According to Peper et al 2010, trees are mini-reservoirs, controlling runoff at the source, thereby reducing runoff volumes and erosion, as well as delaying the onset of peak flows. Trees can reduce runoff in several ways:
Leaves and branch surfaces intercept and store rainfall, thereby reducing runoff volumes and delaying the onset of peak flows.
Roots increase the rate at which rainfall infiltrates soil by creating root channels. This increases the capacity of soil to store water, reducing overland flow.
Tree canopies and litter reduce soil erosion by diminishing the impact of raindrops on bare surfaces.
Transpiration through tree leaves reduces moisture levels in the soil, increasing the soil’s capacity to store rainfall.