Humidity thresholds reverse forest cooling: Empirical evidence challenges thermal buffering paradigms in subtropical mountains

Forest ecosystems regulate microclimates through hydrothermal processes, yet the mechanisms driving phase transitions between cooling and warming states remain poorly quantified, particularly with respect to interactive canopy-atmosphere controls on threshold dynamics. Through six years in situ monitoring in subtropical montane forests, we demonstrate that relative humidity (RH) modulates thermal buffering transitions. We found an asymmetric buffering pattern where intensified nocturnal warming outweighs moderate daytime cooling, culminating in net annual warming. Crucially, RH predominates over soil moisture as the primary thermal regulator, masking soil moisture cooling effects. We further quantify nonlinear threshold responses, showing that rising humidity initially enhances cooling below critical points but suppresses evaporative cooling above these values, thereby triggering warming at seasonally specific thresholds. These biophysical switches provide new empirical evidence of RH-mediated phase transitions in forest microclimate and offer a scalable framework for climate-adaptive hydroclimate management in humid regions.