from the drier shadehouse had the greatest shifts in functional traits, whereas succulent epiphytes released stored leaf water to maintain water status.ConclusionsIndividuals in the drier shadehouse had a substantial reduction in performance, whereas drought‐treated individuals that experienced cloud immersion displayed minimal changes in water status. Our results indicate that projected increases in the cloud base height will reduce growth and performance of epiphytic communities and that nonsucculent epiphytes may be particularly vulnerable.
In tall trees, the mechanisms by which foliage maintains sufficient turgor pressure and water con... more In tall trees, the mechanisms by which foliage maintains sufficient turgor pressure and water content against height-related constraints remain poorly understood. Pressure-volume curves generated from leafy shoots collected crown-wide from 12 large Sequoiadendron giganteum (Lindley) J. Buchholz (giant sequoia) trees provided mechanistic insights into how the components of water potential vary with height in tree and over time. The turgor loss point (TLP) decreased with height at a rate indistinguishable from the gravitational potential gradient and was controlled by changes in tissue osmotica. For all measured shoots, total relative water content at the TLP remained above 75%. This high value has been suggested to help leaves avoid precipitous declines in leaf-level physiological function, and in giant sequoia was controlled by both tissue elasticity and the balance of water between apoplasm and symplasm. Hydraulic capacitance decreased only slightly with height, but importantly this parameter was nearly double in value to that reported for other tree species. Total water storage capacity also decreased with height, but this trend essentially disappeared when considering only water available within the typical range of water potentials experienced by giant sequoia. From summer to fall measurement periods we did not observe osmotic adjustment that would depress the TLP. Instead we observed a proportional shift of water into less mobile apoplastic compartments leading to a reduction in hydraulic capacitance. This collection of foliar traits allows giant sequoia to routinely, but safely, operate close to its TLP, and suggests that gravity plays a major role in the water relations of Earth's largest tree species.
Optimality theory states that whole-tree carbon gain is maximized when leaf N and photosynthetic ... more Optimality theory states that whole-tree carbon gain is maximized when leaf N and photosynthetic capacity profiles are distributed along vertical light gradients such that the marginal gain of nitrogen investment is identical among leaves. However, observed photosynthetic N gradients in trees do not follow this prediction, and the causes for this apparent discrepancy remain uncertain. Our objective was to evaluate how hydraulic limitations potentially modify crown-level optimization in Sequoiadendron giganteum (giant sequoia) trees up to 90 m tall. Leaf water potential (Ψ l ) and branch sap flow closely followed diurnal patterns of solar radiation throughout each tree crown. Minimum leaf water potential correlated negatively with height above ground, while leaf mass per area (LMA), shoot mass per area (SMA), leaf nitrogen content (%N), and bulk leaf stable carbon isotope ratios (δ(13)C) correlated positively with height. We found no significant vertical trends in maximum leaf photosynthesis (A), stomatal conductance (g s), and intrinsic water-use efficiency (A/g s), nor in branch-averaged transpiration (E L), stomatal conductance (G S), and hydraulic conductance (K L). Adjustments in hydraulic architecture appear to partially compensate for increasing hydraulic limitations with height in giant sequoia, allowing them to sustain global maximum summer water use rates exceeding 2000 kg day(-1). However, we found that leaf N and photosynthetic capacity do not follow the vertical light gradient, supporting the hypothesis that increasing limitations on water transport capacity with height modify photosynthetic optimization in tall trees.
This is the author manuscript accepted for publication and has undergone full peer review but has... more This is the author manuscript accepted for publication and has undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as
Water stored in tree stems (i.e., trunks and branches) is an important contributor to transpirati... more Water stored in tree stems (i.e., trunks and branches) is an important contributor to transpiration that can improve photosynthetic carbon gain and reduce the probability of cavitation. However, in tall trees, the capacity to store water may decline with height because of chronically low water potentials associated with the gravitational potential gradient. We quantified the importance of elastic stem water storage in the top 5–6 m of large (4.2–5.0 m diameter at breast height, 82.1–86.3 m tall) Sequoiadendron giganteum (Lindley) J. Buchholz (giant sequoia) trees using a combination of architectural measurements and automated sensors that monitored summertime diel rhythms in sap flow, stem diameter and water potential. Stem water storage contributed 1.5–1.8% of water transpired at the tree tops, and hydraulic capacitance ranged from 2.6 to 4.1 l MPa−1 m−3. These values, which are considerably smaller than reported for shorter trees, may be associated with persistently low water pote...
Epiphyte communities comprise important components of many forest ecosystems in terms of biomass ... more Epiphyte communities comprise important components of many forest ecosystems in terms of biomass and diversity, but little is known regarding trade-offs that underlie diversity and structure in these communities or the impact that microclimate has on epiphyte trait allocation. We measured 22 functional traits in vascular epiphyte communities across six sites that span a microclimatic gradient in a tropical montane cloud forest region in Costa Rica. We quantified traits that relate to carbon and nitrogen allocation, gas exchange, water storage, and drought tolerance. Functional diversity was high in all but the lowest elevation site where drought likely limits the success of certain species with particular trait combinations. For most traits, variation was explained by relationships with other traits (trait co-variance), rather than differences in microclimate across sites. Although there were significant differences in microclimate, epiphyte abundance, and diversity, we found substa...
Proceedings of the National Academy of Sciences, 2021
Significance A fundamental association between sustained water transport and downstream tissue su... more Significance A fundamental association between sustained water transport and downstream tissue survival should select for xylem that avoids embolism in long-lived woody plants. Previous studies suggest that long-vessel species, such as oaks and vines, are more susceptible to drought-induced loss of function than other species. We show that western North American oaks—even those occurring in wet temperate forest—possess xylem capable of tolerating substantial water stress. Evolutionary relationships between drought tolerance traits combined with plant–climate interactions yield positive hydraulic safety margins in oaks from diverse habitats, demonstrating that these key species are not yet on the verge of hydraulically mediated loss of function. Quantifying physical tolerance limits to desiccation is imperative for predicting ecological consequences of future droughts.
Summary Vulnerability to embolism varies between con‐generic species distributed along aridity gr... more Summary Vulnerability to embolism varies between con‐generic species distributed along aridity gradients, yet little is known about intraspecific variation and its drivers. Even less is known about intraspecific variation in tissues other than stems, despite results suggesting that roots, stems and leaves can differ in vulnerability. We hypothesized that intraspecific variation in vulnerability in leaves and stems is adaptive and driven by aridity. We quantified leaf and stem vulnerability of Quercus douglasii using the optical technique. To assess contributions of genetic variation and phenotypic plasticity to within‐species variation, we quantified the vulnerability of individuals growing in a common garden, but originating from populations along an aridity gradient, as well as individuals from the same wild populations. Intraspecific variation in water potential at which 50% of total embolism in a tissue is observed (P50) was explained mostly by differences between individuals (&...
AimWithin C3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stoma... more AimWithin C3 plants, photosynthesis is a balance between CO2 supply from the atmosphere via stomata and demand by enzymes within chloroplasts. This process is dynamic and a complex but crucial aspect of photosynthesis. We sought to understand the spatial pattern in CO2 supply–demand balance on a global scale, via analysis of stable isotopes of carbon within leaves (Δ13C), which provide an integrative record of CO2 drawdown during photosynthesis.LocationGlobalTime period1951–2011.Major taxa studiedVascular plants.MethodsWe assembled a database of leaf carbon isotope ratios containing 3,979 species–site combinations from across the globe, including 3,645 for C3 species. We examined a wide array of potential climate and soil drivers of variation in Δ13C.ResultsThe strongest drivers of carbon isotope discrimination at the global scale included atmospheric pressure, potential evapotranspiration and soil pH, which explained 44% of the variation in Δ13C. Addition of eight more climate and ...
Summary A defining feature of the redwood forest in coastal California is the presence of fog in ... more Summary A defining feature of the redwood forest in coastal California is the presence of fog in the summer months, a time when there is typically little rainfall. Our goal was to determine the role of summer fog in canopy transformation of nitrogen, nitrogen uptake by trees and photosynthesis within a coastal redwood forest ecosystem. We measured horizontal and vertical inputs of nitrogen, the isotopic composition of nitrogen in a variety of atmospheric sources (summer fog, winter rain and throughfall throughout the year), nitrogen pools (soil solution) and plant tissue (roots and foliage), as well as rates of photosynthesis and nitrogen uptake by trees. Throughfall nitrogen fluxes were greater at the forest edge compared to the interior both within the canopy (sampled 10 m above‐ground) and onto the forest floor (sampled 1 m above‐ground; P < 0.05). Similarly, soil solution and total inorganic nitrogen were greater at the forest edge compared to the interior (P = 0.0014 and 0.0...
ABSTRACT The oxygen and hydrogen isotope ratios of water in plants and soils are powerful tools f... more ABSTRACT The oxygen and hydrogen isotope ratios of water in plants and soils are powerful tools for identifying the water sources in terrestrial ecosystems, partitioning evapotranspiration fluxes between evaporation and transpiration, and validating global climate models. To date, water isotope samples have only been collected at very few sites for any particular region or continent and often these collections are not made in coordination with important complementary observations such as eddy covariance measurements of latent and sensible heat fluxes and carbon dioxide exchange. We present data from 2005 and 2006 on the seasonal and interannual variation in the oxygen and hydrogen isotope ratios of leaf, stem and soil water across 12 eddy covariance flux sites comprising the MIBA (Moisture Isotopes in the Biosphere and Atmosphere) network within the continental United States. Values of delta18O in leaf, stem and soil water ranged from -10 to +30°, -16 to -2°, and -16 to 0°, respectively, reflecting the large variation across the major climatic and vegetation zones in the U.S. Stems were often more depleted than soils, but both approached the isotope ratios of local precipitation. As expected leaves were always more enriched than soil or stems and exhibited the greatest seasonal variation. Within each site, variation in deltaD and delta18O were strongly related, yet there were different &quot;evaporation lines&quot; for leaves, stems and soil. Differences in the slopes and intercepts of these evaporation lines appeared to be most strongly related to climatic differences among the sites. We demonstrate the utility of the MIBA data for constraining process-based models by comparing against response functions and spatial and temporal patters predicted by ISOLSM for the continental U.S. We also compare measurements against predictions by the widely used Craig-Gordon model of fractionation during evaporation.
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Papers by Todd Dawson