Isotopic composition of sheep wool records seasonality of climate and diet.

RATIONALE: Hair keratin is a very important material in ecological and archaeological studies because it grows continuously, can be obtained non-invasively, does not require extensive processing prior to analysis and can be found in archaeological sites. Only a few studies have examined seasonal variations in hair isotope values, and there is no published dataset examining the isotope variability recorded in the keratinous tissues of stationary (i.e., non-migrating) domestic mammals. METHODS: Thirty-six Irish sheep were sampled in eight farms every three months between September 2006 and June 2007. A shearing strategy was adopted to sample only the most recently grown wool in order to represent an average of the summer, autumn, winter and spring conditions. The stable isotope ratios of the ground samples were measured using two different stable isotope mass spectrometers operated in dual-inlet (C, N) and continuous-flow (O, H) mode. RESULTS: Wool O isotope ratios are a good proxy for seasonal variability in climate and can be used to anchor a chronology independently of other isotope records (C, N) that are influenced by diet or physiology. By contrast, interpretation of seasonal variations in hair H isotope composition in terms of climate is more complex probably due to the influence of dietary H. The C and N isotope values of grass-fed animals varied seasonally, probably reflecting the annual cycle of seasonal variation in grass isotope values. The highest δ(13) C values were measured in summer-grown wool, while the highest δ(15) N values were measured in winter-grown wool. Supplementation of the sheep diet with concentrates was detected easily and was marked by an increase in δ(13) C values and a decrease in δ(15) N values in winter-grown wool. CONCLUSIONS: The present study demonstrates that time-resolved sampling and stable isotope ratio analysis of sheep wool can be used to identify short-term changes in diet and climate and therefore offer a tool to examine a wide variety of present and past husbandry practices.

Ecophysiology of riparian cottonwood and willow before, during, and after two years of soil water removal.

Riparian cottonwood/willow forest assemblages are highly valued in the southwestern United States for their wildlife habitat, biodiversity, and watershed protection. Yet these forests are under considerable threat from climate change impacts on water resources and land-use activities to support human enterprise. Stream diversions, groundwater pumping, and extended drought have resulted in the decline of cottonwood/willow forests along many riparian corridors in the Southwest and, in many cases, the replacement of these forests with less desirable invasive shrubs and trees. Nevertheless, ecophysiological responses of cottonwood and willow, along with associated ecohydrological feedbacks of soil water depletion, are not well understood. Ecophysiological processes of mature Fremont cottonwood and coyote willow stands were examined over four consecutive growing seasons (2004-2007) near Salt Lake City, Utah, USA. The tree stands occurred near the inlet of a reservoir that was drained in the spring of 2005 and remained empty until mid-summer of 2006, effectively removing the primary water source for most of two growing seasons. Stem sap flux density (Js) in cottonwood was highly correlated with volumetric soil moisture (theta) in the upper 60 cm and decreased sevenfold as soil moisture dropped from 12% to 7% after the reservoir was drained. Conversely, Js in willow was marginally correlated with 0 and decreased by only 25% during the same period. Opposite patterns emerged during the following growing season: willow had a lower whole-plant conductance (kt) in June and higher leaf carbon isotope ratios (delta13C) than cottonwood in August, whereas k(t) and delta13C were otherwise similar between species. Water relations in both species recovered quickly from soil water depletion, with the exception that sapwood area to stem area (As:Ast) was significantly lower in both species after the 2007 growing season compared to 2004. Results suggest that cottonwood has a greater sensitivity to interannual reductions in water availability, while willow is more sensitive to longer periods of soil water depletion. These data shed light on the linkage between soil water deficits and ecophysiological processes of threatened riparian forests given potential land-use and long-term drought impacts on freshwater resources.

Stable isotope analysis of modern human hair collected from Asia (China, India, Mongolia, and Pakistan).

We report isotopic data (delta(2)H, delta(18)O n = 196; delta(13)C, delta(15)N n = 142; delta(34)S n = 85) from human hair and drinking water (delta(2)H, delta(18)O n = 67) collected across China, India, Mongolia, and Pakistan. Hair isotope ratios reflected the large environmental isotopic gradients and dietary differences. Geographic information was recorded in H and O and to a lesser extent, S isotopes. H and O data were entered into a recently developed model describing the relationship between the H and O isotope composition of human hair and drinking water in modern USA and pre-globalized populations. This has anthropological and forensic applications including reconstructing environment and diet in modern and ancient human hair. However, it has not been applied to a modern population outside of the USA, where we expect different diet. Relationships between H and O isotope ratios in drinking water and hair of modern human populations in Asia were different to both modern USA and pre-globalized populations. However, the Asian dataset was closer to the modern USA than to pre-globalized populations. Model parameters suggested slightly higher consumption of locally produced foods in our sampled population than modern USA residents, but lower than pre-globalized populations. The degree of in vivo amino acid synthesis was comparable to both the modern USA and pre-globalized populations. C isotope ratios reflected the predominantly C(3)-based regional agriculture and C(4) consumption in northern China. C, N, and S isotope ratios supported marine food consumption in some coastal locales. N isotope ratios suggested a relatively low consumption of animal-derived products compared to western populations.

Gender-specific patterns of aboveground allocation, canopy conductance and water use in a dominant riparian tree species: Acer negundo.

Acer negundo Sarg. (box elder) is a dioecious tree species that dominates riparian systems at mid elevations throughout the southwest and Intermountain West of the United States. Previous studies have shown that female A. negundo trees occur at higher frequencies along stream margins, whereas males occur at higher frequencies in drier microsites. To better understand the adaptive significance of sex ratio biases and their impact on the ecohydrology of riparian ecosystems, we examined whole-plant water relations and hydraulic properties of mature male and female A. negundo trees occurring within 1 m of a perennial stream channel. We hypothesized that (1) females would have significantly greater canopy water fluxes than males (particularly during periods of seed production: May-June), and (2) xylem in females is more hydraulically efficient but more vulnerable to cavitation than xylem in males. Mean sap flux density (J(s)) during the early growing season (May and June) was 43% higher in female trees than in male trees (n = 6 and 7 trees respectively, P < 0.0001). Mean J(s) in July and August remained 17% higher in females than in males (P = 0.0009). Mean canopy stomatal conductance per unit leaf area (g(s,leaf)) in May and June was on average 140% higher in females than in males (P < 0.0001). Mean g(s,leaf) in July and August remained 69% higher in female trees than in male trees (P < 0.0001). Canopy stomatal conductance scaled to basal area was 90 and 31% higher in females relative to males during May-June and July-August, respectively (P < 0.0001 during both periods). Conversely, there were no apparent differences in either branch hydraulic conductance or branch xylem cavitation vulnerability between genders. These results improve our capacity to describe the adaptive forces that shape the spatial distribution of male and female trees in dioecious species, and their consequences for ecohydrological processes in riparian ecosystems.

Transpiration and hydraulic strategies in a piñon-juniper woodland.

Anthropogenic climate change is likely to alter the patterns of moisture availability globally. The consequences of these changes on species distributions and ecosystem function are largely unknown, but possibly predictable based on key ecophysiological differences among currently coexisting species. In this study, we examined the environmental and biological controls on transpiration from a piñon-juniper (Pinus edulis-Juniperus osteosperma) woodland in southern Utah, USA. The potential for climate-change-associated shifts in moisture inputs could play a critical role in influencing the relative vulnerabilities of piñons and junipers to drought and affecting management decisions regarding the persistence of this dominant landscape type in the Intermountain West. We aimed to assess the sensitivity of this woodland to seasonal variations in moisture and to mechanistically explain the hydraulic strategies of P. edulis and J. osteosperma through the use of a hydraulic transport model. Transpiration from the woodland was highly sensitive to variations in seasonal moisture inputs. There were two distinct seasonal pulses of transpiration: a reliable spring pulse supplied by winter-derived precipitation, and a highly variable summer pulse supplied by monsoonal precipitation. Transpiration of P. edulis and J. osteosperma was well predicted by a mechanistic hydraulic transport model (R2 = 0.83 and 0.92, respectively). Our hydraulic model indicated that isohydric regulation of water potential in P. edulis minimized xylem cavitation during drought, which facilitated drought recovery (94% of pre-drought water uptake) but came at the cost of cessation of gas exchange for potentially extended periods. In contrast, the anisohydric J. osteosperma was able to maintain gas exchange at lower water potentials than P. edulis but experienced greater cavitation over the drought and showed a lesser degree of post-drought recovery (55% of pre-drought uptake). As a result, these species should be differentially affected by shifts in the frequency, duration, and intensity of drought. Our results highlight the sensitivity of this woodland type to potential climate-change-associated shifts in seasonal moisture patterns and demonstrate the utility of mechanistic hydraulic models in explaining differential responses of coexisting species to drought.

Differential summer water use by Pinus edulis and Juniperus osteosperma reflects contrasting hydraulic characteristics.

Previous studies of pinyon-juniper woodlands show that Pinus edulis Engelm. makes better use of soil water from summer precipitation pulses than does co-occurring Juniperus osteosperma (Torr.) Little. To investigate the basis of this difference, we examined seasonal variation in cavitation and hydraulic conductance. Pinus edulis remained isohydric over the growing season. Minimum water potentials never fell below -2.3 MPa, and the extent of xylem cavitation remained near constant during the dry season. In contrast, J. osteosperma was anisohydric, reaching water potentials as low as -6.9 MPa, and experiencing progressively greater xylem cavitation as the dry season progressed despite having more cavitation-resistant xylem than P. edulis. We conducted an irrigation experiment to observe the responses of the study species to a summer pulse of water. Although sap flow increased in both species in response to the 25-mm irrigation pulse, only J. osteosperma responded to the 10-mm pulse. This was inconsistent with the response of P. edulis to light rain events and may have been due to a difference in the distribution of irrigation water and rain water between the under- and between-canopy areas. Whole-plant conductance increased following the 25-mm irrigation in P. edulis but remained constant in J. osteosperma. We hypothesized that this difference was caused, in part, by differential refilling of embolized xylem. Area specific hydraulic conductivity was 66% higher in roots of irrigated P. edulis trees relative to roots of control trees 3 days after the 25-mm irrigation (t = 2.14, P = 0.02, df = 16). There was no change in hydraulic conductivity of the roots of J. osteosperma or in the stems of either species. Our results indicate that the response to an irrigation pulse in P. edulis depended on cavitation avoidance in stems and the reversal of cavitation in roots, resulting in increased whole-plant conductance and water uptake. In contrast, J. osteosperma failed to exploit light summer rain events but was able to extract deep soil water at low water potentials.

Population structure, physiology and ecohydrological impacts of dioecious riparian tree species of western North America.

The global water cycle is intimately linked to vegetation structure and function. Nowhere is this more apparent than in the arid west where riparian forests serve as ribbons of productivity in otherwise mostly unproductive landscapes. Dioecy is common among tree species that make up western North American riparian forests. There are intrinsic physiological differences between male and female dioecious riparian trees that may influence population structure (i.e., the ratio of male to female trees) and impact ecohydrology at large scales. In this paper, we review the current literature on sex ratio patterns and physiology of dioecious riparian tree species. Then develop a conceptual framework of the mechanisms that underlie population structure of dominant riparian tree species. Finally, we identify linkages between population structure and ecohydrological processes such as evapotranspiration and streamflow. A more thorough understanding of the mechanisms that underlie population structure of dominant riparian tree species will enable us to better predict global change impacts on vegetation and water cycling at multiple scales.

Seasonal variations in moisture use in a piñon-juniper woodland.

In water-limited environments of the intermountain region of North America, summer precipitation may play a role in the structure and function of aridland communities and ecosystems. This study examined the potential reliance on summer precipitation of two widespread, coexisting woody species in the southwestern United States, Pinus edulis Englmn. (Colorado piñon) and Juniperus osteosperma (Torr) Little (Utah juniper). The current distributions of P. edulis and J. osteosperma are highly suggestive of different dependencies on summer rainfall. We hypothesized that P. edulis was dependent on summer precipitation, utilizing summer precipitation even during extremely dry summers, whereas J. osteosperma was not dependent, using summer precipitation only when amounts were above some minimum threshold. Using sap flux and stable isotopic methods to assess seasonal water sources and water use efficiency, we examined the response of these two species to seasonal variations in moisture at a site located near the northern limits of the North American monsoon. Both sap flux and isotopic results indicated that P. edulis was responsive to summer rain, while J. osteosperma was not. Following summer rain events, sap flux density increased in P. edulis for several days, but not in J. osteosperma. Isotopic evidence indicated that P. edulis took up summer-derived moisture to a greater extent than J. osteosperma. Values of the natural abundance stable isotope ratio of carbon of leaf soluble carbohydrates increased over the summer for P. edulis, indicative of assimilation at higher water use efficiency, but were invariant for J. osteosperma. Our results supported the hypothesis that P. edulis and J. osteosperma are differentially sensitive to summer precipitation and are discussed in the light of potential changes in the seasonality of precipitation associated with climate change.

Inferring biogenic and anthropogenic carbon dioxide sources across an urban to rural gradient.

We continuously monitored CO(2) concentrations at three locations along an urban-to-rural gradient in the Salt Lake Valley, Utah from 2004 to 2006. The results showed a range of CO(2) concentrations from daily averages exceeding 500 p.p.m. at the city center to much lower concentrations in a non-urbanized, rural region of the valley. The highest values were measured in the wintertime and under stable atmospheric conditions. At all three sites, we utilized weekly measurements of the C and O isotope composition of CO(2) for a 1-year period to evaluate the CO(2) sources underlying spatial and temporal variability in CO(2) concentrations. The results of an inverse analysis of CO(2) sources and the O isotope composition of ecosystem respiration (delta(18) O (R)) showed large contributions (>50%) of natural gas combustion to atmospheric CO(2) in the wintertime, particularly at the city center, and large contributions (>60%) of biogenic respiration to atmospheric CO(2) during the growing season, particularly at the rural site. delta(18) O (R) was most enriched at the rural site and more isotopically depleted at the urban sites due to the effects of irrigation on ecosystem water pools at the urban sites. The results also suggested differences in the role of leaf versus soil respiration between the two urban sites, with seasonal variation in the contribution of leaf respiration at a residential site and relatively constant contributions of leaf respiration at the city center. These results illustrate that spatial and temporal patterns of urban CO(2) concentrations and isotopic composition can be used to infer patterns of energy use by urban residents as well as plant and soil processes in urban areas.

Turnover of stable carbon isotopes in the muscle, liver, and breath CO2 of alpacas (Lama pacos).

Stable carbon isotope analysis of animal liver and muscle has become a widespread tool for investigating dietary ecology. Nonetheless, stable carbon isotope turnover of these tissues has not been studied in large mammals except with isotopically labelled tracer methodologies, which do not produce carbon half-lives analogous to those derived from naturalistic diet-switch experiments. To address this gap, we studied turnover of carbon isotopes in the liver, muscle, and breath CO2 of alpacas (Lama pacos) by switching them from a C3 grass diet to an isonitrogenous C4 grass diet. Breath samples as well as liver and muscle biopsies were collected and analyzed for up to 72 days to monitor the incorporation of the C4-derived carbon. The data suggest half-lives of 2.8, 37.3, and 178.7 days for alpaca breath CO2, liver, and muscle, respectively. Alpaca liver and muscle carbon half-lives are about 6 times longer than those of gerbils, which is about what would be expected given their size. In contrast, breath CO2 turnover does not scale readily with body mass. We also note that the breath CO2 and liver data are better described using a multiple-pool exponential decay model than a single-pool model.

Turnover of carbon isotopes in tail hair and breath CO2 of horses fed an isotopically varied diet.

Temporal stable isotope records derived from animal tissues are increasingly studied to determine dietary and climatic histories. Despite this, the turnover times governing rates of isotope equilibration in specific tissues following a dietary isotope change are poorly known. The dietary isotope changes recorded in the hair and blood bicarbonate of two adult horses in this study are found to be successfully described by a model having three exponential isotope pools. For horse tail hair, the carbon isotope response observed following a dietary change from a C3 to a C4 grass was consistent with a pool having a very fast turnover rate ( t1/2 approximately 0.5 days) that made up approximately 41% of the isotope signal, a pool with an intermediate turnover rate ( t1/2 approximately 4 days) that comprised approximately 15% of the isotope signal, and a pool with very slow turnover rate ( t1/2 approximately 140 days) that made up approximately 44% of the total isotope signal. The carbon isotope signature of horse blood bicarbonate, in contrast, had a different isotopic composition, with approximately 67% of the isotope signal coming from a fast turnover pool ( t1/2 0.2 days), approximately 17% from a pool with an intermediate turnover rate ( t1/2 approximately 3 days) and approximately 16% from a pool with a slow turnover rate ( t1/2 approximately 50 days). The constituent isotope pools probably correspond to one exogenous and two endogenous sources. The exogenous source equates to our fast turnover pool, and the pools with intermediate and slow turnover rates are thought to derive from the turnover of metabolically active tissues and relatively inactive tissues within the body, respectively. It seems that a greater proportion of the amino acids available for hair synthesis come from endogenous sources compared to the compounds undergoing cellular catabolism in the body. Consequently, the isotope composition of blood bicarbonate appears to be much more responsive to dietary isotope changes, whereas the amino acids in the blood exhibit considerable isotopic inertia.

Responses of Acer negundo genders to interannual differences in water availability determined from carbon isotope ratios of tree ring cellulose.

Understanding the responses of riparian trees to water availability is critical for predicting the effects of changes in precipitation on riparian ecosystems. Dioecious Acer negundo L. (box elder) is a common riparian tree that is highly sensitive to water stress. Earlier studies indicated that the genders of A. negundo respond differently to water availability, with males being more conservative in their water use than females. To assess the potential effects of changes in precipitation on the sex ratio of riparian trees, we extended earlier studies of A. negundo by analyzing responses of male and female genotypes to interannual differences in water availability in a common garden. We measured growth of tree rings and used stable carbon isotope analysis of tree ring alpha-cellulose to integrate physiological responses to annual water treatments. During dry years, male and female trees exhibited similar growth and physiological responses. However, during wet years, females exhibited higher growth rates and lower carbon isotope ratios (indicating less conservative water use) than did males. Furthermore, we found that male trees exhibited similar stomatal behavior (inferred from integrated carbon isotope ratios) whether years were wet or dry, whereas females did not exhibit a consistent response to changes in water availability. We predict that with increasing precipitation and soil water availability, the representation of females will be favored because of shifts in the competitive interactions of the genders. Such changes may affect the reproductive output of these riparian trees and may influence overall water flux from riparian ecosystems. In addition, this study demonstrates the utility of carbon isotope analysis for assessing long-term responses of tree populations to shifts in water availability.

Establishing a grassland signature in veins: 18O in the leaf water of C3 and C4 grasses.

We show that 18O evaporative enrichment of bulk leaf water in grass species can be significantly more enriched than predicted by the Craig-Gordon model, with C4 grasses considerably more enriched than C3 grasses. Our results suggest that the unanticipated 18O leaf water enrichment of grasses is attributable to the progressive evaporative enrichment along parallel veins (a function of both leaf length and interveinal distance), a pattern that does not occur in Dicotyledonous species. We propose that the differential 18O enrichment of grasses will result in distinct C18O16O biospheric signals from grassland and forest ecosystems, allowing for further partitioning of terrestrial carbon fluxes.

Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees.

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of -125/-15‰ (δD/δ18O), -48/-6‰, and -67/-7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ18O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ18O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ18O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions.

There is no temperature dependence of net biochemical fractionation of hydrogen and oxygen isotopes in tree-ring cellulose.

The isotopic composition of tree-ring cellulose was obtained over a two-year period from small diameter, riparian zone trees along an elevational transect in Big Cottonwood Canyon, Utah, USA to test for a possible temperature dependence of net biological fractionation during cellulose synthesis. The isotope ratios of stream water varied by only 3.6% and 0.2% in deltaD and delta18O, respectively, over an elevation change of 810m. The similarity in stream water and macroenvironment over the short (13km) transect produced nearly constant stem and leaf water deltaD and delta18O values. In addition, what few seasonal variations observed in the isotopic composition of source water and atmospheric water vapor or in leaf water evaporative enrichment were experienced equally by all sites along the elevational transect. The temperature at each site along the transect spanned a range of > or = 5 degrees C as calculated using the adiabatic lapse rate. Since the deltaD and delta18O values of stem and leaf water varied little for these trees over this elevation/temperature transect, any differences in tree-ring cellulose deltaD and delta18O values should have been associated with temperature effects on net biological fractionation. However, the slopes of the regressions of elevation versus the deltaD and delta18O values of tree-ring cellulose were not significantly different from zero indicating little or no temperature dependence of net biological fractionation. Therefore, cross-site climatic reconstruction studies using the isotope ratios of cellulose need not be concerned that temperatures during the growing season have influenced results.

Carbon dioxide starvation, the development of C4 ecosystems, and mammalian evolution.

The decline of atmospheric CO2 over the last 65 million years (Ma) resulted in the 'CO2-starvation' of terrestrial ecosystems and led to the widespread distribution of C4 plants, which are less sensitive to CO2 levels than are C3 plants. Global expansion of C4 biomass is recorded in the diets of mammals from Asia, Africa, North America, and South America during the interval from about 8 to 5 Ma. This was accompanied by the most significant Cenozoic faunal turnover on each of these continents, indicating that ecological changes at this time were an important factor in mammalian extinction. Further expansion of tropical C4 biomass in Africa also occurred during the last glacial interval confirming the link between atmospheric CO2 levels and C4 biomass response. Changes in fauna and flora at the end of the Miocene, and between the last glacial and interglacial, have previously been attributed to changes in aridity; however, an alternative explanation for a global expansion of C4 biomass is CO2 starvation of C3 plants when atmospheric CO2 levels dropped below a threshold significant to C3 plants. Aridity may also have been a factor in the expansion of C4 ecosystems but one that was secondary to, and perhaps because of, gradually decreasing CO2 concentrations in the atmosphere. Mammalian evolution in the late Neogene, then, may be related to the CO2 starvation of C3 ecosystems.

Ecosystem-atmosphere CO(2) exchange: interpreting signals of change using stable isotope ratios.

Changes in the concentration and stable isotope ratio of atmospheric CO(2) can be used to study variations in the net exchange of carbon dioxide in terrestrial ecosystems (net difference between total photosynthesis and respiration). Changes in the timing of seasonal fluctuations in atmospheric CO(2) concentration have suggested that net uptake of carbon dioxide has been increasing in northern latitude ecosystems in association with warmer temperatures and a lengthening of the growing season. Stable isotope techniques allow a more detailed separation of differences between ecosystem photosynthesis and respiration because these two processes have contrasting effects on both the carbon and oxygen isotope ratio of atmospheric CO(2). Future applications of stable isotope analyses include documenting and monitoring the influence of global environmental change on ecosystem CO(2) exchange at regional scales (10-1000km(2)).

Carbon and oxygen isotope ratios of ecosystem respiration along an Oregon conifer transect: preliminary observations based on small-flask sampling.

Isotope ratio analyses of atmospheric CO(2) at natural abundance have significant potential for contributing to our understanding of photosynthetic and respiration processes in forest ecosystems. Recent advances in isotope ratio mass spectrometry allow for rapid, on-line analysis of small volumes of CO(2) in air, and open new research opportunities at the ecophysiological, whole-organism, and atmospheric levels. Among the immediate applications are the carbon and oxygen isotope ratio analyses of carbon dioxide in atmospheric air. Routine analysis of carbon dioxide in air volumes of approximately 50-300 &mgr;l is accomplished by linking a commercially available, trace gas condenser and gas chromatograph to an isotope ratio mass spectrometer operated in continuous-flow mode. Samples collected in the field are stored in either gas-tight syringes or 100-ml flasks. The small sample volume required makes it possible to subsample the air in flasks for CO(2) and then to sample the remaining air volume for the analysis of the isotopic composition of either methane or nitrous oxide. Reliable delta(13)C and delta(18)O values can be obtained from samples collected and stored for 1-3 days. Longer-term storage, on the order of weeks, is possible for delta(13)C measurements without drift in the isotope ratio signal, and should also be possible for delta(18)O measurements. When linked with an infrared gas analyzer, pump and flask sampling system, it is feasible to sample CO(2) extensively in remote forest locations. The air-sampling system was used to measure the isotope ratios of atmospheric CO(2) and to conduct a regression analysis of the relationship between these two parameters. From the regression, we calculated the delta(13)C of ecosystem respiration of four coniferous ecosystems along a precipitation gradient in central Oregon. The ecosystems along the coast-to-interior Oregon (OTTER) gradient are dominated by spruce-hemlock forests at the wet, coastal sites (> 200 cm precipitation annually) to juniper woodlands (20 cm precipitation) at the interior, dry end of the transect. The delta(13)C values of ecosystem respiration along this transect differed by only 1.3 per thousand (range of -25.2 to -23.9 per thousand ) during August at the peak of the summer drought. Following autumn rains in September, the delta(13)C of ecosystem respiration in the four stands decreased; overall the difference in the carbon isotope ratio of ecosystem respiration among sites increased to 3.9 per thousand (-26.8 to -22.9 per thousand ).

Carbon Isotopic Fractionation Does Not Occur during Dark Respiration in C3 and C4 Plants.

The magnitude of possible carbon isotopic fractionation during dark respiration was investigated with isolated mesophyll cells from mature leaves of common bean (Phaseolus vulgaris L.), a C3 plant, and corn (Zea mays L.), a C4 plant. Mesophyll protoplasts were extracted from greenhouse-grown leaves and incubated in culture solutions containing different carbohydrate substrates (fructose, glucose, and sucrose) with known [delta]13C values. The CO2 produced by protoplasts after incubation in the dark was collected, purified, and analyzed for its carbon isotope ratio. From observations of the isotope ratios of the substrate and respired CO2, we calculated the carbon isotope discrimination associated with metabolism of each of these substrates. In eight of the 10 treatment combinations, the carbon isotope ratio discrimination was not significantly different from 0. In the remaining two treatment combinations, the carbon isotope ratio discrimination was 1[per mille (thousand) sign]. From these results, we conclude that there is no significant carbon isotopic discrimination during mitochondrial dark respiration when fructase, glucose, or sucrose are used as respiratory substrates.