Identifying drivers of leaf water and cellulose stable isotope enrichment in Eucalyptus in northern Australia

Do 2 H and 18 O in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (δ² H and δ¹⁸ O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ² H was more closely correlated with δ² H of xylem water or atmospheric vapour, whereas leaf water δ¹⁸ O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ² H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ¹⁸ O. We next expressed leaf water as isotopic enrichment above xylem water (Δ² H and Δ¹⁸ O) to remove the impact of xylem water isotopic variation. For Δ² H, leaf water still correlated with atmospheric vapour, whereas Δ¹⁸ O showed no such correlation. This was explained by covariance between air relative humidity and the Δ¹⁸ O of atmospheric vapour. This is consistent with a previously observed diurnal correlation between air relative humidity and the deuterium excess of atmospheric vapour across a range of ecosystems. We conclude that ² H and ¹⁸ O in leaf water do indeed reflect the balance of environmental drivers differently; our results have implications for understanding isotopic effects associated with water cycling in terrestrial ecosystems and for inferring environmental change from isotopic biomarkers that act as proxies for leaf water.

Intra-leaf heterogeneities of hydrogen isotope compositions in leaf water and leaf wax of monocots and dicots

Several recent studies showed that leaf wax n-alkane δ²H values (δ²H_wax) within a leaf were heterogeneous in a small number of species. It still remains unclear whether the heterogeneity of intra-leaf δ²H_wax values is general for various species, how δ²H_wax values vary spatially and temporally, and whether there is a common explanation for the intra-leaf δ²H_wax heterogeneity in higher plants. Here we compared the hydrogen isotope compositions of leaf wax and corresponding leaf water (δ²H_lw) across leaf sections among a variety of monocot and dicot plant species. There is significant and consistent heterogeneity for both δ²H_wax and δ²H_lw, i.e., base-to-tip ²H-enrichment for monocots (except Hemerocallis citrina, and Dactylis glomerata) whereas base-to-tip and center-to-edge increases in δ²H_wax and δ²H_lw for dicots. The consistent occurrence of variations of δ²H_lw and δ²H_wax values within a leaf imply that δ²H_wax values probably inherit point-to-pint from in-situ δ²H_lw values, and thus the intra-leaf δ²H_wax heterogeneity mainly results from the spatial pattern of intra-leaf δ²H_lw values associated with veinal structures between dicots and monocots. The general heterogeneity of intra-leaf δ²H_wax values further intensifies that it is necessarily needed for in-depth understanding leaf wax biomarker.

Using δ13C and δ18O to analyze loblolly pine (Pinus taeda L.) response to experimental drought and fertilization

Drought frequency and intensity are projected to increase throughout the southeastern USA, the natural range of loblolly pine (Pinus taeda L.), and are expected to have major ecological and economic implications. We analyzed the carbon and oxygen isotopic compositions in tree ring cellulose of loblolly pine in a factorial drought (~30% throughfall reduction) and fertilization experiment, supplemented with trunk sap flow, allometry and microclimate data. We then simulated leaf temperature and applied a multi-dimensional sensitivity analysis to interpret the changes in the oxygen isotope data. This analysis found that the observed changes in tree ring cellulose could only be accounted for by inferring a change in the isotopic composition of the source water, indicating that the drought treatment increased the uptake of stored moisture from earlier precipitation events. The drought treatment also increased intrinsic water-use efficiency, but had no effect on growth, indicating that photosynthesis remained relatively unaffected despite 19% decrease in canopy conductance. In contrast, fertilization increased growth, but had no effect on the isotopic composition of tree ring cellulose, indicating that the fertilizer gains in biomass were attributable to greater leaf area and not to changes in leaf-level gas exchange. The multi-dimensional sensitivity analysis explored model behavior under different scenarios, highlighting the importance of explicit consideration of leaf temperature in the oxygen isotope discrimination (Δ18Oc) simulation and is expected to expand the inference space of the Δ18Oc models for plant ecophysiological studies.

Seasonal variation of δ18O and δ2H in leaf water of Fagus sylvatica L. and related water compartments

The study aims to assess variability in leaf water isotopic enrichment occurring in the field under natural conditions. We focused on seasonal variation and difference between sun-exposed and shaded leaves. Isotopic composition (δ¹⁸O, δ²H) of leaf water was monitored in a beech tree (Fagus sylvatica L.) growing in the forest-meadow ecotone together with δ¹⁸O (²H) of water compartments which are in close relation to this signal, namely twig and soil water. The sampling was carried out in approximately two-week intervals during five consecutive vegetation seasons. The δ¹⁸O (²H) data showed a distinct seasonal pattern and a consistency in relative differences between the seasons and sample categories. Leaf water was the most isotopically enriched water compartment. The leaf water enrichment decreased toward the autumn reflecting the change in δ¹⁸O (²H) of source water and evaporative demands. The soil and twig water isotopic signal was depleted against current precipitation as it partly retained the isotopic signature from winter precipitation however the seasonal pattern of soil and twig water followed that of precipitation. No significant differences between sun-exposed and shaded samples were detected. Nevertheless, the observed strong seasonal pattern of isotope composition of leaf, twig and soil water should be taken into account when using leaf water enrichment for further calculations or modeling.