Greening boosts transpiration, but water limits are emerging

A recent study in Forest Ecosystems highlights how large-scale vegetation restoration in China’s Three-North region has increased plant transpiration over the past two decades, yet the hydrological benefits of continued greening are diminishing due to soil moisture constraints.

Using satellite observations and ecohydrological models from 2000 to 2023, researchers from Beijing Forestry University examined how rising vegetation cover affects the transpiration fraction (the share of evapotranspiration contributed by plant transpiration). This metric reflects the ecosystem’s role in regulating water vapor exchange with the atmosphere.

The study found that leaf area index (LAI), a measure of vegetation density, increased by about 0.33 m²/m² per decade. This greening trend drove a steady rise in transpiration fraction, averaging 1.44% per decade. However, the sensitivity of transpiration fraction to increasing LAI declined by 32% over 24 years, indicating diminishing marginal hydrological gains. In other words, adding more vegetation now produces smaller increases in transpiration than in the early 2000s.

Soil moisture emerged as the main limiting factor. Regions experiencing soil drying showed the sharpest declines in sensitivity, with reductions up to 8%, while areas with adequate soil wetting buffered or nearly halted this decline. Atmospheric dryness, measured by vapor pressure deficit (VPD), played a secondary role; overall, regional VPD trends remained stable because rising temperatures (which increase atmospheric demand) were offset by increasing humidity.

The researchers also analyzed hydroclimatic variations across the region. Areas that became wetter with lower atmospheric demand showed lower sensitivity, suggesting ecosystems are approaching saturation, while drying regions experienced the steepest reductions in vegetation’s contribution to transpiration. Even in regions where LAI continued to increase, water stress limited the potential hydrological impact.

These findings carry important implications for ecological restoration initiatives such as the Three-North Shelter Forest Program by China in 1978 (one of the world's largest afforestation ecological engineering projects aimed at restoring the ecological environment). While afforestation has improved ecosystem function, the study shows that soil water availability ultimately constrains the capacity of vegetation to regulate water fluxes. As climate change intensifies aridity in northern China, adaptive water management and restoration strategies that account for soil moisture limitations will be critical to ensuring long-term sustainability.

DOI Link:

https://doi.org/10.1016/j.fecs.2025.100422