Summary: Limitations and utility of three measures of water use characteristics were evaluated: water use efficiency (WUE), intrinsic WUE and marginal water cost of carbon gain () estimated, respectively, as ratios of assimilation (A) to transpiration (E), of A to stomatal conductance (g_s) and of sensitivities of E and A with variation in g_s. Only the measure estimates water use strategy in a way that integrates carbon gain relative to water use under varying environmental conditions across scales from leaves to communities. This insight provides updated and simplified ways of estimating and adds depth to understanding ways that plants balance water expenditure against carbon gain, uniquely providing a mechanistic means of predicting water use characteristics under changing environmental scenarios.

A quantitative accounting of how mangrove ecosystems respond to tidal perturbations is needed to anticipate changes in these ecosystems when sea level rises. Here we use long‐term field observations and a two‐source ecohydrological model to reveal specialized characteristics of evapotranspiration (ET), soil surface evaporation (E), and canopy transpiration (T) in three subtropical mangrove ecosystems in southeastern China. Average wintertime ET observed in these three mangrove forests (2.6 mm day^–1) was consistent with values for semiarid ecosystems, while average summertime ET (6.2 mm day^–1) approached that observed in rainforests. By contrast, T fluxes were small year‐round, averaging 1.3 mm day^–1 in winter and 2.5 mm day^–1 in summer. Combining our results with measurements from three Florida mangroves, observed values of T ranged from 350 to 870 mm year⁻¹, varying primarily with salinity, while T/ET increased exponentially from 30% to 70% with rising leaf area index. Simulations of half‐hourly ET and T using a modified two‐source model were highly correlated with eddy covariance observations of ET (I, index of agreement >0.93 at all three sites) and sap flow gauge‐based estimates of T (I = 0.93 at the Yunxiao site). Variations of T in mangrove ecosystems are distinguished from those in terrestrial forests mainly by the sensitivity of stomatal conductance to leaf temperature, with tidal and salinity effects superimposed. Our modified model accounts for these effects and therefore holds promise for improving our understanding of how mangrove ecosystems may respond to changing stress conditions under global warming and sea level rise.

Coastal mangroves are increasingly recognized as valuable natural resources and important sites of water and carbon exchange. In this study, we examine atmospheric water cycling in the boundary layer above a coastal mangrove forest in southern China. We collected site observations of isotopic ratios in water vapor and precipitation along with core meteorological variables during July 2017. Our evaluation of these data highlights the influences of large-scale atmospheric transport and rain–vapor exchange in the boundary layer water budget. Rain–vapor exchange takes different forms for different types of rainfall events. The evolution of isotopic ratios in water vapor suggests that substantial rain recycling occurs during the passage of large-scale organized convective systems, but that this process is much weaker during rainfall associated with less organized events of local origin. We further examine the influences of large-scale transport during the observation period using a Lagrangian trajectory-based moisture source analysis. More than half (63%) of the boundary layer moisture during the study period traced back to the South China Sea, consistent with prevailing southerly to southwesterly flow. Other important moisture sources included mainland Southeast Asia and the Indian Ocean, local land areas (e.g., Hainan Island and the Leizhou Peninsula), and the Pacific Ocean. Together, these five regions contributed more than 90% of the water vapor. The most pronounced changes in isotopic content due to large-scale transport during the study period were related to the passage of Tropical Storm Talas. The outer rain bands of this tropical cyclone passed over the measurement site on 15–17 July, causing a sharp reduction in the heavy isotopic content of boundary layer water vapor and a substantial increase in deuterium excess. These changes are consistent with extensive isotopic distillation and rain–vapor exchange in downdrafts associated with the intense convective systems produced by this storm.