Greening mediates climate and CO2 induced water use efficiency effects on freshwater yield

Forests are crucial for the production of high-quality freshwater resources. Complex interactions between climate change and forest processes can result in uncertainty in the availability of freshwater to downstream communities and the environment. Previous studies reported consistent increasing trends in global river discharge during the last century, which has been explained by either climate factors (usually called “hydrological intensification”) or suppressed transpiration due to CO₂-induced stomatal closure. In this study, we study long-term changes in hydrological partitioning of precipitation between evapotranspiration and runoff generation (as mm per year) along a gradient of forested watershed along the eastern temperate forest biome. The precipitation is increasing at faster rates than runoff at most of these study catchments, which suggests long-term increases in evapotranspiration. These divergent trends in precipitation versus runoff rates are significantly correlated to long-term trends in NDVI and growing season length at the watershed scale, while climate variables cannot provided significant explanation. These findings suggest that the combined effect of increased temperatures and CO₂ fertilization have led to increased leaf area and lengthened growing season, which may counteract the effect of the CO₂-induced stomatal closure across the eastern US. This study emphasizes the importance of understanding vegetation responses to climate change to predict future flow regimes in forested watersheds.