Key role of ambient temperature in modulating leaf water isotopic enrichment seasonality in a humid subtropical climate

Abstract

There is increasing evidence that plants enhance stomatal conductance and transpiration with rising temperatures to prevent leaf overheating especially in environments with ample water availability. We investigated the interplay among environmental parameters, plant water status and leaf physiology as drivers of monthly variations of leaf water oxygen and hydrogen isotopic enrichment above plant source water (Δ¹⁸O_L, Δ²H_L, respectively) over two years in four common tree species with contrasting leaf shapes and water-uptake patterns in a humid subtropical climate in Southwest China. We hypothesized that Δ¹⁸O_L and Δ²H_L variation throughout the year is primarily driven by air temperature seasonality through modulation of stomatal regulation of transpiration at seasonal timescale. We found tight coupling and inverse links of Δ¹⁸O_L and Δ²H_L with air temperature and leaf water content in each target species, which is consistent with the expected effect of enhanced stomatal conductance and transpiration on leaf water isotopic composition under warmer conditions. Leaf dry matter oxygen isotopic data provide further evidence of general increases in time-integrated stomatal conductance and transpiration with increasing ambient air temperature and leaf water content across species. These results support that trees can achieve effective transpirational leaf cooling under high ambient temperatures when soil water availability is also high, at both short and long timescales. Our analysis also shows that the correlations of Δ¹⁸O_L and Δ²H_L with meteorological parameters are largely unaffected by leaf morphology but can be slightly modified by water-uptake pattern among species. Finally, this study highlights the key influence of temperature-modulated seasonal changes in stomatal conductance on leaf water isotopic enrichment fluctuations through time, which sheds light on plant-environment interactions and ecosystem water fluxes in humid subtropical regions.