The ratio of transpiration to evapotranspiration dominates ecosystem water use efficiency response to drought

Abstract

Water use efficiency (WUE) is an important metric for quantifying the trade-off between ecosystem photosynthesis and transpiration, which can reflect how ecosystems respond to extreme climate events (e.g., drought). However, due to the different definitions of WUE indices and the complexity of drought with different dimensions, the responses of ecosystem WUE to drought still remain debated. Here, we use global flux observations to comprehensively examine whether different WUE indices (i.e., WUE_T=GPP/T, WUE_ET=GPP/ET, and uWUE= GPP*VPD^0.5/ET) have consistent responses to drought and how their responses are affected by drought intensity, duration and timing. The results show that WUE_T decreases, WUE_ET changes inapparently in direction of positive or negative, and uWUE increases during drought, although they have the same component GPP/T. The drought responses of ecosystem WUEs are significantly and nonlinearly influenced by drought intensity and duration, but insignificantly by drought timing. Increase in drought intensity leads to a higher reduction in WUE_T but greater increase in WUE_ET and uWUE. Short-term (< 2 months) and long-term droughts (> 6 months) have similar negative effects on WUE_T and WUE_ET, while medium to long-term droughts exert great positive impact on uWUE. The influences of drought intensity and duration are varied with drought timing. Further driver analyses reveal the unchanged GPP and increased T jointly lead to the decline in WUE_T during drought. Meanwhile, drought-driven rises in T/ET and VPD offset the reduced GPP/T, resulting in the inapparent change of WUE_ET and increase of uWUE, respectively. Overall, WUE_T response to drought is determined by T, while both WUE_ET and uWUE are controlled by T/ET. Our study highlights the necessary of disentangling the drought responses of different WUE indices with considering different drought dimensions, and investigating the T/ET variability during drought will provide deeper physiological understanding of ecosystem carbon-water coupling during drought.