Summary Stable carbon isotope ratios (δ13C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ13C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C3 plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO2 concentrations (ci/ca), coordination between stomatal conductance and photosynthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C4 plants, variation in bundle‐sheath leakiness to CO2 can either damp or amplify the effects of ci/ca on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night‐time CO2 fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

The natural ratio of stable carbon isotopes (δC) was compared to leaf structural and chemical characteristics in evergreen conifers in the north-central Rockies, United States. We sought a general model that would explain variation in δC across altitudinal gradients. Because variation in δC is attributed to the shifts between supply and demand for carbon dioxide within the leaf, we measured structural and chemical variables related to supply and demand. We measured stomatal density, which is related to CO supply to the chloroplasts, and leaf nitrogen content, which is related to CO demand. Leaf mass per area was measured as an intermediate between supply and demand. Models were tested on four evergreen conifers: Pseudotsuga menziesii, Abies lasiocarpa, Picea engelmannii, and Pinus contorta, which were sampled across 1800 m of altitude. We found significant variation among species in the rate of δC increase with altitude, ranging from 0.91‰ km for A. lasiocarpa to 2.68‰ km for Pinus contorta. Leaf structure and chemistry also varied with altitude: stomatal density decreased, leaf mass per area increased, but leaf nitrogen content (per unit area) was constant. The regressions on altitude were particularly robust in Pinus contorta. Variables were derived to describe the balance between supply and demand; these variables were stomata per gram of nitrogen and stomata per gram of leaf mass. Both derived variables should be positively related to internal CO supply and thus negatively related to δC. As expected, both derived variables were negatively correlated with δC. In fact, the regression on stomatal density per gram was the best fit in the study (r =0.72, P<0.0001); however, the relationships were species specific. The only general relationship observed was between δC and LMA: δC (‰)=-32.972+ 0.0173×LMA (r =0.45, P<0.0001). We conclude that species specificity of the isotopic shift indicates that evergreen conifers demonstrate varying degrees of functional plasticity across environmental gradients, while the observed convergence of δC with LMA suggests that internal resistance may be the key to understanding inter-specific isotopic variation across altitude.

The leaf area to sapwood area ratio (A :A) of trees has been hypothesized to decrease as trees become older and taller. Theory suggests that A :A must decrease to maintain leaf-specific hydraulic sufficiency as path length, gravity, and tortuosity constrain whole-plant hydraulic conductance. We tested the hypothesis that A :A declines with tree height. Whole-tree A :A was measured on 15 individuals of Douglas-fir (Pseudotsuga menziesii var. menziesii) ranging in height from 13 to 62 m (aged 20-450 years). A :A declined substantially as height increased (P=0.02). Our test of the hypothesis that A :A declines with tree height was extended using a combination of original and published data on nine species across a range of maximum heights and climates. Meta-analysis of 13 whole-tree studies revealed a consistent and significant reduction in A :A with increasing height (P<0.05). However, two species (Picea abies and Abies balsamea) exhibited an increase in A :A with height, although the reason for this is not clear. The slope of the relationship between A :A and tree height (ΔA :A/Δh) was unrelated to mean annual precipitation. Maximum potential height was positively correlated with ΔA :A/Δh. The decrease in A :A with increasing tree size that we observed in the majority of species may be a homeostatic mechanism that partially compensates for decreased hydraulic conductance as trees grow in height.

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The stable isotopes of water (hydrogen and oxygen isotopes) are of utmost interest in ecology and the geosciences. In many cases water has to be extracted directly from a matrix such as soil or plant tissue before isotopes can be analyzed by mass spectrometry. Currently, the most widely used technique for water is cryogenic vacuum extraction. We present a simple and inexpensive modification of this method and document tests conducted with soils of various grain size and tree core replicates taken on four occasions during 2010. The accuracies for sandy soils are between 0.4‰ and 3‰ over a range of 21‰ and 165‰ for δ(18)O and δ(2)H, respectively. Spiking tests with water of known isotope composition were conducted with soil and tree core samples; they indicate reliable precision after an extraction time of 15 min for sandy soils. For clayey soils and tree cores, the deviations were up to 0.63‰ and 4.7‰ for δ(18)O and δ(2)H, respectively. This indicates either that the extraction time should be extended or that mechanisms different from Rayleigh fractionation play a role. The modified protocol allows a fast and reliable extraction of large numbers of water samples from soil and plant material in preparation for stable isotope analyses.

Measurements of the ratio of deuterium to hydrogen (D/H) in stem xylem water were used to determine the relative uptake of summer precipitation by four co‐occurring plant species in southern Utah. The species compared included two trees, Juniperus osteosperma and Pinus edulis, and two shrubs, Artemisia tridentata and Chrysothamnus nauseousus. There were significant differences among species in the relative use of summer precipitation. Chrysothamnus nauseosus had stem water D/H ratios in May through August 1990 that were not significantly different from that of groundwater. In contrast, the other three species had stem water D/H ratios that were intermediate between the groundwater value and summer precipitation values, indicating that a mixture of both precipitation and groundwater was being used by these species. The two tree species generally had higher D/H values than did A. tridentata indicating a higher average uptake of summer precipitation, although the roots of J. osteosperma and P. edulis may not be as responsive to small precipitation events as A. tridentata. There was a strong negative correlation between stem water D/H ratios and predawn water potential, which suggests a relationship between plant rooting pattern and water source use. In addition, water‐use efficiency during photosynthetic gas exchange, calculated from leaf carbon isotope composition, differed among species and was strongly correlated with differences in the relative uptake of summer precipitation.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Abstract. Stable carbon isotope composition was determined on leaves of woody plants sampled along an 800-km transect on the western flank of the Rocky Mountains at altitudes ranging from 610 to 2650 m above mean sea level. Discrimination decreased by 1.20 + 0.1 1%o (mean + 1 SE) per km of altitude (n = 15, Fl 13 = 127.8, P < 0.0001). The change in discrimination was just sufficient to maintain a constant CO2 partial pressure gradient from ambient air to the intercellular spaces within the leaf for both deciduous (P = 0.60) and evergreen (P = 0.90) species. However, the CO2 gradient so maintained was significantly steeper among evergreen (11.31 ± 0.14 Pa) than among deciduous (9.64 ± 0.14 Pa) species (t = 8.4, 27 df, P < 0.0001). As a consequence, the evergreens had lower discrimination than the deciduous species at any given altitude. After the data were corrected for altitude, further analysis revealed significant differences in discrimination and in CO2 partial pressure gradient among species. Thuja plicata (western red-cedar), a scale-leaved evergreen, had lowest mean discrimination (16.67 + 0.50%oo, n = 4) and the steepest CO2 gradient from ambient to intercellular spaces (14.5 + 0.5 Pa). Larix occidentalis (western larch), a deciduous conifer, had the highest discrimination (20.95 + 0.34%o, n = 9) and the flattest CO2 gradient (8.3 ± 0.4 Pa). A simple model of water-use efficiency predicted that evergreen species would average 18 + 2% higher in water-use efficiency at any given altitude and that mean water-use efficiency would triple across a 2000-m altitude gradient. The difference between evergreen and deciduous species is attributable to variation in the CO2 partial pressure gradient, but the tripling with altitude was almost exclusively a consequence of reduced evaporative demand.

In this commentary, we summarize and build upon discussions that emerged during the workshop "Isotopebased studies of water partitioning and plant-soil interactions in forested and agricultural environments" held in San Casciano in Val di Pesa, Italy, in September 2017. Quantifying and understanding how water cycles through the Earth's critical zone is important to provide society and policymakers with the scientific background to manage water resources sustainably, especially considering the ever-increasing worldwide concern about water scarcity. Stable isotopes of hydrogen and oxygen in water have proven to be a powerful tool for tracking water fluxes in the critical zone. However, both mechanistic complexities (e.g. mixing and fractionation processes, heterogeneity of natural systems) and methodological issues (e.g. lack of standard protocols to sample specific compartments, such as soil water and xylem water) limit the application of stable water isotopes in critical-zone science. In this commentary, we examine some Published by Copernicus Publications on behalf of the European Geosciences Union. 6400 D. Penna et al.: Tracing terrestrial ecosystem water fluxes using stable isotopes of the opportunities and critical challenges of isotope-based ecohydrological applications and outline new perspectives focused on interdisciplinary research opportunities for this important tool in water and environmental science.