Soil moisture governs the weakening response of transpiration fraction to leaf area index increase: A spatiotemporal analysis in China's Three-North region

Accurately assessing vegetation-hydrology interactions is crucial for water resource management, especially amidst climate change and ecological restoration. Using remote sensing observations (MODIS LAI) and GLEAM model outputs (evapotranspiration components, soil moisture (SM)) from 2000 to 2023 for China's Three-North (TN) region, we quantified the sensitivity of the transpiration fraction (TF, the ratio of transpiration to total evapotranspiration) to changes in leaf area index (LAI), denoted as θ = ∂TF/∂LAI. We employed an analytical approach combining SM and vapor pressure deficit (VPD) trends to evaluate the mechanisms governing θ′s response to increasing vegetation cover. Results show that while the TN region experienced a significant LAI increase (0.33 m²·m⁻²·decade⁻¹), driving a continuous TF rise (1.44% decade⁻¹), the sensitivity θ markedly decreased (−3.4% year⁻¹), accumulating a 32% decline over 24 years. This reveals a clear diminishing return of LAI increase on enhancing TF. Regional VPD remained stable, with opposing effects from rising temperature and atmospheric moisture largely cancelling out. Crucially, the decline in θ was primarily governed by SM dynamics; θ decreased most sharply under soil drying conditions (Δθ up to −8%), whereas sufficient soil wetting buffered the decline. Sensitivity also varied across different combinations of SM and VPD trends, being lowest where SM increased, and VPD decreased. This study demonstrates a weakening hydrological feedback to vegetation restoration in the TN region, highlighting soil moisture availability as the key constraint limiting the ecosystem's capacity to regulate water vapor fluxes. These findings provide a critical basis for assessing ecological sustainability and informing adaptive water management strategies under future aridification.