Drivers of ecosystem soil water stress response revealed by critical soil moisture thresholds

Understanding the critical soil moisture threshold (Ɵcrit), defined as the volumetric water content triggering plant water stress, is essential for evaluating ecosystem vulnerability to drought. Based on the estimates of the critical soil moisture threshold (Ɵcrit), slope (S), and maximum evaporation fraction (EFmax) derived from the evaporative fraction-soil moisture (EF-SM) coupling, this study aims to investigate the soil water stress responses of various vegetation types and analyzed the dominant mechanisms driving the spatial variations of Ɵcrit, S, and EFmax. The results showed that among vegetation types, evergreen broadleaf forests demonstrated the greatest resistance to soil water stress, characterized by the highest Ɵcrit (0.43 ± 0.036 m3/m3) and lowest S (0.35 ± 0.042). In contrast, deciduous broadleaf forests were more sensitive to soil moisture limitations. Beyond forests, closed shrublands, croplands, and grasslands also exhibited high sensitivity to water stress. Notably, open shrublands were the most vulnerable vegetation type overall, with extremely low Ɵcrit (0.21 ± 0.053 m3/m3) and the highest S values (1.10 ± 0.059) indicating severe water limitation. An analysis of the dominant factors suggests that soil properties serve as the primary determinants of Ɵcrit variation (59 % explained variance), whereas meteorological factors predominantly govern the spatial variation of S (35 % explained variance). The spatial distribution of EFmax is mainly shaped by vegetation characteristics (62 % explained variance). More importantly, the results showed that vegetation physiological traits, such as gross primary productivity, leaf nitrogen content, and vegetation optical depth, further regulate Ɵcrit and S likely through mechanisms involving deep root systems, leaf morphology, and transpiration processes. These findings offer valuable insights into the variability of ecosystem's soil water stress responses, providing a robust scientific basis for enhancing water resource management in the context of climate change.