Ecosystem responses to climatic droughts range from increased frequency of stomatal closure and declines in carbon assimilation to plant mortality and increased vulnerability to insects/pathogens and fire. Forest thinning is often posited as a management practice that can reduce mitigate these forest drought responses and may have implications for groundwater recharge and streamflow. There are still relatively few field-based studies of thinning impacts on forest water use. Most of these studies focus only on responses in the first 1-2 years following thinning and show substantial cross-site differences in responses. To extend and better understand field-based findings, we utilize a fully coupled model of ecosystem carbon cycling and hydrology to estimate forest water use, carbon sequestration, regrowth rates and indicators of water stress for different thinning densities. We compare results across climate, topography and soils. For the California Sierra, results emphasize the importance of plant accessible subsurface water storage capacity (PAWSC). We find that a first-order control on system response to thinning is the interactions between PAWSC and climate. We also show that a critical information gap is estimates of the degree to which neighboring trees share water. We conclude by suggesting model applications and new measurements that could be used to help guide decision-making with respect to thinning by accounting for how these multiple controls on thinning ecohydrology interact.
