Root zone storage capacity reveals ecohydrological turning points in Tibetan Plateau permafrost regions

Permafrost degradation on the Tibetan Plateau (TP) has intensified under recent climate warming, profoundly affecting hydrological processes and ecosystem stability. The root zone represents a key interface where ecological and hydrological processes interact, yet its water storage capacity remain poorly constrained due to the scarcity of direct observations in high-altitude environments. In this study, we employ an observation-based water balance approach to estimate root zone storage capacity ( S _R), defined as the maximum ecosystem-accessible volume that can be accessed by roots to allows plant water use during critical drought periods. Spatial patterns of S _R across the TP display considerable heterogeneity (24–278 mm, 1st–99th percentiles; Mean ± SD = 93 ± 12 mm), governed jointly by hydroclimatic and biological factors. Permafrost regions show significantly lower S _R (67 ± 6 mm) than seasonal-frost regions (110 ± 15 mm). Segmented regression combined with Davies’ supremum test identifies an ecohydrological turning point at an active layer thickness (ALT) of 2.2–2.5 m, beyond which S _R shifts from increasing to decreasing trends. This transition coincides with marked declines in the evaporation ratio, Budyko-Fu’s ω parameter, and measured belowground phytomass, reflecting restricted access of roots to supra-permafrost water. The results highlight a critical threshold linking permafrost degradation to vegetation water use and provide a quantitative basis for understanding and predicting the coupled assessing ecohydrological vulnerability of alpine/high-plateau ecosystems under ongoing climatic warming.