Excess radiation exacerbates drought stress impacts on canopy conductance along aridity gradients

Abstract

Stomatal conductance (gs) of all coexisting species regulates transpiration in arid and semiarid grasslands prone to droughts. However, the effect of drought stress on canopy conductance (Gs) is debated, and the interactive effects of abiotic and biotic constraints on Gs remain poorly understood. Here, we used 18O enrichment above the source water (Δ18O) of leaf organic matter as a proxy for Gs in order to increase the understanding of these effects. Three grassland transects were established along aridity gradients on the Loess Plateau (LP), the Inner Mongolian Plateau (MP), and the Tibetan Plateau (TP), which differ with respect to solar radiation and temperature conditions. Results showed that Gs consistently decreased with increasing aridity within transects. Gs on the TP was lower than that on the other two plateaus for a given level of aridity due to low temperature and high radiation. The primary determinant of drought stress on Gs was soil moisture (SM) on the LP and MP, whereas it was the vapor pressure deficit (VPD) on the TP. Solar radiation exhibited a consistently negative effect on Gs via drought stress within transects, while temperature had negative effects on Gs on the TP but no effect on the LP or MP. Adding the interaction of leaf area and abiotic factors increases the percentage of explained variability in Gs by 17 % and 36 % on the LP and MP, respectively, although this is not the case on the TP, where the climate exerts an overwhelming effect. These results highlight the need to integrate multiple stressors and plant properties to determine spatial variability in Gs.