Changes in wood and foliar δ13C in sugar maple at Gatineau Park,Quebec, Canada

Leaf samples and tree rings formed between the mid‐1960s and mid‐1990s from sugar maple (Acer saccharum Marsh.) at Gatineau Park (45°30′ N, 75°54′ W), Quebec were analysed for δ¹³C. Leaf samples were collected at ground level (1–2 m above ground) at monthly intervals during the summer, whereas tree cores were extracted from the largest trees (d.b.h. > 30 cm) in the young deciduous forest in August 1998. Significant linear decreases in δ¹³C over time were found in foliage and tree rings, but the decrease in δ¹³C was significantly greater in foliage than in the wood. The apparent isotopic discrimination (Δ) of tree rings varied insignificantly around a mean of 18‰, whereas foliar Δ increased significantly from 19‰ in the 1960s to around 23‰ by the mid 1990s, likely as a result of an increasing canopy effect as the forest matured. Using models of carbon discrimination and Δ‐values of the tree rings, we calculate that the intrinsic water use efficiency of mature sugar maple has increased by approximately 4% over the study period.     

Environmental and species‐specific controls on δ13C and δ15N in dominant woody plants from central‐western Argentinian drylands

Spatial variation in mean annual precipitation is the principal driver of plant water and nitrogen status in drylands. The natural abundance of carbon stable isotopes (δ¹³C) in photosynthetic tissues of C3 plants is an indicator of time‐integrated behaviour of stomatal conductance; while that of nitrogen stable isotopes (δ¹⁵N) is an indicator of the main source of plant N (soil N vs. atmospheric N₂). Previous studies in drylands have documented that plant δ¹³C and δ¹⁵N values increase with decreasing mean annual precipitation due to reductions in stomatal conductance, and soil enriched in ¹⁵N, respectively. However, evidence for this comes from studies focused on stable isotopes measurements integrated at the plant community level or on dominant plants at the site level, but little effort has been made to study C and N isotope variations within a species growing along rainfall gradients. We analysed plant δ¹³C, δ¹⁵N and C/N values of three woody species having different phenological leaf traits (deciduous, perennial and aphyllous) along a regional mean annual precipitation gradient from the central‐western Argentinian drylands. Noticeably, plant δ¹³C and δ¹⁵N values in the three woody species did not increase towards sites with low precipitation or at the start of the growing season (drier period), as we expected. These results suggest that environmental factors other than mean annual precipitation may be affecting plant δ¹³C and δ¹⁵N. The short‐term environmental conditions may interact with species‐specific plant traits related to water and nitrogen use strategies and override the predictive influence of the mean annual precipitation on plant δ¹³C and δ¹⁵N widely reported in drylands.     

The legacy of enhanced N and S deposition as revealed by the combined analysis of δ13C, δ18O and δ15N in tree rings

This study aimed to evaluate the effects of long‐term repeated aerial nitrogen (N) and sulphur (S) misting over tree canopies of a Sitka spruce plantation in Scotland. We combined δ¹³C and δ¹⁸O in tree rings to evaluate the changes in CO₂ assimilation (A) and stomatal conductance (g_s) and to assess their contribution to variations in the intrinsic water‐use efficiency (WUE_i, i.e., the A/g_s ratio). Measurements of δ¹⁵N enabled shifts in the ecosystem N cycling following misting to be assessed. We found that: (i) N applications, with or without S, increased the ratio between A and g_s in favour of A, thus supporting a fertilizer effect of added N. (ii) After the treatments ceased, the trees quickly adjusted to the reductions of N deposition, but not to the reduction in S deposition, which had a negative effect on WUE_i by reducing A. This indicates that the beneficial role of N deposition may be negated in forests that previously received a high load of acid rain. (iii) δ¹⁵N in tree rings reflected the N dynamics caused by canopy retention, with the fingerprint also present in the litter, after the experiment stopped. (iv) Both our results (obtained using canopy applications) and a collection of published data (obtained using soil applications) showed that generally WUE_i increased in response to an increase of N applications, with the magnitude of the changes related to soil conditions and the availability of other nutrients. The shifts observed in δ¹⁵N in tree rings also suggest that both the quantity of the applied N and its quality, mediated by processes occurring during canopy N retention, are important determinants of the interactions between N and C cycles. Stable isotopes are useful probes to understand these processes and to put the results of short‐term experiments into context.     

Relationship between δ13C of chironomid remains and methane flux in Swedish lakes

1. Methanogenic carbon can be incorporated by methane‐oxidising bacteria, leading to a ¹³C‐depleted stable carbon isotopic composition (δ¹³C) of chironomids that feed on these microorganisms. This has been shown for the chironomid tribe Chironomini, but very little information is available about the δ¹³C of other abundant chironomid groups and the relationship between chironomid δ¹³C and methane production in lakes. 2. Methane flux was measured at the water surface of seven lakes in Sweden. Furthermore, fluxes from the sediments to the water column were measured in transects in two of the lakes. Methane fluxes were then compared with δ¹³C of chitinous chironomid remains isolated from the lake surface sediments. Several different chironomid groups were examined (Chironomini, Orthocladiinae, Tanypodinae and Tanytarsini). 3. Remains of Orthocladiinae in the seven study lakes had the highest δ¹³C values (?31.3 to ?27.0‰), most likely reflecting δ¹³C of algae and other plant‐derived organic matter. Remains of Chironomini and Tanypodinae had lower δ¹³C values (?33.2 to ?27.6‰ and ?33.6 to ?28.0‰, respectively). A significant negative correlation was observed between methane fluxes at the lake surface and δ¹³C of Chironomini (r = ?0.90, P = 0.006). Methane release from the sediments was also negatively correlated with δ¹³C of Chironomini (r = ?0.67, P = 0.025) in the transect samples obtained from two of the lakes. The remains of other chironomid taxa were only weakly or not correlated with methane fluxes measured in our study lakes (P > 0.05). 4. Selective incorporation of methane‐derived carbon can explain the observed correlations between methane fluxes and δ¹³C values of Chironomini. Remains of this group might therefore have the potential to provide information about past changes in methane availability in lakes using sediment records. However, differences in productivity, algal δ¹³C composition and the importance of allochthonous organic matter input between the studied lakes may also have influenced Chironomini δ¹³C. More detailed studies with a higher number of analysed samples and detailed measurement of δ¹³C of different ecosystem components (e.g. methane, dissolved inorganic carbon) will be necessary to further resolve the relative contribution of different carbon sources to δ¹³C of chironomid remains.     

Effects of light regime on δ13C,photosynthesis and yield of field-grown banana (Musa sp., Musaceae)*

Banana plants (Musa sp., Musaceae) were grown for 2 years in the field in 12×20m plots under irradiance levels incident upon the canopy of 100, 81, 62 and 32% of sunlight. Time-integrated parameters such as leaf δ¹³C, yield and leaf mass to area ratio were linearly correlated with irradiance level (R²>0.9). In contrast, midday CO₂ assimilation and transpiration efficiency (A/g) decreased significantly and substomatal CO₂ concentrations (c_st) increased significantly only at the lowest irradiance levels (below 81% irradiance). Diurnal gas exchange measurements indicated that the linear response of the long-term parameters may be associated with the significant variations in photosynthetic activity among the irradiance treatments observed only in the early morning hours. The linear fit between yield and irradiance level (per cent of control) had a slope of 0.82 (with apparently constant yield to biomass ratio). These results directly demonstrate the significance of variations in incident light, such as may be associated with increasing cloudiness, on productivity of tropical plants such as the banana. The importance of using time-integrated indicators in general, and the reliability of using ¹³C discrimination in particular, in evaluating the responses of plants to changes in incident irradiance is demonstrated.     

Carbon isotope compositions (δ13C) of leaf,wood and holocellulose differ among genotypes of poplar and between previous land uses in a short‐rotation biomass plantation

The efficiency of water use to produce biomass is a key trait in designing sustainable bioenergy‐devoted systems. We characterized variations in the carbon isotope composition (δ¹³C) of leaves, current year wood and holocellulose (as proxies for water use efficiency, WUE) among six poplar genotypes in a short‐rotation plantation. Values of δ¹³C_wood and δ¹³C_{holocellulose} were tightly and positively correlated, but the offset varied significantly among genotypes (0.79–1.01‰). Leaf phenology was strongly correlated with δ¹³C, and genotypes with a longer growing season showed a higher WUE. In contrast, traits related to growth and carbon uptake were poorly linked to δ¹³C. Trees growing on former pasture with higher N‐availability displayed higher δ¹³C as compared with trees growing on former cropland. The positive relationships between δ¹³C_leaf and leaf N suggested that spatial variations in WUE over the plantation were mainly driven by an N‐related effect on photosynthetic capacities. The very coherent genotype ranking obtained with δ¹³C in the different tree compartments has some practical outreach. Because WUE remains largely uncoupled from growth in poplar plantations, there is potential to identify genotypes with satisfactory growth and higher WUE.     

IMPACT OF TAXONOMY,GEOGRAPHY, AND DEPTH ON δ13C AND δ15N VARIATION IN A LARGE COLLECTION OF MACROALGAE1

The natural abundance of carbon stable isotopes (δ¹³C) of marine macrophytes has been measured in previous studies and used to analyze differences in C_i assimilation among the three macroalgal phyla, Chlorophyta, Ochrophyta, and Rhodophyta, and seagrasses, distinguishing diffusive CO₂ entry from the operation of a CO₂‐concentrating mechanisms (CCM). The work reported here further resolves the patterns of δ¹³C variation in aquatic macrophytes related to their taxonomy, geographic location (and consequently climatic conditions), and vertical zonation. Analyses of δ¹³C for 87 species are reported, including eight that have not been previously examined, belonging to taxa in the three macroalgal phyla, plus two species of seagrasses, collected at different latitudes. For one species of each phylum, analyses were also conducted through a vertical depth gradient. Representative species were used in a pH drift experiment, in order to compare the mechanism of C_i acquisition for photosynthesis with the δ¹³C subsequently determined on the same specimen. Our results suggest that the δ¹³C values were mostly determined by taxonomy. Depth effects on C stable isotope composition differed among taxa. The parallel measurements of δ¹⁵N are more difficult to interpret mechanistically; there are no robust phylogenetic and large‐scale biogeographic correlations; local factors of natural (e.g., upwellings) and anthropogenic (e.g., sewage outfall) inputs predominate in determining the macrophyte δ¹⁵N.     

Examining the response of needle carbohydrates from Siberian larch trees to climate using compound‐specific δ13C and concentration analyses

Little is known about the dynamics of concentrations and carbon isotope ratios of individual carbohydrates in leaves in response to climatic and physiological factors. Improved knowledge of the isotopic ratio in sugars will enhance our understanding of the tree ring isotope ratio and will help to decipher environmental conditions in retrospect more reliably. Carbohydrate samples from larch (Larix gmelinii) needles of two sites in the continuous permafrost zone of Siberia with differing growth conditions were analysed with the Compound‐Specific Isotope Analysis (CSIA). We compared concentrations and carbon isotope values (δ¹³C) of sucrose, fructose, glucose and pinitol combined with phenological data. The results for the variability of the needle carbohydrates show high dynamics with distinct seasonal characteristics between and within the studied years with a clear link to the climatic conditions, particularly vapour pressure deficit. Compound‐specific differences in δ¹³C values as a response to climate were detected. The δ¹³C of pinitol, which contributes up to 50% of total soluble carbohydrates, was almost invariant during the whole growing season. Our study provides the first in‐depth characterization of compound‐specific needle carbohydrate isotope variability, identifies involved mechanisms and shows the potential of such results for linking tree physiological responses to different climatic conditions.     

Relationships between growth traits and scale stable isotopes (δ13C, δ15N) of adult chum salmon Oncorhynchus keta in Hokkaido,Japan

Stable isotope analysis (SIA) in combination with growth analysis using scales collected from adult chum salmon Oncorhynchus keta migrating back to Hokkaido, Japan, was performed to describe the variation of isotopic composition of carbon (δ¹³C) and nitrogen (δ¹⁵N) in scales and to examine relationships with growth traits [age, fork length (FL), and relative growth ratio in the last growth period [(RGR^last)]. Scale stable isotope (SI) values in 3‐ to 6‐year‐old fish were highly variable, ranging from ?17.6‰ to ?14.3‰ for δ¹³C and 9.5‰ to 13.4‰ for δ¹⁵N. The δ¹⁵N was positively correlated with FL, and this tendency may indicate changes in trophic level with growth. Significant effect was not detected between δ¹⁵N and RGR^last, it can be inferred that factors potentially yielding high δ¹⁵N may not necessarily result in higher growth rates. No trend found between FL and δ¹³C may imply that there is no clear segregation in feeding locations between the 3‐ to 6‐year groups. This study provided basic information for scale SI values of chum salmon adults and indicated that SIA using scales could be a new approach to elucidating the trophic ecology of chum salmon.     

VARIATION IN INTERNAL δ15N AND δ13C DISTRIBUTIONS AND THEIR BULK VALUES IN THE BROWN MACROALGA PADINA AUSTRALIS GROWING IN SUBTROPICAL OLIGOTROPHIC WATERS1

The utility of δ¹⁵N measurements in Padina australis Hauck as a probe for its external nitrogen (N) sources was tested by monitoring the bulk values of chemical components [δ¹⁵N, δ¹³C, and N and carbon (C) contents] and their internal distributions during a 12 d incubation in a controlled environment. Under the saturated conditions of isotopically heavier nitrate than that of original algal tissue, the bulk δ¹⁵N in P. australis was enriched, but less than what was predicted from a simple mixing model, signaling possible isotopic discrimination during N assimilation and subsequent N efflux from the cells. The enhanced N content (%), which occurred simultaneously with this δ¹⁵N shift, was a useful signal indicating this phenomenon. Bulk δ¹⁵N was enriched, especially around the meristem, in tissues growing under conditions of higher irradiance and temperature, probably due in part to dissolved organic nitrogen (DON) excretion. The δ¹³C enhancement in bulk algal tissues, also associated with high photosynthetic activity, may be an additional signal indicating this unbalanced internal δ¹⁵N distribution. However, in summer and winter environmental conditions with periodic nitrate supplies simulating typical fringing reef waters, the difference in measured algal bulk δ¹⁵N from theoretical predictions was within ±1.0‰. This difference is very small compared with the variation in δ¹⁵N in possible N sources in coastal areas. In the field, therefore, δ¹⁵N in Padina can be used effectively to trace N sources in both space and time after determining algal N content and δ¹³C to determine whether large alterations occur in algal δ¹⁵N.     

Decrypting stable‐isotope (δ13C and δ15N) variability in aquatic plants

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Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Comparing the performance of different stomatal conductance models using modelled and measured plant carbon isotope ratios (δ13C): implications for assessing physiological forcing

Accurate modelling of long‐term changes in plant stomatal functioning is vital to global climate change studies because changes in evapotranspiration influence temperature via physiological forcing of the climate. Various stomatal models are included in land surface schemes, but their robustness over longer timescales is difficult to validate. We compare the performance of three stomatal models, varying in their degree of complexity, and coupled to a land surface model. This is carried out by simulating the carbon isotope ratio of tree leaves (δ¹³C_leaf) over a period of 53 years, and comparing the results with carbon isotope ratios obtained from tree rings (δ¹³C_stem) measured at six sites in northern Europe. All three stomatal models fail to capture the observed interannual variability in the measured δ¹³C_stem time series. However, the Soil‐Plant‐Atmosphere (SPA) model performs significantly better than the Ball‐Berry (BB) or COX models when tested for goodness‐of‐fit against measured δ¹³C_stem. The δ¹³C_leaf time series simulated using the SPA model are significantly positively correlated (P < 0.05) with measured results over the full time period tested, at all six sites. The SPA model underestimates interannual variability measured in δ¹³C_stem, but is no worse than the BB model and significantly better than the COX model. The inability of current models to adequately replicate changes in stomatal response to rising levels of CO₂ concentrations, and thus to quantify the associated physiological forcing, warrants further investigation.     

Transpiration and canopy conductance in a pristine broad-leaved forest of Nothofagus: an analysis of xylem sap flow and eddy correlation measurements

Summary Tree transpiration was determined by xylem sap flow and eddy correlation measurements in a temperate broad-leaved forest of Nothofagus in New Zealand (tree height: up to 36 m, one-sided leaf area index: 7). Measurements were carried out on a plot which had similar stem circumference and basal area per ground area as the stand. Plot sap flux density agreed with tree canopy transpiration rate determined by the difference between above-canopy eddy correlation and forest floor lysimeter evaporation measurements. Daily sap flux varied by an order of magnitude among trees (2 to 87 kg day^–1 tree^–1). Over 50% of plot sap flux density originated from 3 of 14 trees which emerged 2 to 5 m above the canopy. Maximum tree transpiration rate was significantly correlated with tree height, stem sapwood area, and stem circumference. Use of water stored in the trees was minimal. It is estimated that during growth and crown development, Nothofagus allocates about 0.06 m of circumference of main tree trunk or 0.01 m² of sapwood per kg of water transpired over one hour.Maximum total conductance for water vapour transfer (including canopy and aerodynamic conductance) of emergent trees, calculated from sap flux density and humidity measurements, was 9.5 mm s^–1 that is equivalent to 112 mmol m^–2 s^–1 at the scale of the leaf. Artificially illuminated shoots measured in the stand with gas exchange chambers had maximum stomatal conductances of 280 mmol m^–2 s^–1 at the top and 150 mmol m^–2 s^–1 at the bottom of the canopy. The difference between canopy and leaf-level measurements is discussed with respect to effects of transpiration on humidity within the canopy. Maximum total conductance was significantly correlated with leaf nitrogen content. Mean carbon isotope ratio was –27.76±0.27 (average ±s.e.) indicating a moist environment. The effects of interactions between the canopy and the atmosphere on forest water use dynamics are shown by a fourfold variation in coupling of the tree canopy air saturation deficit to that of the overhead atmosphere on a typical fine day due to changes in stomatal conductance.This paper is dedicated to Prof. Dr. O.L. Lange on the occasion of his 65th birthday     

Late-stage canopy tree species with extremely low δ13C and high stomatal sensitivity to seasonal soil drought in the tropical rainforest of French Guiana

We assessed the daily time‐courses of CO₂ assimilation rate (A), leaf transpiration rate (E), stomatal conductance for water vapour (g_s), leaf water potential ( Ψ _w) and tree transpiration in a wet and a dry season for three late‐stage canopy rainforest tree species in French Guiana differing in leaf carbon isotope composition ( δ ¹³C). The lower sunlit leaf δ ¹³C values found in Virola surinamensis ( ? 29·9‰) and in Diplotropis purpurea ( ? 30·9‰), two light‐demanding species, as compared to Eperua falcata ( ? 28·6‰), a shade‐semi‐tolerant species, were clearly associated with higher maximum g_s values of sunlit leaves in the two former species. These two species were also characterized by a high sensitivity of g_s, sap flow density (Ju) and canopy conductance (g_c) to seasonal soil drought, allowing maintenance of high midday Ψ _w values in the dry season. The data for Diplotropis provided an original picture of increasing midday Ψ _w with increasing soil drought. In Virola, stomata were extremely sensitive to seasonal soil drought, leading to a dramatic decrease in leaf and tree transpiration in the dry season, whereas midday Ψ _w remained close to ? 0·3 MPa. The mechanisms underlying such an extremely high sensitivity of stomata to soil drought remain unknown. In Eperua, g_s of sunlit leaves was non‐responsive to seasonal drought, whereas Ju and g_c were lower in the dry season. This suggests a higher stomatal sensitivity to seasonal drought in shaded leaves than in sunlit ones in this species.     

Mathematical modeling of soil carbon turnover in natural Podocarpus forest and Eucalyptus plantation in Ethiopia using compound specific δ13C analysis

Forest soils exhibit huge potential in storing carbon, but may also release large amounts of it if they undergo major changes in land use and environmental conditions. Biogeochemical processes controlling accumulation and release of soil organic carbon (SOC) are not yet sufficiently understood. We investigate the dynamics of SOC depending on its chemical composition below a natural forest (Podocarpus falcatus dominated) and a plantation (Eucalyptus saligna) growing on Nitisols in southern Ethiopia. Soils at the study‐site show a huge shift to less negative δ¹³C values at a depth of 20–30 cm, indicating a change from C4 savanna to C3 forest during the late Holocene. Total organic carbon (TOC), black carbon (BC), and sugars from microbial (rhamnose, fucose) and plant origin (xylose, arabinose) are subjected to compound‐specific stable isotope analysis. Turnover characteristics are calculated using a numerical advection–diffusion–decomposition model. Our measurements show significant differences in carbon storage (P<0.05) for both sites (Podocarpus 23.5 ± 3.2 kg SOC m^?3; Eucalyptus 18.6 ± 2.7 kg SOC m^?3). These differences can be explained with an initial loss of 15–26% of TOC about 50 years ago, induced by clearing the natural forest. After canopy closure, the carbon input below Eucalyptus is 15–34% less than below natural forest. At present, mean residence times (MRTs) of the investigated compounds do not differ between both stands. Sugars show the shortest MRTs in the topsoil with 2–7 years (xylose) and 5–13 years (arabinose) and have been affected the most by clear‐cutting. TOC and BC show MRTs of 13–25 years and 20–34 years, respectively. Old C4 carbon below 20 cm has merely been affected by the land use change. Contrary to expectation, our study does not indicate a pronounced recalcitrance of BC.     

Nitrogen fertilization enhances water‐use efficiency in a saline environment

Effects of salinity and nutrients on carbon gain in relation to water use were studied in the grey mangrove, Avicennia marina, growing along a natural salinity gradient in south‐eastern Australia. Tall trees characterized areas of seawater salinities (fringe zone) and stunted trees dominated landward hypersaline areas (scrub zone). Trees were fertilized with nitrogen (+N) or phosphorus (+P) or unfertilized. There was no significant effect of +P on shoot growth, whereas +N enhanced canopy development, particularly in scrub trees. Scrub trees maintained greater CO₂ assimilation per unit water transpired (water‐use efficiency, WUE) and had lower nitrogen‐use efficiency (NUE; CO₂ assimilation rate per unit leaf nitrogen) than fringe trees. The CO₂ assimilation rates of +N trees were similar to those in other treatments, but were achieved at lower transpiration rates, stomatal conductance and intercellular CO₂ concentrations. Maintaining comparable assimilation rates at lower stomatal conductance requires greater ribulose 1·5‐bisphosphate carboxylase/oxygenase activity, consistent with greater N content per unit leaf area in +N trees. Hence, +N enhanced WUE at the expense of NUE. Instantaneous WUE estimates were supported by less negative foliar δ¹³C values for +N trees and scrub control trees. Thus, nutrient enrichment may alter the structure and function of mangrove forests along salinity gradients.     

Environmental sensitivity of gas exchange in different-sized trees

The carbon isotope signature (δ¹³C) of foliar cellulose from sunlit tops of trees typically becomes enriched as trees of the same species in similar environments grow taller, indicative of size-related changes in leaf gas exchange. However, direct measurements of gas exchange in common environmental conditions do not always reveal size-related differences, even when there is a distinct size-related trend in δ¹³C of the very foliage used for the gas exchange measurements. Since δ¹³C of foliage predominately reflects gas exchange during spring when carbon is incorporated into leaf cellulose, this implies that gas exchange differences in different-sized trees are most likely to occur in favorable environmental conditions during spring. If gas exchange differs with tree size during wet but not dry conditions, then this further implies that environmental sensitivity of leaf gas exchange varies as a function of tree size. These implications are consistent with theoretical relationships among height, hydraulic conductance and gas exchange. We investigated the environmental sensitivity of gas exchange in different-sized Douglas-fir (Pseudotsuga menziesii) via a detailed process model that specifically incorporates size-related hydraulic conductance [soil–plant–atmosphere (SPA)], and empirical measurements from both wet and dry periods. SPA predicted, and the empirical measurements verified, that differences in gas exchange associated with tree size are greatest in wet and mild environmental conditions and minimal during drought. The results support the hypothesis that annual net carbon assimilation and transpiration of trees are limited by hydraulic capacity as tree size increases, even though at particular points in time there may be no difference in gas exchange between different-sized trees. Maximum net ecosystem exchange occurs in spring in Pacific Northwest forests; therefore, the presence of hydraulic limitations during this period may play a large role in carbon uptake differences with stand-age. The results also imply that the impacts of climate change on the growth and physiology of forest trees will vary depending on the age and size of the forest.     

Leaf stable carbon isotope composition reflects transpiration efficiency in Zea mays

The increasing demand for food production and predicted climate change scenarios highlight the need for improvements in crop sustainability. The efficient use of water will become increasingly important for rain‐fed agricultural crops even in fertile regions that have historically received ample precipitation. Improvements in water‐use efficiency in Zea mays have been limited, and warrant a renewed effort aided by molecular breeding approaches. Progress has been constrained by the difficulty of measuring water‐use in a field environment. The stable carbon isotope composition (δ¹³C) of the leaf has been proposed as an integrated signature of carbon fixation with a link to stomatal conductance. However, additional factors affecting leaf δ¹³C exist, and a limited number of studies have explored this trait in Z. mays. Here we present an extensive characterization of leaf δ¹³C in Z. mays. Significant variation in leaf δ¹³C exists across diverse lines of Z. mays, which we show to be heritable across several environments. Furthermore, we examine temporal and spatial variation in leaf δ¹³C to determine the optimum sampling time to maximize the use of leaf δ¹³C as a trait. Finally, our results demonstrate the relationship between transpiration and leaf δ¹³C in the field and the greenhouse. Decreasing transpiration and soil moisture are associated with decreasing leaf δ¹³C. Taken together these results outline a strategy for using leaf δ¹³C and reveal its usefulness as a measure of transpiration efficiency under well‐watered conditions rather than a predictor of performance under drought.     

A genetic link between leaf carbon isotope composition and whole‐plant water use efficiency in the C4 grass Setaria

Genetic selection for whole‐plant water use efficiency (yield per transpiration; WUE_plant) in any crop‐breeding programme requires high‐throughput phenotyping of component traits of WUE_plant such as intrinsic water use efficiency (WUE_i; CO₂ assimilation rate per stomatal conductance). Measuring WUE_i by gas exchange measurements is laborious and time consuming and may not reflect an integrated WUE_i over the life of the leaf. Alternatively, leaf carbon stable isotope composition (δ¹³C_leaf) has been suggested as a potential time‐integrated proxy for WUE_i that may provide a tool to screen for WUE_plant. However, a genetic link between δ¹³C_leaf and WUE_plant in a C₄ species has not been well established. Therefore, to determine if there is a genetic relationship in a C₄ plant between δ¹³C_leaf and WUE_plant under well watered and water‐limited growth conditions, a high‐throughput phenotyping facility was used to measure WUE_plant in a recombinant inbred line (RIL) population created between the C₄ grasses Setaria viridis and S. italica. Three quantitative trait loci (QTL) for δ¹³C_leaf were found and co‐localized with transpiration, biomass accumulation, and WUE_plant. Additionally, WUE_plant for each of the δ¹³C_leaf QTL allele classes was negatively correlated with δ¹³C_leaf, as would be predicted when WUE_i influences WUE_plant. These results demonstrate that δ¹³C_leaf is genetically linked to WUE_plant, likely to be through their relationship with WUE_i, and can be used as a high‐throughput proxy to screen for WUE_plant in these C₄ species.