Continuous Observations of Leaf Transpiration Isotopic Composition in Two Dryland Species Highlight Sensitivity to Changes in Irradiance and Soil Water Content.

Abstract:

Advances in optical isotope techniques have enabled near-continuous determination of leaf transpiration isotopic composition (δ_T), allowing researchers to trace water movement through the soil-plant-atmosphere continuum at fine temporal scales. However, to date very few direct measurements of δ_T are available. We applied a novel flow-through chamber method developed by Wang et al. (2011) to monitor short-term variability in leaf transpired water isotopic composition of two African savanna trees, Vachiella tortillis and Senegalia mellifera, in response to variable irradiance and soil water content. Single leaves were inserted into a custom cuvette connected to an off-axis integrated cavity output spectroscopy (IA-ICOS) water vapor isotope analyzer and allowed to reach isotopic steady-state. Measurements were taken on alternating species every 10 minutes for two hours during mid-morning when plants were most active. Above-canopy irradiance and soil moisture at rooting depth were recorded daily at 1- and 10-minute intervals, respectively. We found that shading initiated rapid depletion of δ²H and δ¹⁸O in both species, though differences were more pronounced in V. tortillis. Similarly, an increase in irradiance caused δ¹⁸O enrichment of transpired water by up to 5‰ on the order of minutes. Interestingly, soil water content was positively correlated with δ²H: in drier conditions transpired water was more depleted relative to wetter conditions. Our results suggest that the isotopic composition of leaf water within a canopy may vary considerably depending on leaf location and amount of radiation received. These findings have important implications for models of plant water isotopes and underscore the need for additional direct observations of δ_T .