Tree planting strongly attenuates storm runoff response on the Chinese Loess Plateau

Tree planting, widely promoted as a climate change mitigation strategy, has been shown to reduce average streamflows. Its effects on storm runoff, however, remain unclear. In the early 2000's, China’s “Grain for Green Program” almost doubled vegetation cover on the Loess Plateau. Here we show that this tree planting reduced peak runoff responses by up to 86%, proportional to the percentage growth in Leaf Area Index, across five mesoscale basins with different degrees of afforestation. Surprisingly, this effect was larger during higher-intensity rainfall, implying that it cannot arise primarily from increased canopy interception or soil moisture depletion, which would have relatively larger effects during low-intensity precipitation. Instead, we hypothesize that the main mechanisms are reductions in runoff-generating areas and increases in infiltration rates. Consistent with this hypothesis, afforestation increased low flows up to 25%. These substantial and persistent hydrological consequences of tree planting can inform future climate change mitigation projects.