Seasonal and multi-year ecohydrologic responses to forest thinning

In the water-limited forested watersheds of the Western US, a critical eco-hydrology question is how does thinning influence streamflow and water-stress for remaining vegetation. We summarize recent literature on field-based examples of analyses which show both increases and decreases in streamflow following moderate thinning either through drought-related mortality or forest management. We use RHESSys, a coupled eco-hydrologic model, to disentangle the controls that may influence thinning responses. We examine both within watershed (topography, water holding capacity) and between watersheds differences in climate and their influence on thinning effects. We use the model to develop probability distributions of streamflow and productivity responses across inter-annual variation in meteorology forcing in the year following thinning. We also look at longer-term, multi-year response to examine potential impact of thinning on subsequent drought vulnerability and mortality risk. A somewhat surprising result from this analysis is that there are scenarios where thinning can actually increase subsequent vulnerability of water resources to drought stress.