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Schwinning2000 Schwinning2001, Sharifi1990' Sparks1997"Thorburn1995?#Williams1996Williams20006Williams2000Williams20006Williams2000Williams20006Williams2000Williams2000Williams2000Williams20006Williams2000Williams2000Williams2000Williams20006Williams2000Williams2000Williams2000Williams2000Williams2000Williams2000Williams2000Williams20006Williams2000Williams2000Williams20006Williams2000Williams2000Williams2000Williams20006Williams2000Williams2000Williams20002000200020002000ms200020001996Williams David1996Williams David1996Williams David19961996Williams David19961Williams David1996Williams David1996Williams David199619961996     AuthorsJournalsKeywords         0*Comstock, Jonathan P. Ehleringer, James R. 1992>8Plant adaptation in the Great Basin and Colorado PlateauGreat Basin Naturalist523195-215Adapative features of plants of the Great Basin are reviewed. The combination of cold winters and an arid to semiarid precipitation regime results in the distinguishing features of the vegetation in the Greatest Basin and Colorado Plateau. The primary effects of these climatic features arise from how they structure the hydrologic regime. Water is the most limiting factor to plant growth, and water is most reliably available in the early spring after winter recharge of soil moisture. This factor determines many characteristics of root morphology, growth phenology of roots and shoots, and photosynthetic physiology. Since winters are typically cold enough to suppress growth, and drought limits growth during the summer, the cool temperatures characteristic of the peak growing season are the second most important climatic factor influencing plant habit and performance. The combination of several distinct stress periods, including low-temperature stress in winter and spring and high-temperature stress combined with drought in summer, appears to have limited plant habit to a greater degree than found in the warm deserts to the south. Nonetheless, cool growing conditions and a more reliable spring growing season result in higher water-use efficiency and productivity in the vegetation of the cold desert than in warm deserts with equivalent total rainfall amounts. Edaphic factors are also important in structuring communities in these regions, and halophytic communities dominate many landscapes. These communities of the cold desert share more species common with warm deserts than do the nonsaline communities. The Colorado Plateau differs from the Great Basin in having greater amounts of summer rainfall, in some regions less predictable rainfall, sandier soils, and streams which drain into river systems rather than closed basins and salt playas. One result of these climatic and edaphic differences is a more important summer growing season on the Colorado Plateau and a somewhat greater diversification of plant habit, phenology, and physiology.b\Ecophysiological differences among juvenile and reproductive plants of several woody species& Donovan, L. A. Ehleringer, J. R. Oecologia 864A594-597. 1991Photosynthetic and water relations characteristics of small juvenile and large reproductive plants were investigated during one growing season for four woody species native to Red Butte Canyon, Utah, USA: Acer negundo, Artemisia tridentata, Chrysothamnus nauseosus, and Salix exigua. For all species, juvenile plants differed from reproductive plants in at least one of the following characters: water potential, stomatal conductance, photosynthetic rate, or water-use efficiency. Late in the growing season, mortality occurred within juvenile plants (apparently due to a lack of water), but not within reproductive plants. The observed differences between juvenile and reproductive classes are discussed in terms of environment, development, and mortality selection.y^WContrasting water-use patterns among size and life-history classes of a semi-arid shrube& Donovan, L. A. Ehleringer, J. R.Functional Ecology64482-488o 19920)Ecophysiological characteristics of plants from different size and life-history classes were investigated in a field population of Chrysothamnus nauseosus. Juvenile plants ( gt 1-year-old but pre-reproductive) had higher rates of photosynthesis, stomatal conductance and transpiration than reproductive adults, even though pre-dawn xylem pressure potentials of juveniles were slightly lower. Juveniles were also less water-use efficient than adults based on instantaneous gas exchange (photosynthesis/transpiration) and carbon isotope discrimination (DELTA). A comparison of DELTA-values indicated a gradient of water-use efficiency that was correlated with size: Seedlings lt Juveniles = Small Adults lt Large Adults. Reproductive status did not account for any variation independent of size. Small establishing plants may experience short-term environmental conditions and long-term selective pressures different from those of larger reproductive plants. The pattern of smaller plants having higher rates of gas exchange and less efficient use of water in the absence of higher pre-dawn xylem pressure potentials suggests a developmental progression in ecophysiological parameters with increasing size, in addition to the environmentally induced variation that has been documented for these characters for many species.<6Donovan, Lisa A. Mausberg, James Ehleringer, James, R. 1993<6Seedling size and survival for Chrysothamnus nauseosusGreat Basin Naturalist533237-245p$Seedling size and survival in relation to summer drought were examined for Chrysothamnus nauseosus growing under field and greenhouse conditions. In the field, summer survival rates were less than 2% annually for the three years monitored. The effect of initial seedling height on subsequent survival was examined in both the field and greenhouse by grouping seedlings into live and dead categories on each census date and comparing the initial heights for seedlings in these categories. For a majority of the census dates, the initial height of surviving seedlings was greater than the initial height of those that subsequently died (significant differences ranged from 1 to 8 mm), indicating that seedlings that were taller at the initiation of the drought period had a higher probability of survival. In the greenhouse, taller seedlings had greater shoot and root biomass and rooting depth. Seedlings that are larger (i.e., taller and have greater aboveground biomass) in late spring appear to have a higher probability of surviving the summer drought due to greater rooting depth and hence increased access to moisture in deeper soil layers. Seed availability and safe sites for germination were probably not limiting since large numbers of seedlings successfully germinated in a patchy pattern during the study period. Seedling size and probability of survival were not related to either seedling density or the distance to nearest seedling neighbor. Survival through summer drought appears to be the main limitations to seedling recruitment in this population.& Donovan, L. A. Ehleringer, J. R. 1994TMWater stress and use of summer precipitation in a Great Basin shrub communityaFunctional Ecology8 32289-297c1. Seasonal patterns of water stress (determined from predawn xylem pressure potentials) and relative use of summer precipitation (determined from hydrogen isotope composition) varied intraspecifically and interspecifically in a Great Basin shrub community. 2. The development of water stress during the dry season from June to July was positively correlated with the use of summer precipitation in August. The smallest plants generally developed the greatest water stress and took up the most summer precipitation, presumably due to being more dependent on shallow roots. 3. Among mature shrubs examined, moisture from summer precipitation was taken up by Chrysothamnus viscidiflorus and Artemisia tridentata, but not by C. nauseosus, Juniperus osteosperma and Tetradymia canescens. 4. For shrub species that are deeply rooted at maturity, such as C. nauseosus, the quantity and timing of available soil moisture is different for small establishing plants as compared to large reproductively mature plants.g:L,%Donovan, Lisa A. Ehleringer, James R.o 1994ngCarbon isotope discrimination, water-use efficiency, growth and mortality in a natural shrub populationn Oecologia 1003347-354voIn order to scale up from the ecophysiological characters of individual plants to population-level questions, we need to determine if character patterns in natural populations are stable through time, and if the characters are related to growth and survival. We investigated these questions in a 3-year study for one character, integrated water-use efficiency (WUE) as estimated by carbon isotope discrimination (DELTA) in a population of the Great Basin shrub, Chrysothamnus nauseosus. WUE was a conservative character for a given plant within and across seasons, and a previously documented difference between two size classes (represented by juveniles and adults) was maintained-, smaller juveniles had a lower WUE than larger adults. The lower WUE of juveniles was often accompanied by higher rates of photosynthesis and stomatal conductance as compared to adults even though juveniles generally had more negative xylem pressure potentials. Although many discussions of the role of WUE in natural populations have been based on the expectation that higher WUE (lower DELTA) is generally associated with less growth, we found no such relationship for juvenile plants in this population (i.e. DELTA was not positively correlated with height increase). In addition, juvenile plant mortality was not correlated with DELTA. Although there were stable patterns of WUE for plants in this population, the positive correlation between WUE and size, and the lack of a negative correlation between WUE and height growth. make it unlikely that the WUE of an individual plant will be related in a simple manner to its growth and survival in the population.g@9Differential utilization of summer rains by desert plantsLEEhleringer, J. R. Phillips, S. L. Schuster, W. S. F. Sandquist, D. R.i Oecologiai883430-434 1991tmSeasonal changes in the hydrogen isotope ratios of xylem waters were measured to determine water sources used for growth in desert plants of southern Utah. While all species used winter-spring recharge precipitation for spring growth, utilization of summer rains was life-form dependent. Annuals and succulent pereninals exhibited a complete dependence on summer precipitation. Herbaceous and woody perennial species simultaneously utilized both summer precipitation and remaining winter-spring precipitation, with herbaceous species much more reliant on the summer precipitation component. Several of the woody perennials exhibited no response to summer precipitation. Currently, precipitation in southern Utah is evenly partitioned between winter and summer time periods; however, global circulation models predict that summer precipitation will increase in response to anticipated climate change. Our data indicate that components within the community will differentially respond to the change in precipitation patterns. These results are discussed in relation to impact on competition and possible changes in community structure." J't&Sparks, J. P. Ehleringer, J. R. 1997lfLeaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects Oecologia 1093362-367aLeaf carbon isotope discrimination (DELTA), seasonal estimates of the leaf-to-air water vapor gradient on a molar basis (omega), and leaf nitrogen contents were examined in three riparian tree species (Populus fremontii, P. angustifolia and Salix exigua) along elevational transects in northern and southern Utah USA (1500-2670 m and 600-1820 m elevational gradients, respectively). The co values decreased with elevation for all species along transects. Plants growing at higher elevations exhibited lower A values than plants at lower elevations (P. fremontii, 22.9%. and 19.5 permill , respectively; P. angustifolia, 23.2 permill and 19.2 permill , respectively; and S. exigua, 21.1 permill and 19.1 permill , respectively). Leaf nitrogen content increased with elevation for all species, suggesting that photosynthetic capacity at a given intercellular carbon dioxide concentration was greater at higher elevations. Leaf A and nitrogen content values were highly correlated, implying that leaves with higher photosynthetic capacities also had lower intercellular carbon dioxide concentrations. No significant interannual differences were detected in carbon isotope discrimination..'Thorburn, Peter J. Ehleringer, James R.r 1995hbRoot water uptake of field-growing plants indicated by measurements of natural-abundance deuteriumPlant and Soil 1772225-2333 t nMeasurements of stable-isotope ratios of water extracted from stems and, in some studies, soils are increasingly being used to study the integrated root function of field-growing plants. This study explored if additional measurements on water extracted from roots could indicate the activity of roots in different areas of the soil profile and their influence on canopy water sources, so providing advantages over more common sampling strategies. Studies were conducted on trees and shrubs located in diverse habitats: a saline, semi-arid floodplain, a subhumid mountain-range front and a cold desert. At each site, roots, soil immediately surrounding the roots, and plant stems were sampled. Roots were taken from different depths in the soil, to approximately 2 m at one site. Overall, 80% of roots sampled had H isotope ratios different from the surrounding soil. The differences up to 37 permill , were significant (p lt 0.05) at two of the sites. Thus water in most of the roots sampled did not come entirely, if at all, from the surrounding soil, illustrating movement and possible mixing of water within the root system. This condition was not simply related to the availability of water surrounding the soil, which was also measured. There were also differences in root and stem H isotope ratios (up to 17 permill ) in 67% of samples, although the difference was only significant in shallow samples from the floodplain. The general similarity in stem and root delta-2H values indicates that most roots sampled were involved in the main supply of water to the canopy. Patterns of root function varied between the individual sites. Trees were primarily using groundwater at the floodplain and mountain front sites, as the surface soils had mean matric potentials of -1800 kPa. At the mountain front site, the surface roots were transporting groundwater to the canopy in isolation form the surrounding soil. In contrast, surface roots at the floodplain were taking up water from the surrounding soil, although this water was not a significant source in the trees' overall water supply. This activity of surface roots would not have been evident from the delta-2H data without the root samples. At the cold desert the roots in moist surface soil provided the main source of water for the shrubs. There too the root data indicated different water uptake patterns than otherwise would have been assumed. The root data showed that groundwater could not have been a water source, a conclusion which had been reached in a previous study. Thus measurements of stable isotope ratios in root water may be a valuable tool in assessing water uptake patterns and root function. X* B20*Flanagan, Lawrence B. Ehleringer, James R. 1998d]Ecosystem-atmosphere CO2 exchange: Interpreting signals of change using stable isotope ratiosB&Trends in Ecology and Evolution-131 10-14A0*Gebauer, Renate L. E. Ehleringer, James R. 2000^WWater and nitrogen uptake patterns following moisture pulses in a cold desert communitytEcologyF815 1415-1424aztVariation in the ability to utilize pulses of both water and nitrogen (N) is one possible mechanism allowing the coexistence of species in the cold desert community on the Colorado Plateau. We simulated 25-mm precipitation events and used stable isotope tracers (2H and 15N) to follow water and N uptake patterns in six dominant perennials (Artemisia filifolia, Coleogyne ramosissima, Cryptantha flava, Ephedra viridis, Quercus havardii, and Vanclevea stylosa) at different times of the growing season. Water pulse utilization varied on a seasonal basis and was to some extent different among species during the summer. Carbon isotope discrimination was negatively related to both plant use of moisture in upper soil layers and foliar N concentration. Species that were similar in water pulse utilization patterns differed in the natural abundances of 15N, suggesting partitioning in N sources. All species were able to utilize N pulses after rain events, but there were temporal differences in the responses among species. We also found that water and N uptake in shallow roots do not necessarily occur simultaneously. Artemisia, Cryptantha, and Quercus showed significant uptake of both water and N from the upper soil layers. In contrast, Coleogyne and Ephedra showed the capacity to utilize the water pulse, but not the N pulse. Vanclevea only took up N. The results indicate that different parts of the root system may be responsible for the acquisition of water and N. Our results also suggest that N and water partitioning could contribute to the coexistence of species in highly variable environments such as the Colorado Plateau desert system.\VJackson, R. B. Canadell, J. Ehleringer, J. R. Mooney, H. A. Sala, O. E. Schulze, E. D. 1996D>A global analysis of root distributions for terrestrial biomes Oecologia 1083a389-411o Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient beta to the data for each biome (Gale and Grigal 1987). beta is a simple numerical index of rooting distribution based on the asymptotic equation Y= 1-beta-d, where d = depth and Y = the proportion of roots from the surface to depth d. High values of beta correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (beta = 0.913, 0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (beta = 0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil (beta = 0.952), while shrubs had only 21% in the same depth increment (beta = 0.978). The rooting distribution of all temperate and tropical trees was beta = 0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was beta = 0.966 (r-2 = 0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.<5Lin, Guanghui Phillips, Susan L. Ehleringer, James R. 1996d^Monsoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau Oecologia 106e1 8-17 zSouth-eastern Utah forms a northern border for the region currently influenced by the Arizona monsoonal system, which feeds moisture and summer precipitation into western North America. One major consequence predicted by global climate change scenarios is an intensification of monsoonal (summer) precipitation in the arid land areas of the western United States. We examined the capacity of dominant perennial shrubs in a Colorado Plateau cold desert ecosystem of southern Utah, United States, to use summer moisture inputs. We simulated increases of 25 and 50 mm summer rain events on Atriplex canescens, Artemisia filifolia, Chrysothamnus nauseosus, Coleogyne ramosissima, and Vanclevea stylosa, in July and September with an isotopically enriched water (enriched in deuterium but not 180). The uptake of this artificial water source was estimated by analyzing hydrogen and oxygen isotope ratios of stem water. The predawn and midday xylem water potentials and foliar carbon isotope discrimination were measured to estimate changes in water status and water-use efficiency. At. canescens and Ch. nauseosus showed little if any uptake of summer rains in either July or September. The predawn and midday xylem water potentials for control and treatment plants of these two species were not significantly different from each other. For A. filifolia and V styles, up to 50% of xylem water was from the simulated summer rain, but the predawn and midday xylem water potentials were not significantly affected by the additional summer moisture input. In contrast, C. ramosissima showed significant uptake of the simulated summer rain ( gt 50% of xylem water was from the artificial summer rain) and an increase in both predawn and midday water potentials. The percent uptake of simulated summer rain was greater when those rains were applied in September than in July, implying that high soil temperature in midsummer may in some way inhibit water uptake. Foliar carbon isotope discrimination increased significantly in the three shrubs taking up simulated summer rain, but pre-treatment differences in the absolute discrimination values were maintained among species. The ecological implications of our results are discussed in terms of the dynamics of this desert community in response to changes in the frequency and dependability of summer rains that might be associated with a northward shift in the Arizona monsoon boundary.Heritability of carbon isotope discrimination in Gutierrezia microcephala (Asteraceae)LESchuster, W. S. F. Phillips, S. L. Sandquist, D. R. Ehleringer, J. R. American Journal of Botany792216-221 1992D=The carbon isotope composition (13C/12C) of C-3 plant tissues provides a long-term, integrated mea,%Donovan, Lisa A. Ehleringer, James R. 1994nhPotential for selection on plants for water-use efficiency as estimated by carbon isotope discrimination American Journal of Botany817927-935Water-use efficiency is thought to be related to plant performance and natural selection for plants in arid habitats, based on a general expectation that increased water-use efficiency is associated with decreased carbon gain and biomass accumulation. Using leaf carbon isotope discrimination (DELTA) to determine integrated water-use efficiency, we estimated genetic variance for, and examined the relationships among DELTA biomass, and gas exchange characters for full-sibling families of the woody shrub, Chrysothamnus nauseosus, grown from seed collected at Tintic, Utah. In both well-watered greenhouse and common garden experiments, and water-limited common garden experiments, there were significant family differences for DELTA, biomass, and morphological characters, indicating a potential for genetic change in response to selection. However, estimates of broadsense heritabilities for DELTA were low, indicating that the rate of change in response to selection would be relatively slow. This was consistent with the large amount of phenotypic plasticity observed for DELTA as it differed with water treatment and year in the garden experiment. Phenotypically, aboveground biomass and DELTA were negatively correlated within the well-watered treatments (i.e., more water-use efficient plants were larger), not correlated within the water-limited treatment, and positively correlated for combined well-watered and water-limited garden treatments, suggesting that variation in both photosynthetic capacity and stomatal limitation contribute to the variation in DELTA. In contrast to the phenotypic correlations, genetic correlations for biomass and DELTA were consistently negative within each treatment, and selection for higher water-use efficiency through low DELTA for C. nauseosus plants in this population would tend to shift populations toward larger plants. For C. nauseosus, increased water-use efficiency is not necessarily associated with decreased carbon gain.LFSeasonal variation in the carbon isotopic composition of desert plants81Ehleringer, J. R. Phillips, S. L. Comstock, J. P.iFunctional Ecology6u4.396-404i 1992Carbon isotope discrimination (DELTA) has been shown to be a valuable tool for long-term estimates of water-use efficiency, because this parameter is influenced by the ratio of intercellular to ambient CO-2 levels, which is in turn related to water-use efficiency. The objective of this study was to examine variation in DELTA-values of field materials and the extent to which changes in DELTA might be interpreted as changes in water-use efficiency. On-line discrimination observations with four desert species confirmed theoretically expected relationships between DELTA and the ratio of intercellular to ambient CO-2 concentrations, a prerequisite to applying DELTA-values to water-use efficiency. Seasonal changes in DELTA-values were measured in 15 desert species over the course of two growing seasons in western Arizona (USA). Two major patterns appeared: in leaves of some species there were 2-3% changes during the year, whereas in others the changes in isotopic composition were less than 1 permill . When carbon isotope analyses are applied to water-use efficiency extrapolations, it has been assumed that leaf temperatures are equivalent among the species being compared. An error analysis of this assumption indicated a near-linear relationship between leaf temperature differential and the difference in isotopic composition necessary to distinguish differences in water-use efficiency. When leaf temperature differentials between plants were ltoreq 2 cntdot 5 degree C, a 1 permill difference in DELTA was sufficient to rank plants correctly and unambiguously with respect to water-use efficiency, indicating the extent to which changes in intercellular CO-2 can offset possible differences in the evaporative gradient among plants when calculating water-use efficiencies. As leaf DELTA-values among different species often range 4% on a given sampling date, carbon isotope discrimination appears to be a feasible approach for ranking relative water-use efficiency differences among arid-land plants.6) !0)Sandquist, Darren R. Ehleringer, James R. 1995f_Carbon isotope discrimination in the C-4 shrub Atriplex confertifolia along a salinity gradientGreat Basin Naturalist552o135-141aCarbon isotope discrimination (DELTA) was measured for leaves of Atriplex confertifolia along a salinity gradient in northern Utah. Over this gradient, the variation of DELTA values was high for a C-4 species, and the DELTA values were positively correlated with salinity in both years of the study. Of the possible explanations for this pattern, the DELTA results are consistent with the notion that salinity induces an increase in the bundle sheath leakiness of these C-4 plants.hSchulze, E. D. Mooney, H. A. Sala, O. E. Jobbagy, E. Buchmann, N. Bauer, G. Canadell, J. Jackson, R. B. Loreti, J. Oesterheld, M. Ehleringer, J. R.f 1996d^Rooting depth, water availability, and vegetation cover along an aridity gradient in Patagonia Oecologia 108.3503-511Above- and belowground biomass distribution, isotopic composition of soil and xylem water, and carbon isotope ratios were studied along an aridity gradient in Patagonia (44-45 degree S). Sites, ranging from those with Nothofagus forest with high annual rainfall (770 mm) to Nothofagus scrub (520 mm), Festuca (290 mm) and Stipa (160 mm) grasslands and into desert vegetation (125 mm), were chosen to test whether rooting depth compensates for low rainfall. Along this gradient, both mean above- and belowground biomass and leaf area index decreased, but average carbon isotope ratios of sun leaves remained constant (at -27 permill ), indicating no major differences in the ratio of assimilation to stomatal conductance at the time of leaf growth. The depth of the soil horizon that contained 90% of the root biomass was similar for forests and grasslands (about 0.80-0.50 m), but was shallower in the desert (0.30 m). In all habitats, roots reached water-saturated soils or ground water at 2-3 m depth. The depth profile of oxygen and hydrogen isotope ratios of soil water corresponded inversely to volumetric soil water contents and showed distinct patterns throughout the soil profile due to evaporation, water uptake and rainfall events of the past year. The isotope ratios of soil water indicated that high soil moisture at 2-3 m soil depth had originated from rainy periods earlier in the season or even from past rainy seasons. Hydrogen and oxygen isotope ratios of xylem water revealed that all plants used water from recent rain events in the topsoil and not from water-saturated soils at greater depth. However, this study cannot explain the vegetation zonation along the transect on the basis of water supply to the existing plant cover. Although water was accessible to roots in deeper soil layers in all habitats, as demonstrated by high soil moisture, earlier rain events were not fully utilized by the current plant cover during summer drought. The role of seedling establishment in determining species composition and vegetation type, and the indirect effect of seedling establishment on the use of water by fully developed plant cover, are discussed in relation to climate change and vegetation modelling.\VHeritability of carbon isotope discrimination in Gutierrezia microcephala (Asteraceae)LESchuster, W. S. F. Phillips, S. L. Sandquist, D. R. Ehleringer, J. R. American Journal of Botany792m216-221s 1992D=The carbon isotope composition (13C/12C) of C-3 plant tissues provides a long-term, integrated measure of photosynthetic metabolism. Quantitative genetic methods were used to study the inheritance of carbon isotope composition and several morphological characters in Guiterrezia microcephala, a short-liver desert perennial. Open-pollinated seed was collected from a population located near Lee's Ferry, Arizona, in an area that was disturbed approximately 20 years ago. Seeds were germinated and seedlings grown in a common greenhouse environment. Carbon isotope discrimination (DELTA) and all morphological characters varied significantly among maternal families. Heritability of DELTA was estimated as 81% in the greenhouse and a 92% using the regression of maily mean DELTA on parent DELTA from field samples. Using both field and greenhouse data, we estimated a lower bound for heritability in nature of 54%. Offspring size and biomass both showed significant, positive correlations with DELTA. The corresponding genetic correlations were similar in magnitude and direction, but did not differ significantly from zero. These results imply the existence of heritable differences in physiology associated with carbon assimilation and water loss within populations, and the potential for microevolutionary change through natural selection.i ^\LZTSchuster, William S. F. Sandquist, Darren R. Phillips, Susan L. Ehleringer, James R. 1992rkComparisons of carbon isotope discrimination in populations of aridland plant species differing in lifespan Oecologia913332-337F@Carbon isotope discrimination (DELTA) was compared between populations of dominant perennial plant species, differing in life expectancy, in two deserts with contrasting vegetation types. In both deserts, plants of the shorter-lived species showed significantly higher DELTA and greater intrapopulation variance in this character compared to the long-lived species. These results indicate underlying differences in gas-exchange physiology, and suggest a positive correlation between water-use efficiency and lifespan in desert plants. Differences in variance for this character may reflect greater microenvironmental variation experienced by shorter-lived plants and/or different forms of selection acting on water-use traits. Spatial distributions were significantly clustered for the shorter-lived species and significantly uniform for the long-lived species, indicating that competition has been important the development of the long-lived populations. The long-lived Larrea tridentata showed a significant, negative correlation between DELTA and Thiessen polygon area, suggesting a positive relationship between water use efficiency and longevity within this species. This relationship was weakly supported in the other warm desert species, Encelia farinosa, but was not observed within populations of the cold desert species, Gutierrezia microcephala and Coleogyne ramosissima. These results suggest that DELTA reflects key aspects of plant metabolism related to lifespan; these differences may ultimately influence interactions among desert plants and the structure of desert plant communities.ZTSchuster, William S. F. Sandquist, Darren R. Phillips, Susan L. Ehleringer, James R. 1994hbHigh levels of genetic variation in populations of four dominant aridland plant species in Arizona"Journal of Arid Environments272 159-167dNGIntrapopulation genetic variation in four dominant, perennial plant species from the deserts of southwestern North America was assessed and compared using isozyme techniques. In each of two localities, a warm desert and a cold desert environment, one population of the most common long-lived perennial was compared with a nearby population of a dominant, comparatively short-lived perennial. The warm desert species were Encelia farinosa and Larrea tridentata while Gutierrezia microcephala and Coleogyne ramosissima were examined at the cold desert site. All samples were electrophoresed and stained for 18 enzyme systems. Mean values for these four species were 0.173 for gene diversity, 66.9% for polymorphism, and 2.02 for number of alleles per locus. These values are significantly greater than published means from a recent survey of the plant isozyme literature, and are comparable to or higher than levels of genetic variation in other widespread plant species. Differences in level of variation among the species were not significant, thus failing to indicate any relationship between life expectancy and genetic variation. These results demonstrate that aridland plant populations can harbor high levels of genetic variation, and suggest that environmental heterogeneity may be important in the development and maintenance of this diversity.t\USchuster, William S. F. Sandquist, Darren, R. Phillips, Susan L. Ehleringer, James R.t 1994hbHigh levels of genetic variation in populations of four dominant aridland plant species in Arizona"Journal of Arid Environments272159-167 NGIntrapopulation genetic variation in four dominant, perennial plant species from the deserts of southwestern North America was assessed and compared using isozyme techniques. In each of two localities, a warm desert and a cold desert environment, one population of the most common long-lived perennial was compared with a nearby population of a dominant, comparatively short-lived perennial. The warm desert species were Encelia farinosa and Larrea tridentata while Gutierrezia microcephala and Coleogyne ramosissima were examined at the cold desert site. All samples were electrophoresed and stained for 18 enzyme systems. Mean values for these four species were 0.173 for gene diversity, 66.9% for polymorphism, and 2.02 for number of alleles per locus. These values are significantly greater than published means from a recent survey of the plant isozyme literature, and are comparable to or higher than levels of genetic variation in other widespread plant species. Differences in level of variation among the species were not significant, thus failing to indicate any relationship between life expectancy and genetic variation. These results demonstrate that aridland plant populations can harbor high levels of genetic variation, and suggest that environmental heterogeneity may be important in the development and maintenance of this diversity.s.(Schwinning, Susanne Ehleringer, James R. 2001RLWater use trade-offs and optimal adaptations to pulse-driven arid ecosystemsJournal of Ecology893t464-480 $1 We introduce a hydraulic soil-plant model with water uptake from two soil layers; one a pulse-dominated shallow soil layer, the other a deeper soil layer with continuous, but generally less than saturated soil moisture. Water uptake is linked to photosynthetic carbon assimilation through a photosynthesis model for C3 plants. 2 A genetic algorithm is used to identify character suites that maximize photosynthetic carbon gain for plants that experience a particular soil moisture pattern. The character suites include allocation fraction to stem, leaves and shallow root, stem capacitance and stem water storage capacity, maximal leaf conductance and sensitivity of leaf conductance to plant water potential, and a critical soil water potential at which shallow roots cease to transfer water. 3 We find that if pulse water is a more important water source than deeper soil water in the environment, optimal phenotypes lean towards adaptations that maximize pulse water use (small root:shoot ratio, predominantly shallow root system, high leaf conductance with high stomatal sensitivity to plant water status). If deeper soil water is more important, phenotypes lean towards adaptations that maximize deeper soil water use (large root:shoot ratio, predominantly deep root system, lower leaf conductance with low stomatal sensitivity). Stem succulence is adaptive only when deeper soil water is unavailable. 4 From among the continuum of derived phenotypes, four phenotypes are selected that resemble the character suites of winter annuals, drought-deciduous perennials, evergreen perennials and stem succulents. Under common conditions, these phenotypes reproduce many of the responses to drought and water pulse observed in their respective life-form counterparts. The comparison also highlights the differences in plant life-form sensitivity to summer and winter drought conditions. 5 Based on these results, we discuss the possible role of annual precipitation patterns in shaping plant adaptations and determining the plant composition of arid and semi-arid environments.i > 3d-4.Lack of nitrogen cycling in the Atacama DesertXREhleringer, J. R. Mooney, H. A. Rundel, P. W. Evans, R. D. Palma, B. Delatorre, J. Nature 359 6393316-318 1992VOMesquite (Prosopis) trees growing in the rainless region of the Atacama Desert (Chile) produce leaves that abscise and accumulate on a concrete-like carbonate surface, often attaining litter depths of 45 cm. The virtual lack of surface moisture inhibits leaf decomposition, and prevents cycling of nitrogen, the mineral most often limiting plant growth. Leaves in the midpoint of a litter profile were aged to pre-bomb dates (older than 1950) and had both high nitrogen concentrations and a carbon to nitrogen ratio comparable to that of live leaves. The thick carbonate layer prevents root growth into the litter. Prosopis appear to persist by having roots that fix nitrogen in moist subsurface layers and by extracting water and other nutrients from ground water, allowing plants to persist in an ecosystem in which there is no nitrogen cycling.NGEhleringer, James R. Rundel, Philip W. Palma, Beatriz Mooney, Harold A. 1998f_Carbon isotope ratios of Atacama Desert plants reflect hyperaridity of region in northern Chile*#Revista Chilena de Historia Natural17117 79-86e^WLeaf carbon isotope ratios were measured on plants from the coastal portions of the Atacama Desert at Pan de Azucar and Paposo, Chile. Most species possessed C3 photosynthesis, although there were several CAM species, indications of some facultative CAM species, and only one C4 species. The carbon isotope ratios of the C3 plants are unusually high, even for species from and ecosystems. These observations indicated that C3 species of the Atacama Desert were characterized by very low intercellular CO2 concentration that averaged between 159 and 190 ppm, depending on the severity of the drought.e2,Ehleringer, J. R. Schwinning, S. Gebauer, R. 2000(!Water use in arid land ecosystemsf 82Malcolm C. Press Julie D. Scholes Martin G. Barker"Physiological Plant Ecology Boston Blackwell Sciencet347-365i$Evans, R. D. Ehleringer, J. R. 1993VOA break in the nitrogen cycle in arid lands? Evidence from nitrogen-15 of soils Oecologia943314-317We examined the content and isotopic composition of nitrogen within soils of a juniper woodland and found that a cryptobiotic crust composed of cyanobacteria, lichens, and mosses was the predominant source of nitrogen for this ecosystem. Disturbance of the crust has resulted in considerable spatial variability in soil nitrogen content and isotopic composition; inter-canopy soils were significantly depleted in nitrogen and had greater abundance of 15N compared to intra-canopy soils. Variations in the 15N/14N ratio for inter- and intra-canopy locations followed similar Rayleigh distillation curves, indicating that the greater 15N/14N ratios for inter-canopy soils were due to relatively greater net nitrogen loss. Coverage of cryptobiotic crusts has been reduced by anthropogenic activities during the past century, and our results suggest that destruction of the cryptobiotic crust may ultimately result in ecosystem degradation through elimination of the predominant source of nitrogen input.$Evans, R. D. Ehleringer, J. R. 19946/Water and nitrogen dynamics in an arid woodland Oecologia99 3-4233-242Arid environments are characterized by spatial and temporal variation in water and nitrogen availability. Differences in delta-15N and delta-D of four co-occurring species reveal contrasting patterns of plant resource acquisition in response to this variation. Mineralization potential and nitrogen concentration of surface soils associated with plant canopies were greater than inter-canopy locations, and values decreased with increasing depth in both locations. Mineralization potential and nitrogen concentration were both negatively correlated with soil delta-15N. The spatial variation in soil delta-15N caused corresponding changes in plant delta-15N such that plant delta-15N values were negatively correlated with nitrogen concentration of surface soils. Plants occurring on soils with relatively high nitrogen concentrations had lower delta-15N, and higher leaf nitrogen concentrations, than plants occurring on soils with relatively low nitrogen concentrations. Two general temporal patterns of water and nitrogen use were apparent. Three species (Juniperus, Pinus and Artemisia) relied on the episodic availability of water and nitrogen at the soil surface. delta-15N values did not vary through the year, while xylem pressure potentials and stem-water delta-D values fluctuated with changes in soil moisture at the soil surface. In contrast, Chrysothamnus switched to a more stable water and nitrogen source during drought. delta-15N values of Chrysothamnus increased throughout the year, while xylem pressure potentials and stem-water delta-D values remained constant. The contrasting patterns of resource acquisition have important implications for community stability following disturbance. Disturbance can cause a decrease in nitrogen concentration at the soil surface, and so plants that rely on surface water and nitrogen may be more susceptible than those that switch to more stable water and nitrogen sources at depth during drought.%& F2,Buchmann, Nina Kao Wen, Yuan Ehleringer, Jim 1997Influence of stand structure on carbon-13 of vegetation, soils, and canopy air within deciduous and evergreen forests, in Utah, United States OecologiaA 110o1109-119y D >Carbon isotope ratios (delta-13C) were studied in evergreen and deciduous forest ecosystems in semi-arid Utah (Pinus contorta, Populus tremuloides, Acer negundo and Acer grandidentatum). Measurements were taken in four to five stands of each forest ecosystem differing in overstory leaf area index (LAI) during two consecutive growing seasons. The delta-13C-leaf (and carbon isotope discrimination) of understory vegetation in the evergreen stands (LAI 1.5-2.2) did not differ among canopies with increasing LAI, whereas understory in the deciduous stands (LAI 1.5-4.5) exhibited strongly decreasing delta-13C-leaf values (increasing carbon isotope discrimination) with increasing LAI. The delta-13C values of needles and leaves at the top of the canopy were relatively constant over the entire LAI range, indicating no change in intrinsic water-use efficiency with overstory LAI. In all canopies, delta-13C-leaf decreased with decreasing height above the forest floor, primarily due to physiological changes affecting c-i/c-a ( gt 60%) and to a minor extent due to delta-13C of canopy air ( lt 40%). This intra-canopy depletion of delta-13C-leaf was lowest in the open stand (1 permill ) and greatest in the denser stands (4.5 permill ). Although overstory leaf-delta-13C did not change with canopy LAI, delta-13C of soil organic carbon increased with increasing LAI in Pinus contorta and Populus tremuloides ecosystems. In addition, delta-13C of decomposing organic carbon became increasingly enriched over time (by 1.7-2.9 permill ) for all deciduous and evergreen dry temperate forests. The delta-13C-canopy of CO-2 in canopy air varied temporally and spatially in all forest stands. Vertical canopy gradients of delta-13C-canopy, and (CO-2)-canopy were larger in the deciduous Populus tremuloides than in the evergreen Pinus contorta stands of similar LAI. In a very wet and cool year, ecosystem discrimination (DELTA-e) was similar for both deciduous Populus tremuloides (18.0 +- 0.7 permill ) and evergreen Pinus contorta (18.3 +- 0.9 permill ) stands. Gradients of delta-13C-canopy and (CO-2)-canopy were larger in denser Acer spp. stands than those in the open stand. However 13C enrichment above and photosynthetic draw-down of (CO-2)-canopy below tropospheric baseline values were larger in the open than in the dense stands, due to the presence of a vigorous understory vegetation. Seasonal patterns of the relationship delta-13C-canopy versus 1/(CO-2)-canopy were strongly influenced by precipitation and air temperature during the growing season. Estimates of DELTA-e for Acer spp. did not show a significant effect of stand structure, and averaged 16.8 +- 0.5 permill in 1933 and 17.4 +- 0.7 permill in 1994. However, DELTA-e, varied seasonally with small fluctuations for the open stand (2 permill ), but more pronounced changes for the dense stand (5 permill ).\VCanadell, J. Jackson, R. B. Ehleringer, J. R. Mooney, H. A. Sala, O. E. Schulze, E. D. 1996D=Maximum rooting depth of vegetation types at the global scale Oecologia 108-4583-594aThe depth at which plants are able to grow roots has important implications for the whole ecosystem hydrological balance, as well as for carbon and nutrient cycling. Here we summarize what we know about the maximum rooting depth of species belonging to the major terrestrial biomes. We found 290 observations of maximum rooting depth in the literature which covered 253 woody and herbaceous species. Maximum rooting depth ranged from 0.3 m for some tundra species to 68 m for Boscia albitrunca in the central Kalahari; 194 species had roots at least 2 m deep, 50 species had roots at a depth of 5 m or more, and 22 species had roots as deep as 10 m or more. The average for the globe was 4.6+-0.5 m. Maximum rooting depth by biome was 2.0+-0.3 m for boreal forest, 2.1+-0.2 m for cropland, 9.5+-2.4 m for desert, 5.2+-0.8 m for sclerophyllous shrubland and forest, 3.9+-0.4 m for temperate coniferous forest, 2.9+-0.2 m for temperate deciduous forest, 2.6+-0.2 m for temperate grassland, 3.7+-0.5 m for tropical deciduous forest, 7.3+-2.8 m for tropical evergreen forest, 15.0+-5.4 m for tropical grassland/savanna, and 0.5+-0.1 m for tundra. Grouping all the species across biomes (except croplands) by three basic functional groups: trees, shrubs, and herbaceous plants, the maximum rooting depth was 7.0+-1.2 m for trees, 5.1+-0.8 m for shrubs, and 2.6+-0.1 m for herbaceous plants. These data show that deep root habits are quite common in woody and herbaceous species across most of the terrestrial biomes, far deeper than the traditional view has held up to now. This finding has important implications for a better understanding of ecosystem function and its application in developing ecosystem models.p+ JPJBrooks, J. Renee Flanagan, Lawrence B. Buchmann, Nina Ehleringer, James R. 1997TNCarbon isotope composition of boreal plants: Functional grouping of life forms Oecologia 1103301-311i j cWe tested the hypothesis that life forms (trees, shrubs, forbs, and mosses; deciduous or evergreen) can be used to group plants with similar physiological characteristics. Carbon isotope ratios (delta-13C) and carbon isotope discrimination (A) were used as functional characteristics because delta-13C and DELTA integrate information about CO-2 and water fluxes, and so are useful in global change and scaling studies. We examined delta-13C values of the dominant species in three boreal forest ecosystems: wet Picea mariana stands, mesic Populus tremuloides stands, and dry Pinus banksiana stands. Life form groups explained a significant fraction of the variation in leaf carbon isotope composition; seven life-form categories explained 50% of the variation in delta-13C and 42% of the variation in DELTA and 52% of the variance not due to intraspecific genetic differences (n=335). The life forms were ranked in the following order based on their values: evergreen trees lt deciduous trees=evergreen and deciduous shrubs=evergreen forbs lt deciduous forbs=mosses. This ranking of the life forms differed between deciduous (Populus) and evergreen (Pinus and Picea) ecosystems. Furthermore, life forms in the Populus ecosystem had higher discrimination values than life forms in the dry Pinus ecosystem; the Picea ecosystem had intermediate DELTA values. These correlations between DELTA and life form were related to differences in plant stature and leaf longevity. Shorter plants had lower A values than taller plants, resulting from reduced light intensity at lower levels in the forest. After height differences were accounted for, deciduous leaves had higher discrimination values than evergreen leaves, indicating that deciduous leaves maintained higher ratios of intracellular to ambient CO-2 (c-i/c-a) than did evergreen leaves in a similar environment within these boreal ecosystems. We found the same pattern of carbon isotope discrimination in a year with above-average precipitation as in a year with below-average precipitation, indicating that environmental fluctuations did not affect the ranking of life forms. Furthermore, plants from sites near the northern and southern boundaries of the boreal forest had similar patterns of discrimination. We concluded that life forms are robust indicators of functional groups that are related to carbon and water fluxes within boreal ecosystems.X,JXQWater relations of stem succulent trees in north-central Baja California (Mexico)RKNilsen, E. T. Sharifi, M. R. Rundel, P. W. Forseth, I. N. Ehleringer, J. R. Oecologia823299-303 1990f_Water relations of several stem succulent trees were measured in north-central Baja California in comparisons to other growth forms in the same habitat. Our research concentrated on three stem succulent species (Idria collumnaris, Pachycormus discolor and Bursera microphylla) each with a different succulent stem morphology. The stem succulent trees had 1 5 to 4 kg H-2O/m-3 of trunk while the other trees and shrubs in the same habitat had 0.6 to 0.8 kg H-2O/m-3. The diurnal and seasonal variation in leaf water potential was small for the stem succulent species in comparison to deciduous and evergreen species as a consequence of the stem-water, buffering capacity. In addition, the leaf conductance of the stem succulent species was low (60 mmol m-2 s-1) and yet, the leaf conductance decreased through the day similar to adjacent evergreen and deciduous species. The leaves of the stem succulent trees lost turgor at low saturated water deficits (0.06 to 0.14), had comparatively high osmotic potentials, and high values of leastic modulus in comparison to adjacent evergreen and deciduous species. The stem acts as an important buffering mechanisms allowing for the maintenance of leaf turgor in these stem succulent trees. The low transporiation rates of the stem succulent trees may be a mechanism to minimize leaf saturated water deficit and extend leaf longevity.s.'Phillips, Susan L. Ehleringer, James R.h 1995|Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quercus gambelii Nutt) Treesn9e4 214-219cWinter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (delta-D) and oxygen (delta-18O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem delta-D and delta-18O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. delta-D values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.oleSandquist, Darren R. Schuster, William S. F. Donovan, Lisa A. Phillips, Susan L. Ehleringer, James R. 1993xqDifferences in carbon isotope discrimination between seedlings and adults of southwestern desert perennial plants1Southwestern NaturalistG383s212-217l,%Carbon isotope discrimination (DELTA) is associated with long-term water-use efficiency (ratio of photosynthesis to transpiration), and varies within and among desert plant species. The variation of DELTA within aridland perennial plants may be related to life-history class attributes, while differences in DELTA between long- and shorter-lived species may be based on life-form dependent tradeoffs of growth and survival. We examined these patterns in two desert environments and asked if life-history class and life-form related differences in DELTA occurred in Coleogyne ramosissima, Larrea tridentata, Encelia farinosa, ambrosia dumosa and Gutierrezia microcephala. The mean DELTA values of seedlings were greater than those of adults for all species except G. microcephala. The difference between life-history classes may represent a shift from low water-use efficiency (high DELTA) during establishment to high water-use efficiency (low DELTA) as an adult. Our results suggest that this shift is based on genetically- and/or environmentally-induced ontogenetic changes in plant metabolism. Such changes may be beneficial if a high DELTA and low DELTA are favored at different developmental stages. Interspecific comparisons showed that adult long-lived plants had lower DELTA values than adults of shorter-lived species in the same community, but for seedlings the same pattern was found for only one pair of species. Thus the previously shown trend for long-lived species to have lower DELTA values than shorter-lived species may depend on the stage of development.e#.'Williams, David G. Ehleringer, James R. 1996`ZCarbon isotope discrimination in three semi-arid woodland species along a monsoon gradient Oecologia 1064455-4602,Leaf carbon isotope discrimination (DELTA) was measured for three dominant, semi-arid woodland species along a summer monsoon gradient in the southwestern United States over a 2-year period. We tested the hypothesis that decreased humidity levels during the growing season along this gradient resulted in lower leaf DELTA values. Sites of similar elevation along the transect were selected and the, range in monsoon contribution to overall annual precipitation varied from 18 to 58%, while total annual precipitation differed by a maximum of only 25% across this gradient. Leaf DELTA values in Quercus gambelii were negatively correlated with omega, a seasonally weighted estimate of the evaporative humidity gradient, suggesting that stomatal conductance declined as transpiration potential increased. For two other trees that co-occurred along this gradient, Pinus edulis and Juniperus osteosperma, DELTA remained relatively constant despite large variation in omega. These woodland species represent the full spectrum of responses of carbon isotope discrimination to increases in evaporative potential; that of decline where c-i/c-a (ratio of internal to ambient CO-2 concentration) and presumably stomatal conductance decrease, and that of constancy where whole plant internal adjustments allow c-i/c-a to remain stable..'Williams, David G. Ehleringer, James R. 2000b[Intra- and interspecific variation for summer precipitation use in pinyon-juniper woodlandsEcological Monographs704517-537-In the arid southwest of North America, winter precipitation penetrates to deep soil layers, whereas summer "monsoon" precipitation generally wets only surface layers. Use of these spatially separated water sources was determined for three dominant tree species of the pinyon-juniper ecosystem at six sites along a gradient of increasing summer precipitation in Utah and Arizona. Mean summer precipitation ranged from 79 to 286 mm, or from 18% to 60% of the annual total across the gradient. We predicted that, along this summer rainfall gradient, populations of dominant tree species would exhibit a clinal off-on response for use of water from upper soil layers, responding at particular threshold levels of summer precipitation input. This prediction was largely supported by our observations of tree water source use over a two-year period and from irrigation experiments. Hydrogen and oxygen stable isotope ratios (deltaD and delta18O) of tree xylem water were compared to that of precipitation, groundwater, and deep and shallow soil water to distinguish among possible tree water sources. deltaD-delta18O relationships and seasonal xylem water potential changes revealed that trees of this ecosystem used a mixture of soil water and recent precipitation, but not groundwater. During the monsoon period, a large proportion of xylem water in Pinus edulis and Juniperus osteosperma was from monsoon precipitation, but use of this precipitation declined sharply with decreasing summer rain input at sites near the regional monsoon boundary in Utah. Quercus gambelii at most sites along the gradient used only deep soil water even following substantial inputs of summer rain. Populations of Quercus at sites with the highest average summer precipitation input, however, predominantly used water in upper soil layers from recent summer rain events. Soil temperature correlated with patterns of summer precipitation use across the gradient; high soil temperatures north of the monsoon boundary may have inhibited surface root activity for some or all of the three tree species. Irrigation experiments with deuterium-labeled water revealed that Quercus gambelii in northern Arizona and southern Utah did not use water from surface layers. In contrast, Juniperus osteosperma at these sites responded significantly to the irrigations: between 37% and 41% of xylem water originated from irrigations that wetted only the top 30 cm of soil. Responses by Pinus edulis to these irrigations were variable; uptake of labeled water by this species was greater in September at the end of the summer than during the hot midsummer period. Inactivity of Pinus roots in midsummer supports the hypothesis that root activity in this species is sensitive to soil temperature. Seasonal patterns of leaf gas exchange and plant water potential corresponded to the seasonality of rainfall at different sites. However, no correlation between a species' ability to use summer rainfall and its tolerance to water deficits at the leaf level was found. Midday stomatal conductance (gs) for Pinus needles approached zero at predawn water potentials near -2 MPa, whereas gs in Quercus and Juniperus declined to zero at -2.8 and -3.7 MPa, respectively. The relationship between photosynthesis (A) and gs was similar among the three species, although Quercus maintained higher overall rates of gas exchange and tended to operate higher on the A/gs curve than the two conifers. At sites in eastern Arizona where Quercus fully used moisture from summer rains, leaf gas exchange characteristics were similar to those of Pinus and Juniperus.a