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.
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...
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 ...
Despite its importance for urban planning, landscape management, and water management, there are ... more Despite its importance for urban planning, landscape management, and water management, there are very few in situ estimates of urban-forest transpiration. Because urban forests contain an unusual and diverse mix of species from many regions worldwide, we hypothesized that species composition would be a more important driver of spatial variability in urban-forest transpiration than meteorological variables in the Los Angeles (California, USA) region. We used constant-heat sap-flow sensors to monitor urban tree water use for 15 species at six locations throughout the Los Angeles metropolitan area. For many of these species no previous data on sap flux, water use, or water relations were available in the literature. To scale sap-flux measurements to whole trees we conducted a literature survey of radial trends in sap flux across multiple species and found consistent relationships for angiosperms vs. gymnosperms. We applied this relationship to our measurements and estimated whole-tree and plot-level transpiration at our sites. The results supported very large species differences in transpiration, with estimates ranging from 3.2 6 2.3 kgÁtree À1 Ád À1 in unirrigated Pinus canariensis (Canary Island pine) to 176.9 6 75.2 kgÁtree À1 Ád À1 in Platanus hybrida (London planetree) in the month of August. Other species with high daily transpiration rates included Ficus microcarpa (laurel fig), Gleditsia triacanthos (honeylocust), and Platanus racemosa (California sycamore). Despite irrigation and relatively large tree size, Brachychiton populneas (kurrajong), B. discolor (lacebark), Sequoia sempervirens (redwood), and Eucalyptus grandis (grand Eucalyptus) showed relatively low rates of transpiration, with values , 45 kgÁtree À1 Ád À1 . When scaled to the plot level, transpiration rates were as high as 2 mm/d for sites that contained both species with high transpiration rates and high densities of planted trees. Because plot-level transpiration is highly dependent on tree density, we modeled transpiration as a function of both species and density to evaluate a likely range of values in irrigated urban forests. The results show that urban forests in irrigated, semi-arid regions can constitute a significant use of water, but water use can be mitigated by appropriate selection of site, management method, and species.
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 ...
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.
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.
Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban wat... more Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban water budgets in semi-arid regions. A detailed understanding of plot-scale ET and its sensitivity to plant species composition is necessary to improve estimates of urban water vapour fluxes and water balance. We used portable enclosed chambers and empirical equations to quantify ET from (1) unshaded urban lawns covered exclusively by turfgrass and (2) urban lawns comprised of open-grown trees and turfgrass groundcover in the Los Angeles Metropolitan area. Turfgrass at all locations had a non-limiting supply of soil water because of regular sprinkler irrigation. ET of irrigated turfgrass reached a maximum of 10. 4 ± 1·3 mm d À1 and was always higher than plotscale tree transpiration, which did not exceed 1 mm d À1 . In summer, total plot ET of the lawns with trees was lower than lawns without trees by 0·9-3·9 mm d À1 . Turfgrass ET was highly sensitive to solar radiation, and the ratio of ET of lawns with trees to ET of lawns without trees decreased with tree canopy cover. Hence, reductions in turfgrass ET caused by shading effects of open-grown trees were more important in influencing total landscape ET than the addition of tree transpiration. This suggests that low-density planting of trees that partially shade irrigated urban lawns may be a water-saving measure in semi-arid irrigated environments.
In the United States, turfgrasses cover an area larger than any cultivated crop. In arid and semi... more In the United States, turfgrasses cover an area larger than any cultivated crop. In arid and semiarid cities, where up to 75% of household water is used for irrigation, turfgrass may contribute to a substantial portion of evapotranspiration (ET) from urban landscapes. However, urban ET and the role of turfgrass have seldom been directly quantified. Using portable chambers, we directly measured ET from irrigated turfgrass in 8 locations in the Los Angeles metropolitan area. We addressed the following questions: (1) How does ET from irrigated turfgrass compare with transpiration of urban trees in the study region? (2) What are the major environmental controls on turfgrass ET? (3) How variable is turfgrass ET spatially and temporally? We found summertime ET from irrigated turfgrass to vary from 2.2 ± 0.2 mm/d to 8.8 ± 0.8 mm/d, which exceeded the maximum, plot-level transpiration of irrigated trees in the study region. The main driver of spatial variability of turfgrass ET was incoming...
Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban wat... more Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban water budgets in semi-arid regions. A detailed understanding of plot-scale ET and its sensitivity to plant species composition is necessary to improve estimates of urban water vapour fluxes and water balance. We used portable enclosed chambers and empirical equations to quantify ET from (1) unshaded urban lawns covered exclusively by turfgrass and (2) urban lawns comprised of open-grown trees and turfgrass groundcover in the Los Angeles Metropolitan area. Turfgrass at all locations had a non-limiting supply of soil water because of regular sprinkler irrigation. ET of irrigated turfgrass reached a maximum of 10. 4 ± 1·3 mm d À1 and was always higher than plotscale tree transpiration, which did not exceed 1 mm d À1 . In summer, total plot ET of the lawns with trees was lower than lawns without trees by 0·9-3·9 mm d À1 . Turfgrass ET was highly sensitive to solar radiation, and the ratio of ET of lawns with trees to ET of lawns without trees decreased with tree canopy cover. Hence, reductions in turfgrass ET caused by shading effects of open-grown trees were more important in influencing total landscape ET than the addition of tree transpiration. This suggests that low-density planting of trees that partially shade irrigated urban lawns may be a water-saving measure in semi-arid irrigated environments.
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 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.
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...
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 ...
Despite its importance for urban planning, landscape management, and water management, there are ... more Despite its importance for urban planning, landscape management, and water management, there are very few in situ estimates of urban-forest transpiration. Because urban forests contain an unusual and diverse mix of species from many regions worldwide, we hypothesized that species composition would be a more important driver of spatial variability in urban-forest transpiration than meteorological variables in the Los Angeles (California, USA) region. We used constant-heat sap-flow sensors to monitor urban tree water use for 15 species at six locations throughout the Los Angeles metropolitan area. For many of these species no previous data on sap flux, water use, or water relations were available in the literature. To scale sap-flux measurements to whole trees we conducted a literature survey of radial trends in sap flux across multiple species and found consistent relationships for angiosperms vs. gymnosperms. We applied this relationship to our measurements and estimated whole-tree and plot-level transpiration at our sites. The results supported very large species differences in transpiration, with estimates ranging from 3.2 6 2.3 kgÁtree À1 Ád À1 in unirrigated Pinus canariensis (Canary Island pine) to 176.9 6 75.2 kgÁtree À1 Ád À1 in Platanus hybrida (London planetree) in the month of August. Other species with high daily transpiration rates included Ficus microcarpa (laurel fig), Gleditsia triacanthos (honeylocust), and Platanus racemosa (California sycamore). Despite irrigation and relatively large tree size, Brachychiton populneas (kurrajong), B. discolor (lacebark), Sequoia sempervirens (redwood), and Eucalyptus grandis (grand Eucalyptus) showed relatively low rates of transpiration, with values , 45 kgÁtree À1 Ád À1 . When scaled to the plot level, transpiration rates were as high as 2 mm/d for sites that contained both species with high transpiration rates and high densities of planted trees. Because plot-level transpiration is highly dependent on tree density, we modeled transpiration as a function of both species and density to evaluate a likely range of values in irrigated urban forests. The results show that urban forests in irrigated, semi-arid regions can constitute a significant use of water, but water use can be mitigated by appropriate selection of site, management method, and species.
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 ...
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.
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.
Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban wat... more Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban water budgets in semi-arid regions. A detailed understanding of plot-scale ET and its sensitivity to plant species composition is necessary to improve estimates of urban water vapour fluxes and water balance. We used portable enclosed chambers and empirical equations to quantify ET from (1) unshaded urban lawns covered exclusively by turfgrass and (2) urban lawns comprised of open-grown trees and turfgrass groundcover in the Los Angeles Metropolitan area. Turfgrass at all locations had a non-limiting supply of soil water because of regular sprinkler irrigation. ET of irrigated turfgrass reached a maximum of 10. 4 ± 1·3 mm d À1 and was always higher than plotscale tree transpiration, which did not exceed 1 mm d À1 . In summer, total plot ET of the lawns with trees was lower than lawns without trees by 0·9-3·9 mm d À1 . Turfgrass ET was highly sensitive to solar radiation, and the ratio of ET of lawns with trees to ET of lawns without trees decreased with tree canopy cover. Hence, reductions in turfgrass ET caused by shading effects of open-grown trees were more important in influencing total landscape ET than the addition of tree transpiration. This suggests that low-density planting of trees that partially shade irrigated urban lawns may be a water-saving measure in semi-arid irrigated environments.
In the United States, turfgrasses cover an area larger than any cultivated crop. In arid and semi... more In the United States, turfgrasses cover an area larger than any cultivated crop. In arid and semiarid cities, where up to 75% of household water is used for irrigation, turfgrass may contribute to a substantial portion of evapotranspiration (ET) from urban landscapes. However, urban ET and the role of turfgrass have seldom been directly quantified. Using portable chambers, we directly measured ET from irrigated turfgrass in 8 locations in the Los Angeles metropolitan area. We addressed the following questions: (1) How does ET from irrigated turfgrass compare with transpiration of urban trees in the study region? (2) What are the major environmental controls on turfgrass ET? (3) How variable is turfgrass ET spatially and temporally? We found summertime ET from irrigated turfgrass to vary from 2.2 ± 0.2 mm/d to 8.8 ± 0.8 mm/d, which exceeded the maximum, plot-level transpiration of irrigated trees in the study region. The main driver of spatial variability of turfgrass ET was incoming...
Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban wat... more Evapotranspiration (ET) of irrigated urban plants is a large yet uncertain component of urban water budgets in semi-arid regions. A detailed understanding of plot-scale ET and its sensitivity to plant species composition is necessary to improve estimates of urban water vapour fluxes and water balance. We used portable enclosed chambers and empirical equations to quantify ET from (1) unshaded urban lawns covered exclusively by turfgrass and (2) urban lawns comprised of open-grown trees and turfgrass groundcover in the Los Angeles Metropolitan area. Turfgrass at all locations had a non-limiting supply of soil water because of regular sprinkler irrigation. ET of irrigated turfgrass reached a maximum of 10. 4 ± 1·3 mm d À1 and was always higher than plotscale tree transpiration, which did not exceed 1 mm d À1 . In summer, total plot ET of the lawns with trees was lower than lawns without trees by 0·9-3·9 mm d À1 . Turfgrass ET was highly sensitive to solar radiation, and the ratio of ET of lawns with trees to ET of lawns without trees decreased with tree canopy cover. Hence, reductions in turfgrass ET caused by shading effects of open-grown trees were more important in influencing total landscape ET than the addition of tree transpiration. This suggests that low-density planting of trees that partially shade irrigated urban lawns may be a water-saving measure in semi-arid irrigated environments.
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 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...
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