Evaluating the impact of an experimental harvest on subsurface water storage in a coast redwood forest

Abstract

Forested watersheds are a vital water source for our drinking water and irrigation as well as for plants, streamflow, and riparian habitats. However, as droughts increase worldwide, forested watersheds are becoming increasingly threatened, leading to the question — how can we increase forest drought resilience? Forest harvesting or thinning is one management technique that has been proposed to potentially increase drought resilience in forests by increasing sub-surface water storage (e.g., soil moisture, groundwater, and rock moisture). Though subsurface water is vital for ecosystems, few studies have examined the impacts of harvesting on soil moisture and groundwater dynamics. We used a paired watershed study design in a coast redwood forest in northern California to answer how thinning might impact subsurface storage dynamics, specifically: (1) Does thinning impact soil moisture dynamics, specifically hydraulic redistribution? (2) Does thinning impact groundwater levels? and (3) Does thinning affect groundwater responsiveness to precipitation? We used groundwater and soil moisture measurements from four sub-watersheds and five hillslope positions taken during a pre-harvest and post-harvest period to answer these questions. We found that thinning did not significantly impact hydraulic redistribution, but hillslope position did have a significant effect. Thinning also did not significantly impact groundwater level, rather soil depth was more important. Finally, we found that thinning did not impact groundwater responsiveness to precipitation, but it might increase the connection between groundwater and streamflow. Thus, watershed managers hoping to use forest harvesting as a tool to increase drought resilience might be advised to examine non-vegetative characteristics and their importance for subsurface storage before thinning.