Increasing atmospheric dryness exacerbates mangrove carbon–water decoupling

Mangroves maintain high productivity due to conservative water use, even under physiological drought stress. However, the temporal variability of this unique carbon-water relation across time scales remains less explored. Here, seven-year eddy covariance measurements were used to examine ecosystem-level gross primary productivity (GPP), transpiration (T), water-use efficiency (WUE), and their environmental controls in a subtropical mangrove in China. We found mangrove WUE followed a U-shaped seasonal pattern (i.e. lower in summer) over the years, while its diurnal pattern changed from a U-shaped one (i.e. lower at noon) in winter to an L-shaped one (i.e. low and stable in the afternoon) in summer. Asynchronous GPP and T with higher morning WUE led to diurnal hysteresis or decoupling, the degree of which co-varied seasonally and annually with vapor pressure deficit (VPD). This diurnal hysteresis between GPP and T can be well reduced by applying a VPD-adjusted hysteresis formulation (i.e. T∝GPP·VPD^0.78). These results suggest increasing atmospheric dryness exacerbates mangrove carbon-water decoupling, with their diurnal hysteresis being highly sensitive to VPD. This study also proposes a validated diurnal hysteresis formulation of mangrove carbon-water relation, which helps to project mangrove carbon and water fluxes in a future warmer and drier climate.