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.