Salinity stress and atmospheric dryness co-limit evapotranspiration in a subtropical monsoonal estuarine mangrove wetland

Abstract

Physiological drought stresses induced by high salinity and atmospheric dryness exert negative effects on mangrove growth by constraining surface conductance (Gs) and evapotranspiration (ET). However, accurate assessments of mangrove vulnerability under changing climate are hindered by limited understanding of the relative importance of these drought stresses. To close this knowledge gap, we utilized the eddy covariance (EC) approach to acquire a 7-year (2017-2023) time series of ET and auxiliary measurements over a subtropical monsoonal estuarine mangrove in southeast China, aiming to examine the temporal patterns of mangrove ET across time scales and its environmental controls, in particular for drought stresses. The results indicated that (a) over the study period, mangrove ET showed strong temporal variations at diurnal and seasonal scales, which were strongly correlated with photosynthetically active radiation, air temperature, and vapor pressure deficit (VPD); (b) the seasonality of mangrove ET followed the changes in monthly air temperature and rainfall, reflecting the temporal pattern of typical monsoonal climate; (c) both increasing salinity and VPD were found to significantly constrain mangrove Gs when salinity and VPD were decoupled; (d) the variability of rainfall exerted a similar constraining effect on mangrove Gs, with annual maximum sustained rainfall accounting for 65% of the inter-annual variability of ET. These findings suggest that salinity stress (or less rainfall) and atmospheric dryness co-limit the temporal variability of ET in subtropical monsoonal mangroves. Future climate change with warmer air temperatures (negative effect) and more extreme rainfall (positive effect) could counteract each other in affecting mangrove ET.