Compound heatwave-drought alters eco-hydrological processes in subtropical evergreen and deciduous forests

In recent years, extreme climate events have occurred globally with increasing frequency, posing severe challenges to forest water cycles. Particularly during the summer of 2022, an unprecedented compound heatwave-drought (CHD) event swept through southern China, while the potential effect on subtropical forest ecosystems remains unclear. On the basis of three-year continuous eddy covariance (EC)-based water flux and climate observations at the subtropical evergreen and deciduous forests between 2021 and 2023, this study addressed to quantify the process and the degree of influence of such CHD event on the ratio of transpiration (T) to evapotranspiration (ET), T, ET, and cumulative (precipitation (P) ​− ​ET), respectively. In contrast to the adjacent years, water contents at different soil depths in the two forest types declined sharply during the summer of 2022. Large differences in the variation ranges of T/ET were revealed between the two forest types, and the evergreen forest (EBF) exhibited relatively mild seasonal fluctuations, whereas the deciduous forest (DBF) showed relatively higher T/ET, T, and ET in summertime. Subsequent analysis revealed that MODIS EVI time-series effectively captured the variability in these eco-hydrological parameters. Furthermore, divergent differences were observed about the CHD-induced stress. For the DBF, both ET and T increased significantly, resulting in a severe water deficit (cumulative (P ​− ​ET)) of approximately −116.31 ​mm in 2022. In contrast, the EBF experienced a substantial reduction in both ET and T, with a water deficit of only −26.34 ​mm in 2022. All these analyses provide mechanistic evidence of the divergent drought response strategies between subtropical evergreen and deciduous forests, and offer scientific support for optimizing forest water resource management and enhancing climate resilience.