Uneven decline in the hydrological efficiency of China's natural and plantation forests

Abstract. The vegetation transpiration fraction (TF) links terrestrial water and carbon cycles, yet the sensitivity of TF to changes in leaf area index (LAI) (θ) and its hydroclimatic controls remain poorly constrained, particularly in China's contrasting natural forests (NF) and plantation forests (PF). Using forest-type maps to identify natural forests (NF) and plantation forests (PF), together with GLEAM-derived evapotranspiration variables and ERA5-Land hydroclimatic data from 1990–2020, we quantified the spatiotemporal patterns of θ and evaluated the relative roles of soil moisture (SM) and vapor pressure deficit (VPD). We found that θ increased spatially from humid to semi-arid regions and was consistently higher in NF than in PF, but declined significantly over time in both forest types, with a larger decrease in PF. Along the joint SM–VPD gradient, θ exhibited a nonlinear response surface, with higher values concentrated under moderate SM and intermediate-to-high VPD conditions. Although VPD still explained a slightly larger share of the present spatial pattern of θ, the influence of SM strengthened over time, indicating increasing soil moisture limitation. Overall, China's forests appear to be shifting toward a more conservative water-use regime, in which the capacity of additional leaf area to enhance TF has weakened. These results provide new insights into how contrasting forest types regulate water use under concurrent warming and greening, with implications for climate-adaptive forest and water-resource management.