A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C... more A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that ...
Trees planted in urban landscapes in southern California are often exposed to an unusual combinat... more Trees planted in urban landscapes in southern California are often exposed to an unusual combination of high atmospheric evaporative demand and moist soil conditions caused by irrigation. The water relations of species transplanted into these conditions are uncertain. We investigated the water relations of coast redwood (Sequoia sempervirens) planted in the urbanized semi-arid Los Angeles Basin, where it often experiences leaf chlorosis and senescence. We measured the sap flux (JO) and hydraulic properties of irrigated trees at three sites in the Los Angeles region. We observed relatively strong stomatal regulation in response to atmospheric vapour pressure deficit (D; JO saturated at D < 1 kPa), and a linear response of JO to photosynthetically active radiation. Total tree water use by coast redwood was relatively low, with plot-level transpiration rates below 1 mm d -1 . There was some evidence of xylem cavitation during the summer, which appeared to be reversed in fall and early winter. We conclude that water stress was not a direct factor in causing leaf chlorosis and senescence as has been proposed. Instead, the relatively strong stomatal control that is adaptive in the native habitat of coast redwood may lead to carbon limitation and other stresses in semi-arid, irrigated habitats.
Establishing quantitative links between plant hydraulic properties and the response of transpirat... more Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m T ; slope of the relationship between tree transpiration and ln D) and transpiration at D = 1 kPa (E Tref ) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m T was significantly related to vulnerability to cavitation of branches (P 50 ). While m T did not appear to differ between ring-and diffuseporous species, the relationship between m T and P 50 was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring-versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.
Proceedings of the National Academy of Sciences, 2006
The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2 co... more The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2 concentration ([CO2]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO 2 enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO 2] and to predict the likely effect of elevated [CO2]. We show that at a FACE site where leaf area index (L) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (L D) to account for differences in growing-season length. Moreover, elevated [CO 2]-induced enhancement of TBCA in the combined data decreased from Ϸ50% (700 g C m ؊2 y ؊1 ) at the lowest LD to Ϸ30% (200 g C m ؊2 y ؊1 ) at the highest L D. The consistency of the trend in TBCA with L and its response to [CO 2] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.
Although outdoor water use is thought to constitute a large fraction of urban water budgets in se... more Although outdoor water use is thought to constitute a large fraction of urban water budgets in semi-arid regions, the amount of water actually used by irrigated urban vegetation is not well quantified. In southern California, urban forests are very diverse, and general relationships between water relations parameters would simplify predictions of tree water use for a wide range of species. In this study, we measured sap flux and xylem vulnerability to cavitation of 16 commonly planted irrigated tree species in the Los Angeles Basin, in order to address the following questions: (1) Is stomatal sensitivity to atmospheric vapor pressure deficit systematically different between ring-porous and diffuse-porous trees, as has been previously reported? (2) Is stomatal sensitivity within species consistent across different sites? (3) Can stomatal sensitivity of irrigated trees be predicted based on their vulnerability to cavitation? Contrary to our expectations, we found that ring-porous and ...
Trees planted in urban landscapes in southern California are often exposed to an unusual combinat... more Trees planted in urban landscapes in southern California are often exposed to an unusual combination of high atmospheric evaporative demand and moist soil conditions caused by irrigation. The water relations of species transplanted into these conditions are uncertain. We investigated the water relations of coast redwood (Sequoia sempervirens) planted in the urbanized semi-arid Los Angeles Basin, where it often experiences leaf chlorosis and senescence. We measured the sap flux (JO) and hydraulic properties of irrigated trees at three sites in the Los Angeles region. We observed relatively strong stomatal regulation in response to atmospheric vapour pressure deficit (D; JO saturated at D < 1 kPa), and a linear response of JO to photosynthetically active radiation. Total tree water use by coast redwood was relatively low, with plot-level transpiration rates below 1 mm d -1 . There was some evidence of xylem cavitation during the summer, which appeared to be reversed in fall and early winter. We conclude that water stress was not a direct factor in causing leaf chlorosis and senescence as has been proposed. Instead, the relatively strong stomatal control that is adaptive in the native habitat of coast redwood may lead to carbon limitation and other stresses in semi-arid, irrigated habitats.
Background/Question/Methods Trees in urban ecosystems provide valuable ecosystem services. Howeve... more Background/Question/Methods Trees in urban ecosystems provide valuable ecosystem services. However, in arid regions which are not naturally forested, the irrigation required to sustain urban trees constitutes an ecosystem disservice. A better understanding of the factors that control urban tree water use, growth, water use efficiency and drought resistance is necessary to maximize ecosystem services and minimize costs. To address this issue, a variety of physiological measurements were made (including sapflux, isotope, leaf level photosynthesis, growth rate, stem vulnerability to cavitation, water potential, and leaf nutrient measurements) spanning more than 35 species in the Los Angeles Basin. These data were used in combination with tree inventory data to examine patterns in urban tree and forest function. We wished to answer the questions: What is the relative importance of climate, species composition, and management in determining plant function? How do patterns of tree functio...
Where does all the water go? The ecohydrology of Los Angeles. Diane E. Pataki 1 , Chris Boone 2 ,... more Where does all the water go? The ecohydrology of Los Angeles. Diane E. Pataki 1 , Chris Boone 2 , Terri S. Hogue 3 , G. Darrel Jenerette 4 , Heather R. McCarthy 5 , Joseph P. McFadden 6 , Caroline Mini 3 and Stephanie Pincetl ...
In arid regions which are not naturally forested, urban trees are sustained through the redistrib... more In arid regions which are not naturally forested, urban trees are sustained through the redistribution of water resources as irrigation. Assessments of outdoor water use in Southwestern US cities have shown that not only is 30-75% of residential water use expended on outdoor landscapes, but that irrigation is frequently in excess of estimated plant demand. Thus, there is a need to understand the factors which influence the magnitude and variability of water use of urban trees. A complicating factor in assessing urban tree water use is the widely recognized heterogeneity of urban environments. Human choices and decision-making result in a landscape with significant variability in water and nutrient inputs, microclimate, biotic inputs and vegetation composition. In order to quantify urban tree water use and explain variation in water use resulting from variability in resource availability and species composition, we have conducted a combination of sapflux, growth and isotopic studies ...
Background/Question/Methods Coast redwood, a popular landscape tree in Southern California, is ex... more Background/Question/Methods Coast redwood, a popular landscape tree in Southern California, is exposed to altered environmental conditions compared to its native habitat, where precipitation and fog drip are essential sources of water. In the semi-arid climate of the Los Angeles Basin, where few tree species grow naturally, cultivated coast redwood trees are irrigated, yet exposed to unusually high atmospheric vapor pressure deficit (VPD). As coast redwood trees are reported to have poor stomatal regulation in their native habitat, high VPD may cause a loss of xylem hydraulic conductivity due to air embolism. Observations that irrigated coast redwood in the Los Angeles Basin are difficult to grow led to the following questions: Is VPD the major driver of sap flow rate in this species? Does weak stomatal regulation lead to excessive air embolism of xylem, and water stress? If so, what are the relative roles of atmospheric and soil water stress? To address these questions, we measured...
Establishing quantitative links between plant hydraulic properties and the response of transpirat... more Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m T ; slope of the relationship between tree transpiration and ln D) and transpiration at D = 1 kPa (E Tref ) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m T was significantly related to vulnerability to cavitation of branches (P 50 ). While m T did not appear to differ between ring-and diffuseporous species, the relationship between m T and P 50 was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring-versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.
Under elevated CO2, the enhancement in the amount of carbon (C) sequestered in woody biomass of f... more Under elevated CO2, the enhancement in the amount of carbon (C) sequestered in woody biomass of forests is controlled by availability of other resources that influence growth, such as nutrients and water. At the Duke Forest FACE site, a large growth response of woody tissue in a loblolly pine forest was transient, settling after three years at a marginal gain.
Orthonormal wavelet transformation (OWT) is a computationally efficient technique for quantifying... more Orthonormal wavelet transformation (OWT) is a computationally efficient technique for quantifying underlying frequencies in nonstationary and gap-infested time series, such as eddy-covariance-measured net ecosystem exchange of CO2 (NEE). We employed OWT to analyze the frequency characteristics of synchronously measured and modeled NEE at adjacent pine (PP) and hardwood (HW) ecosystems. Wavelet cospectral analysis showed that NEE at PP was more correlated to light and vapor pressure deficit at the daily time scale, and NEE at HW was more correlated to leaf area index (LAI) and temperature, especially soil temperature, at seasonal time scales. Models were required to disentangle the impacts of environmental drivers on the components of NEE, ecosystem carbon assimilation (Ac) and ecosystem respiration (RE). Sensitivity analyses revealed that using air temperature rather than soil temperature in RE models improved the modeled wavelet spectral frequency response on time scales longer tha...
Net primary productivity (NPP) is enhanced under future atmospheric [CO 2] in temperate forests r... more Net primary productivity (NPP) is enhanced under future atmospheric [CO 2] in temperate forests representing a broad range of productivity. Yet questions remain in regard to how elevated [CO 2]-induced NPP enhancement may be affected by climatic variations and limiting nutrient resources, as well as how this additional production is distributed among carbon (C) pools of different longevities. Using 10 years of data from the Duke free-air CO 2 enrichment (Duke FACE) site, we show that spatially, the major control of NPP was nitrogen (N) availability, through its control on canopy leaf area index (L). Elevated CO 2 levels resulted in greater L, and thus greater NPP. After canopy closure had occurred, elevated [CO 2] did not enhance NPP at a given L, regardless of soil water availability. Additionally, using published data from three other forest FACE sites and replacing L with leaf area duration (L D) to account for differences in growing season length, we show that aboveground NPP responded to [CO 2] only through the enhancement of L D. For broadleaf forests, the fraction of aboveground NPP partitioned to wood biomass saturated with increasing L D and was not enhanced by [CO 2], whereas it linearly decreased for the conifer forest but was enhanced by [CO 2]. These results underscore the importance of resolving [CO 2] effects on L to assess the response of NPP and C allocation. Further study is necessary to elucidate the mechanisms that control the differential allocation of C among aboveground pools in different forest types. carbon allocation ͉ global change ͉ nitrogen availability ͉ pine plantation C urrent studies and modeling exercises indicate a very signif-
Proceedings of The National Academy of Sciences, 2006
Net primary productivity (NPP) is enhanced under future atmospheric [CO₂] in temperate forests re... more Net primary productivity (NPP) is enhanced under future atmospheric [CO₂] in temperate forests representing a broad range of productivity. Yet questions remain in regard to how elevated [CO₂]-induced NPP enhancement may be affected by climatic variations and limiting nutrient resources, as well as how this additional production is distributed among carbon (C) pools of different longevities. Using 10 years of
The potential for elevated [CO 2 ]-induced changes to plant carbon (C) storage, through modificat... more The potential for elevated [CO 2 ]-induced changes to plant carbon (C) storage, through modifications in plant production and allocation of C among plant pools, is an important source of uncertainty when predicting future forest function. Utilizing 10 yr of data from the Duke free-air CO 2 enrichment site, we evaluated the dynamics and distribution of plant C.
Trees in urban ecosystems are valued for shade and cooling effects, reduction of CO 2 emissions a... more Trees in urban ecosystems are valued for shade and cooling effects, reduction of CO 2 emissions and pollution, and aesthetics. However, in arid and semi-arid regions, urban trees must be maintained through supplemental irrigation, in competition with other water needs. Currently, a comprehensive understanding of the factors which influence water use of urban tree species is lacking. In order to study the drivers of whole tree water use of two common species in the Los Angeles Basin urban forest, four sites in Los Angeles and Orange County were instrumented with sap flow and meteorological sensors. These sites allowed comparisons of the water use of a native riparian (Platanus racemosa Nutt.; California sycamore) and non-native (Pinus canariensis C. Sm.; Canary Island pine) Mediterranean species, as well as the spatial variability in water use under different environmental and management conditions. We found higher rates of sapflux (J O ) in native California sycamore as compared to non-native Canary Island pine. Within each species, we found considerable site-to-site variability in the magnitude and seasonality of J O . For Canary Island pine, the majority of inter-site variability derived from differences in water availability: response to vapor pressure deficit was similar during a period without water limitations. In contrast, California sycamore did not appear to experience water limitation at any site; however, there was considerable spatial variability in water use, potentially linked to differences in nutrient availability. Whole tree transpiration (E) was similar for the two species when water was not limiting, but Canary Island pine was able to withstand unirrigated conditions with a very low E. These results add to the currently small pool of data on urban tree water use and ecophysiology, and contribute to establishing a more quantitative understanding of urban tree function.
In semi-arid cities, urban trees are often irrigated, but may also utilize natural water sources ... more In semi-arid cities, urban trees are often irrigated, but may also utilize natural water sources such as groundwater. Consequently, the sources of water for urban tree transpiration may be uncertain, complicating efforts to efficiently manage water resources. We used a novel approach based on stable isotopes to determine tree water sources in the Los Angeles basin, where we hypothesized that trees would rely on irrigation water in the soil rather than develop deep roots to tap into groundwater. We evaluated the oxygen (δ 18 O) and hydrogen (δD) isotope ratios of xylem water, irrigation water, soil water, and groundwater in a study of temporal patterns in water sources at two urban sites, and a study of spatial patterns at nine urban sites and one "natural" riparian forest. Contrary to our hypothesis, we found that despite frequent irrigation, some trees tap into groundwater, although in most species this was a small water source. Some trees appeared to be using very shallow soil water at <30 cm depth, suggesting that these mature urban trees were quite shallowly rooted. In the natural site, trees appeared to be using urban runoff in addition to shallow soil water. We were able to identify tree uptake of precipitation at only 3 sites. The results show that some irrigated trees utilize groundwater and do not rely solely on irrigation water, which may make them able to withstand drought and/or water conservation measures. However, some irrigated trees may develop very shallow root systems, which may make them more susceptible.
A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C... more A highly controversial issue in global biogeochemistry is the regulation of terrestrial carbon (C) sequestration by soil nitrogen (N) availability. This controversy translates into great uncertainty in predicting future global terrestrial C sequestration. We propose a new framework that ...
Trees planted in urban landscapes in southern California are often exposed to an unusual combinat... more Trees planted in urban landscapes in southern California are often exposed to an unusual combination of high atmospheric evaporative demand and moist soil conditions caused by irrigation. The water relations of species transplanted into these conditions are uncertain. We investigated the water relations of coast redwood (Sequoia sempervirens) planted in the urbanized semi-arid Los Angeles Basin, where it often experiences leaf chlorosis and senescence. We measured the sap flux (JO) and hydraulic properties of irrigated trees at three sites in the Los Angeles region. We observed relatively strong stomatal regulation in response to atmospheric vapour pressure deficit (D; JO saturated at D < 1 kPa), and a linear response of JO to photosynthetically active radiation. Total tree water use by coast redwood was relatively low, with plot-level transpiration rates below 1 mm d -1 . There was some evidence of xylem cavitation during the summer, which appeared to be reversed in fall and early winter. We conclude that water stress was not a direct factor in causing leaf chlorosis and senescence as has been proposed. Instead, the relatively strong stomatal control that is adaptive in the native habitat of coast redwood may lead to carbon limitation and other stresses in semi-arid, irrigated habitats.
Establishing quantitative links between plant hydraulic properties and the response of transpirat... more Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m T ; slope of the relationship between tree transpiration and ln D) and transpiration at D = 1 kPa (E Tref ) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m T was significantly related to vulnerability to cavitation of branches (P 50 ). While m T did not appear to differ between ring-and diffuseporous species, the relationship between m T and P 50 was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring-versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.
Proceedings of the National Academy of Sciences, 2006
The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2 co... more The partitioning among carbon (C) pools of the extra C captured under elevated atmospheric CO2 concentration ([CO2]) determines the enhancement in C sequestration, yet no clear partitioning rules exist. Here, we used first principles and published data from four free-air CO 2 enrichment (FACE) experiments on forest tree species to conceptualize the total allocation of C to below ground (TBCA) under current [CO 2] and to predict the likely effect of elevated [CO2]. We show that at a FACE site where leaf area index (L) of Pinus taeda L. was altered through nitrogen fertilization, ice-storm damage, and droughts, changes in L, reflecting the aboveground sink for net primary productivity, were accompanied by opposite changes in TBCA. A similar pattern emerged when data were combined from the four FACE experiments, using leaf area duration (L D) to account for differences in growing-season length. Moreover, elevated [CO 2]-induced enhancement of TBCA in the combined data decreased from Ϸ50% (700 g C m ؊2 y ؊1 ) at the lowest LD to Ϸ30% (200 g C m ؊2 y ؊1 ) at the highest L D. The consistency of the trend in TBCA with L and its response to [CO 2] across the sites provides a norm for predictions of ecosystem C cycling, and is particularly useful for models that use L to estimate components of the terrestrial C balance.
Although outdoor water use is thought to constitute a large fraction of urban water budgets in se... more Although outdoor water use is thought to constitute a large fraction of urban water budgets in semi-arid regions, the amount of water actually used by irrigated urban vegetation is not well quantified. In southern California, urban forests are very diverse, and general relationships between water relations parameters would simplify predictions of tree water use for a wide range of species. In this study, we measured sap flux and xylem vulnerability to cavitation of 16 commonly planted irrigated tree species in the Los Angeles Basin, in order to address the following questions: (1) Is stomatal sensitivity to atmospheric vapor pressure deficit systematically different between ring-porous and diffuse-porous trees, as has been previously reported? (2) Is stomatal sensitivity within species consistent across different sites? (3) Can stomatal sensitivity of irrigated trees be predicted based on their vulnerability to cavitation? Contrary to our expectations, we found that ring-porous and ...
Trees planted in urban landscapes in southern California are often exposed to an unusual combinat... more Trees planted in urban landscapes in southern California are often exposed to an unusual combination of high atmospheric evaporative demand and moist soil conditions caused by irrigation. The water relations of species transplanted into these conditions are uncertain. We investigated the water relations of coast redwood (Sequoia sempervirens) planted in the urbanized semi-arid Los Angeles Basin, where it often experiences leaf chlorosis and senescence. We measured the sap flux (JO) and hydraulic properties of irrigated trees at three sites in the Los Angeles region. We observed relatively strong stomatal regulation in response to atmospheric vapour pressure deficit (D; JO saturated at D < 1 kPa), and a linear response of JO to photosynthetically active radiation. Total tree water use by coast redwood was relatively low, with plot-level transpiration rates below 1 mm d -1 . There was some evidence of xylem cavitation during the summer, which appeared to be reversed in fall and early winter. We conclude that water stress was not a direct factor in causing leaf chlorosis and senescence as has been proposed. Instead, the relatively strong stomatal control that is adaptive in the native habitat of coast redwood may lead to carbon limitation and other stresses in semi-arid, irrigated habitats.
Background/Question/Methods Trees in urban ecosystems provide valuable ecosystem services. Howeve... more Background/Question/Methods Trees in urban ecosystems provide valuable ecosystem services. However, in arid regions which are not naturally forested, the irrigation required to sustain urban trees constitutes an ecosystem disservice. A better understanding of the factors that control urban tree water use, growth, water use efficiency and drought resistance is necessary to maximize ecosystem services and minimize costs. To address this issue, a variety of physiological measurements were made (including sapflux, isotope, leaf level photosynthesis, growth rate, stem vulnerability to cavitation, water potential, and leaf nutrient measurements) spanning more than 35 species in the Los Angeles Basin. These data were used in combination with tree inventory data to examine patterns in urban tree and forest function. We wished to answer the questions: What is the relative importance of climate, species composition, and management in determining plant function? How do patterns of tree functio...
Where does all the water go? The ecohydrology of Los Angeles. Diane E. Pataki 1 , Chris Boone 2 ,... more Where does all the water go? The ecohydrology of Los Angeles. Diane E. Pataki 1 , Chris Boone 2 , Terri S. Hogue 3 , G. Darrel Jenerette 4 , Heather R. McCarthy 5 , Joseph P. McFadden 6 , Caroline Mini 3 and Stephanie Pincetl ...
In arid regions which are not naturally forested, urban trees are sustained through the redistrib... more In arid regions which are not naturally forested, urban trees are sustained through the redistribution of water resources as irrigation. Assessments of outdoor water use in Southwestern US cities have shown that not only is 30-75% of residential water use expended on outdoor landscapes, but that irrigation is frequently in excess of estimated plant demand. Thus, there is a need to understand the factors which influence the magnitude and variability of water use of urban trees. A complicating factor in assessing urban tree water use is the widely recognized heterogeneity of urban environments. Human choices and decision-making result in a landscape with significant variability in water and nutrient inputs, microclimate, biotic inputs and vegetation composition. In order to quantify urban tree water use and explain variation in water use resulting from variability in resource availability and species composition, we have conducted a combination of sapflux, growth and isotopic studies ...
Background/Question/Methods Coast redwood, a popular landscape tree in Southern California, is ex... more Background/Question/Methods Coast redwood, a popular landscape tree in Southern California, is exposed to altered environmental conditions compared to its native habitat, where precipitation and fog drip are essential sources of water. In the semi-arid climate of the Los Angeles Basin, where few tree species grow naturally, cultivated coast redwood trees are irrigated, yet exposed to unusually high atmospheric vapor pressure deficit (VPD). As coast redwood trees are reported to have poor stomatal regulation in their native habitat, high VPD may cause a loss of xylem hydraulic conductivity due to air embolism. Observations that irrigated coast redwood in the Los Angeles Basin are difficult to grow led to the following questions: Is VPD the major driver of sap flow rate in this species? Does weak stomatal regulation lead to excessive air embolism of xylem, and water stress? If so, what are the relative roles of atmospheric and soil water stress? To address these questions, we measured...
Establishing quantitative links between plant hydraulic properties and the response of transpirat... more Establishing quantitative links between plant hydraulic properties and the response of transpiration to environmental factors such as atmospheric vapor pressure deficit (D) is essential for improving our ability to understand plant water relations across a wide range of species and environmental conditions. We studied stomatal responses to D in irrigated trees in the urban landscape of Los Angeles, California. We found a strong linear relationship between the sensitivity of tree-level transpiration estimated from sap flux (m T ; slope of the relationship between tree transpiration and ln D) and transpiration at D = 1 kPa (E Tref ) that was similar to previous surveys of stomatal behavior in natural environments. In addition, m T was significantly related to vulnerability to cavitation of branches (P 50 ). While m T did not appear to differ between ring-and diffuseporous species, the relationship between m T and P 50 was distinct by wood anatomy. Therefore, our study confirms systematic differences in water relations in ring-versus diffuse-porous species, but these differences appear to be more strongly related to the relationship between stomatal sensitivity to D and vulnerability to cavitation rather than to stomatal sensitivity per se.
Under elevated CO2, the enhancement in the amount of carbon (C) sequestered in woody biomass of f... more Under elevated CO2, the enhancement in the amount of carbon (C) sequestered in woody biomass of forests is controlled by availability of other resources that influence growth, such as nutrients and water. At the Duke Forest FACE site, a large growth response of woody tissue in a loblolly pine forest was transient, settling after three years at a marginal gain.
Orthonormal wavelet transformation (OWT) is a computationally efficient technique for quantifying... more Orthonormal wavelet transformation (OWT) is a computationally efficient technique for quantifying underlying frequencies in nonstationary and gap-infested time series, such as eddy-covariance-measured net ecosystem exchange of CO2 (NEE). We employed OWT to analyze the frequency characteristics of synchronously measured and modeled NEE at adjacent pine (PP) and hardwood (HW) ecosystems. Wavelet cospectral analysis showed that NEE at PP was more correlated to light and vapor pressure deficit at the daily time scale, and NEE at HW was more correlated to leaf area index (LAI) and temperature, especially soil temperature, at seasonal time scales. Models were required to disentangle the impacts of environmental drivers on the components of NEE, ecosystem carbon assimilation (Ac) and ecosystem respiration (RE). Sensitivity analyses revealed that using air temperature rather than soil temperature in RE models improved the modeled wavelet spectral frequency response on time scales longer tha...
Net primary productivity (NPP) is enhanced under future atmospheric [CO 2] in temperate forests r... more Net primary productivity (NPP) is enhanced under future atmospheric [CO 2] in temperate forests representing a broad range of productivity. Yet questions remain in regard to how elevated [CO 2]-induced NPP enhancement may be affected by climatic variations and limiting nutrient resources, as well as how this additional production is distributed among carbon (C) pools of different longevities. Using 10 years of data from the Duke free-air CO 2 enrichment (Duke FACE) site, we show that spatially, the major control of NPP was nitrogen (N) availability, through its control on canopy leaf area index (L). Elevated CO 2 levels resulted in greater L, and thus greater NPP. After canopy closure had occurred, elevated [CO 2] did not enhance NPP at a given L, regardless of soil water availability. Additionally, using published data from three other forest FACE sites and replacing L with leaf area duration (L D) to account for differences in growing season length, we show that aboveground NPP responded to [CO 2] only through the enhancement of L D. For broadleaf forests, the fraction of aboveground NPP partitioned to wood biomass saturated with increasing L D and was not enhanced by [CO 2], whereas it linearly decreased for the conifer forest but was enhanced by [CO 2]. These results underscore the importance of resolving [CO 2] effects on L to assess the response of NPP and C allocation. Further study is necessary to elucidate the mechanisms that control the differential allocation of C among aboveground pools in different forest types. carbon allocation ͉ global change ͉ nitrogen availability ͉ pine plantation C urrent studies and modeling exercises indicate a very signif-
Proceedings of The National Academy of Sciences, 2006
Net primary productivity (NPP) is enhanced under future atmospheric [CO₂] in temperate forests re... more Net primary productivity (NPP) is enhanced under future atmospheric [CO₂] in temperate forests representing a broad range of productivity. Yet questions remain in regard to how elevated [CO₂]-induced NPP enhancement may be affected by climatic variations and limiting nutrient resources, as well as how this additional production is distributed among carbon (C) pools of different longevities. Using 10 years of
The potential for elevated [CO 2 ]-induced changes to plant carbon (C) storage, through modificat... more The potential for elevated [CO 2 ]-induced changes to plant carbon (C) storage, through modifications in plant production and allocation of C among plant pools, is an important source of uncertainty when predicting future forest function. Utilizing 10 yr of data from the Duke free-air CO 2 enrichment site, we evaluated the dynamics and distribution of plant C.
Trees in urban ecosystems are valued for shade and cooling effects, reduction of CO 2 emissions a... more Trees in urban ecosystems are valued for shade and cooling effects, reduction of CO 2 emissions and pollution, and aesthetics. However, in arid and semi-arid regions, urban trees must be maintained through supplemental irrigation, in competition with other water needs. Currently, a comprehensive understanding of the factors which influence water use of urban tree species is lacking. In order to study the drivers of whole tree water use of two common species in the Los Angeles Basin urban forest, four sites in Los Angeles and Orange County were instrumented with sap flow and meteorological sensors. These sites allowed comparisons of the water use of a native riparian (Platanus racemosa Nutt.; California sycamore) and non-native (Pinus canariensis C. Sm.; Canary Island pine) Mediterranean species, as well as the spatial variability in water use under different environmental and management conditions. We found higher rates of sapflux (J O ) in native California sycamore as compared to non-native Canary Island pine. Within each species, we found considerable site-to-site variability in the magnitude and seasonality of J O . For Canary Island pine, the majority of inter-site variability derived from differences in water availability: response to vapor pressure deficit was similar during a period without water limitations. In contrast, California sycamore did not appear to experience water limitation at any site; however, there was considerable spatial variability in water use, potentially linked to differences in nutrient availability. Whole tree transpiration (E) was similar for the two species when water was not limiting, but Canary Island pine was able to withstand unirrigated conditions with a very low E. These results add to the currently small pool of data on urban tree water use and ecophysiology, and contribute to establishing a more quantitative understanding of urban tree function.
In semi-arid cities, urban trees are often irrigated, but may also utilize natural water sources ... more In semi-arid cities, urban trees are often irrigated, but may also utilize natural water sources such as groundwater. Consequently, the sources of water for urban tree transpiration may be uncertain, complicating efforts to efficiently manage water resources. We used a novel approach based on stable isotopes to determine tree water sources in the Los Angeles basin, where we hypothesized that trees would rely on irrigation water in the soil rather than develop deep roots to tap into groundwater. We evaluated the oxygen (δ 18 O) and hydrogen (δD) isotope ratios of xylem water, irrigation water, soil water, and groundwater in a study of temporal patterns in water sources at two urban sites, and a study of spatial patterns at nine urban sites and one "natural" riparian forest. Contrary to our hypothesis, we found that despite frequent irrigation, some trees tap into groundwater, although in most species this was a small water source. Some trees appeared to be using very shallow soil water at <30 cm depth, suggesting that these mature urban trees were quite shallowly rooted. In the natural site, trees appeared to be using urban runoff in addition to shallow soil water. We were able to identify tree uptake of precipitation at only 3 sites. The results show that some irrigated trees utilize groundwater and do not rely solely on irrigation water, which may make them able to withstand drought and/or water conservation measures. However, some irrigated trees may develop very shallow root systems, which may make them more susceptible.
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Papers by Heather McCarthy