In the Southwest, increasing temperatures and changes in precipitation can have profound impacts on ecosystem function and, in particular, on ecosystem water use. 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. These forest responses to drought have also been linked with increased risk of severe fires.
Forest thinning is often posited as a management practice that can reduce the risk of these forest drought responses and may have implications for groundwater recharge and streamflow. Empirical studies have shown declines in fire severity in treated areas, particularly in regions heavily impacted by decades of fire suppression. There are still relatively few studies of thinning impacts on forest water use in the Southwest and most of these studies focus only on responses for 1-2 years following thinning. Studies also show substantial cross-site differences in responses. Conceptual models are needed that can help to explain and ultimately predict how ecohydrologic responses to thinning may vary of space and time and under a warming climate. In this study, we utilize an ecohydrologic model to disentangle the multiple factors that influence forest responses to thinning.
Results/Conclusions
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. Our 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 among PAWSC, climate and assumptions about the degree to which neighboring trees share water. We show that in sites with greater PAWSC thinning typically leads to small reductions in forest water use for the first 3-6 years following thinning but has relatively little impact on drought vulnerability. On the other hand, for sites with shallower soils, and where neighboring plants share water, thinning may substantially improve productivity of neighboring trees, but may also increase overall forest water use. Sensitivity to inter-annual variation in climate during the post-thinning regrowth period is also generally higher for sites with lower PAWSC. 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 interact.