Responses of deep-rooting vegetation to extreme dry spell: Both carbon assimilation and water use efficiency are promoted in a karstic climax forest

The experimental site is in the Mulun National Nature Reserve (107°54′–108°05′ E, 25°07′–25°12′ N), which is a subtropical karst forest in southwestern China, a famous karst region over the world. We investigated the carbon-water dynamics (e.g., gross primary productivity (GPP), evapotranspiration (ET), and water use efficiency (WUE)) and the underlying factors in a karst climax forest during an extreme dry spell, of which the return period is of 66 years. We combined an eddy covariance system and automatic meteorological station for monitoring water and carbon fluxes and meteorological variables. We found that the ET was slightly constrained by water deficit, but the GPP remained relatively high levels throughout the dry spell. Both ET and GPP were positively (generally, r > 0.7, P < 0.05) correlated to climatic factors (e.g., net solar radiation, air temperature, water vapour pressure deficit and wind speed). What’s interesting is that, both ET and GPP exhibited higher correlations with climatic factors when soil water content decreased; and surprisingly the WUE exhibited positive correlations ( P < 0.05) with the climatic factors. Besides these, we also found that the WUE (16.0 ± 11.9 mg CO 2 g −1 H 2 O) was enhanced substantially, being twice as much as the mean value in the similar regions. These findings highlight a unique hydrological buffering capacity of this karstic ecosystem, suggesting that regional models must integrate such deep-rooted vegetation dynamics to accurately project ecohydrological responses to climate extremes. • Deep-rooted karst forest maintained high carbon uptake and transpiration during an extreme dry spell. • Both ET and GPP are more closely correlated to climatic factors when SWC decreased. • Water use efficiency shows positive and linear relationships with key meteorological drivers