Todd Dawson
University of California, Berkeley, Integrative Biology, Faculty Member
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... 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
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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,... 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 ...
Research Interests: Environmental Science, Biology, Nitrogen Cycle, Biological Sciences, Environmental Sciences, and 15 moreStable Isotope, Ecosystems, Nitrogen, Nitrogen Uptake, Oak, Forest ecosystem, Tsuga, Aboveground biomass, Beech, Tsuga Canadensis, Sugar Maple, Forest Floor, Eastern Hemlock, Tree Species, and Quercus rubra
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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... 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.
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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... 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.
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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... 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...
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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... 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...
Research Interests: Biology, Ecology, Medicine, Trait, Root-Mean Square Error, and 4 moreOecologia, Microclimate, Cloud Forest, and Epiphyte
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... 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.
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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... 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 (&...
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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... 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 ...
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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... 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...
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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... 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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During daylight hours, the isotope composition of leaf water generally approximates steady‐state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady‐state... more
During daylight hours, the isotope composition of leaf water generally approximates steady‐state leaf water isotope enrichment model predictions. However, until very recently there was little direct confirmation that isotopic steady‐state (ISS) transpiration in fact exists. Using isotope ratio infrared spectroscopy (IRIS) and leaf gas exchange systems we evaluated the isotope composition of transpiration and the rate of change in leaf water isotopologue storage (isostorage) when leaves were exposed to variable environments. In doing so, we developed a method for controlling the absolute humidity entering the gas exchange cuvette for a wide range of concentrations without changing the isotope composition of water vapour. The measurement system allowed estimation of 18O enrichment both at the evaporation site and for bulk leaf water, in the steady state and the non‐steady state. We show that non–steady‐state effects dominate the transpiration isoflux even when leaves are at physiologi...
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ABSTRACT Understanding the relationship between carbon assimilation and water consumption by natural vegetation is needed to assess how changes in climate will affect plant carbon and water exchange as well as the energy fluxes of... more
ABSTRACT Understanding the relationship between carbon assimilation and water consumption by natural vegetation is needed to assess how changes in climate will affect plant carbon and water exchange as well as the energy fluxes of ecosystems. While climate change is expected to cause significant warming, most models also suggest changes in the timing and amount of precipitation received; thus implications of this type of change are particularly acute in Mediterranean regions of the world. Blue oak savannas are already exposed to broad variation in water availability and to severe droughts during the summer months. Our objective was to evaluate the trade-off between carbon gain and water loss (Water Use Efficiency) in this ecosystem at both the leaf and at the ecosystem scales. We monitored the ratio of the partial pressures of CO2 inside the leaf (Ci) and in the outside air (Ca) or Ci/Ca, during the summer months of three subsequent years. This ratio is determined by the balance between photosynthetic capacity and stomatal conductance to water loss. Leaf-level estimates for individual trees were based on the carbon isotope composition (delta13C) of bulk leaf tissue and of recently fixed carbohydrates (leaf soluble sugars). These leaf and individual tree based estimates were then compared with canopy-level estimates derived from continuous eddy covariance measurements of fluxes of CO2, water vapor and meteorological variables from two eddy covariance systems, one above (23m) and one below (2m) the tree canopy. We found that savanna Blue oak trees cope with severe drought through coordinated down-regulation of carbon and water fluxes, i.e. the ratio Ci/Ca remained stable over four summer months, despite decreasing soil water content and leaf water potentials. Stable C isotope composition of leaf soluble sugars is the most robust measure of Ci/Ca because it reflects the initial discrimination of photosynthetic products, without the confounding effects ascribed to storage, tissue chemical composition and time of tissue formation. Our findings at the leaf-level were confirmed at the ecosystem-level by using a two tower (above and below canopy) eddy covariance method.
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Significance For most of its path through plant bodies, water moves in conduits in the wood. Plant water conduction is crucial for Earth’s biogeochemical cycles, making it important to understand how natural selection shapes conduit... more
Significance For most of its path through plant bodies, water moves in conduits in the wood. Plant water conduction is crucial for Earth’s biogeochemical cycles, making it important to understand how natural selection shapes conduit diameters along the entire lengths of plant stems. Can mathematical modeling and global sampling explain how wood conduits ought to widen from the tip of a plant to its trunk base? This question is evolutionarily important because xylem conduits should widen in a way that keeps water supply constant to the leaves as a plant grows taller. Moreover, selection should act on economy of construction costs of the conducting system. This issue is ecologically important because it helps suggest why climate change alters vegetation height worldwide.
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Current models used for predicting vegetation responses to climate change are often guided by the dichotomous needs to resolve either (i) internal plant water status as a proxy for physiological vulnerability or (ii) external water and... more
Current models used for predicting vegetation responses to climate change are often guided by the dichotomous needs to resolve either (i) internal plant water status as a proxy for physiological vulnerability or (ii) external water and carbon fluxes and atmospheric feedbacks. Yet accurate representation of fluxes does not always equate to accurate predictions of vulnerability. We resolve this discrepancy by using a hydrodynamic framework that simultaneously tracks plant water status and water uptake. We couple a minimal plant hydraulics model with a soil moisture model and, for the first time, translate rainfall variability at multiple timescales-with explicit descriptions at daily, seasonal, and interannual timescales-into a physiologically meaningful metric for the risk of hydraulic failure. The model, parameterized with measured traits from chaparral species native to southern California, shows that apparently similar transpiration patterns throughout the dry season can emerge fr...
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Many animals are considered to be specialists because they have feeding structures that are fine-tuned for consuming specific prey. For example, "smasher" mantis shrimp have highly specialized predatory appendages that generate... more
Many animals are considered to be specialists because they have feeding structures that are fine-tuned for consuming specific prey. For example, "smasher" mantis shrimp have highly specialized predatory appendages that generate forceful strikes to break apart hard-shelled prey. Anecdotal observations suggest, however, that the diet of smashers may include soft-bodied prey as well. Our goal was to examine the diet breadth of the smasher mantis shrimp, Neogonodactylus bredini, to determine whether it has a narrow diet of hard-shelled prey. We combined studies of prey abundance, feeding behavior, and stable isotope analyses of diet in both seagrass and coral rubble to determine if N. bredini's diet was consistent across different habitat types. The abundances of hard-shelled and soft-bodied prey varied between habitats. In feeding experiments, N. bredini consumed both prey types. N. bredini consumed a range of different prey in the field as well and, unexpectedly, the sta...
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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... 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 photos...
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Background/Question/Methods The global increase in drought-related tree mortality has sparked renewed interest in mortality mechanisms in plants. However, the impact of drought on highly diverse ecosystems is not well understood.... more
Background/Question/Methods The global increase in drought-related tree mortality has sparked renewed interest in mortality mechanisms in plants. However, the impact of drought on highly diverse ecosystems is not well understood. Drought-related plant mortality is of particular concern in the Fynbos Biome of South Africa, a well-known global biodiversity hotspot. This winter-wet, summer-dry region is predicted to experience an increase in regionally acute drought, with unknown consequences for the endemic flora. We asked whether drought impacted key functional types in this diverse ecosystem in a predictable manner, and if these patterns fit existing frameworks of drought mortality mechanisms in plants. We conducted two independent, multi-year drought studies in Mountain Fynbos. The first was a 3-year rain-exclusion experiment on the Cape Peninsula. The second was a 2-year sapflow study conducted 100km inland from the former study site, but in a similar vegetation type. For both stu...
Background/Question/Methods The Earth’s largest tree species, giant sequoia (Sequoiadendron giganteum), is restricted to locations in the Sierra Nevada Mountains containing abundant melt water from snowpack, but climate models for... more
Background/Question/Methods The Earth’s largest tree species, giant sequoia (Sequoiadendron giganteum), is restricted to locations in the Sierra Nevada Mountains containing abundant melt water from snowpack, but climate models for California consistently predict a substantial decline in this critical water source in the coming decades. To help maintain favorable hydration status in a changing environment, trees build leaf tissues to reduce the probability that leaf water potential will fall below the threshold where physiological damage is irreversible. This threshold, known as the turgor loss point (TLP), is a strong metric of drought tolerance that varies both within and among species. However, it is virtually unknown how the TLP varies within individuals and especially in very large trees. Our objectives were to (1) understand how close to the TLP giant sequoia operates, and (2) explore within-crown variation of the TLP. Using a pressure-volume curve approach, we quantified TLPs ...
Background/Question/Methods Attempts to understand the mechanisms underlying plant mortality during drought have led to the emergence of a hydraulic framework describing distinct hydraulic strategies. This framework distinguishes species... more
Background/Question/Methods Attempts to understand the mechanisms underlying plant mortality during drought have led to the emergence of a hydraulic framework describing distinct hydraulic strategies. This framework distinguishes species that respond to dehydration by rapidly decreasing stomatal conductance (gs), thereby maintaining high water potential (isohydric), from those that maintain high gs, thereby maintaining carbon assimilation albeit at the cost of hydraulic dysfunction (anisohydric). However, portraying the hydraulic framework as a dichotomy between isohydry and anisohydry may obscure important continuous variation in hydraulic regulation, particularly in complex communities with high functional trait diversity. The purpose of this study was to investigate the range of hydraulic strategies present across a large sample of species from a diverse community within South Africa’s Cape Floristic Region. Using stomatal response curves from laboratory-based drydowns for 40 spe...
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The California Mediterranean savanna has harsh summer conditions with minimal soil moisture, high temperature, high incoming solar radiation and little or no precipitation. Deciduous blue oaks, Quercus douglasii Hook. and Arn., are... more
The California Mediterranean savanna has harsh summer conditions with minimal soil moisture, high temperature, high incoming solar radiation and little or no precipitation. Deciduous blue oaks, Quercus douglasii Hook. and Arn., are winter-deciduous obligate phreatophytes, transpiring mostly groundwater throughout the summer drought. The objective of this work is to fully characterize the seasonal trends of photosynthesis in blue oaks as well as the mechanistic relationships between leaf structure and function. We estimate radiative load of the leaves via the FLiES model and perform in situ measurements of leaf water potential, leaf nitrogen content, an index of chlorophyll content (SPAD readings), photosynthetic and electron transport capacity, and instantaneous rates of CO2 assimilation and electron transport. We measured multiple trees over 3 years providing data from a range of conditions. Our study included one individual that demonstrated strong drought stress as indicated by c...
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Trees are spectacular organisms that can accumulate a large amount of biomass, live for millennia, grow in stressful environments, and have global importance to organisms and human society as a result of their roles in contributing to... more
Trees are spectacular organisms that can accumulate a large amount of biomass, live for millennia, grow in stressful environments, and have global importance to organisms and human society as a result of their roles in contributing to diverse ecosystem services. They survive in an enormous range of environments with disturbances of various frequencies and severities, and by their very nature
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Recent studies have shown that natural abundance stable carbon isotopes can be used to partition measurements of net ecosystem CO 2 exchange into gross fluxes of photosynthesis and respiration. This chapter explores the related... more
Recent studies have shown that natural abundance stable carbon isotopes can be used to partition measurements of net ecosystem CO 2 exchange into gross fluxes of photosynthesis and respiration. This chapter explores the related application of the natural abundance stable carbon isotope ratio of respired CO 2 to partitioning components of ecosystem respiration. Although, the exploitation of the natural variation in stable isotope abundance in respired CO 2 holds the most promise for widespread application among existing measurement networks (e.g., BASIN, FLUXNET, SIBAE) and for comparative studies focused on ecosystem C balance, the reliance on natural variation may also have limitations from the standpoint that such variation is typically small. Recent evidence suggests, however, that variation can be large and significant such that measurable differences among respiration signatures can allow partitioning of ecosystem respiration and therefore inform ecosystem C-balance studies.
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Cellulose is one of the most abundant plant compounds on Earth, and contains oxygen that ultimately has its origin from water. The stable oxygen isotope composition, delta18O, of water is strongly associated with environmental conditions.... more
Cellulose is one of the most abundant plant compounds on Earth, and contains oxygen that ultimately has its origin from water. The stable oxygen isotope composition, delta18O, of water is strongly associated with environmental conditions. It follows therefore that the delta18O of leaf (delta18OLC) and stem (delta18OSC) cellulose is regarded as a powerful indicator of environmental conditions. Interpretations of what
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Fog is thought to influence ecological function in coastal forests worldwide, yet few data are available that illuminate the mechanisms underlying this influence. In a California redwood forest we measured water fluxes from horizontally... more
Fog is thought to influence ecological function in coastal forests worldwide, yet few data are available that illuminate the mechanisms underlying this influence. In a California redwood forest we measured water fluxes from horizontally moving fog and vertically delivered rain as well as redwood tree function. The spatial heterogeneity of water fluxes, water availability, tree water use, and water movement