Impacts of Forest Management on Hydrologic Processes
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Since 2010, Lassen National Forest has implemented a fuels reduction and forest restoration program that uses Diversity Thinning and Group Selection treatments with the goal of re-establishing a healthy, fire-adapted and resilient landscape. The purpose of this study is to better understand how forest management impacts hydrologic processes. It is well documented that forests can influence the hydrologic cycle. Removing the forest canopy and modifying forest structure impacts the local climate, precipitation reaching the soil surface, and evapotranspiration. However, there is still uncertainty in the extent that forest management practices impact hydrological processes in a particular forest, due to different site variables and analytical methods, and a site specific understanding is needed. Long term monitoring is present in Lassen National Forest to better understand forest-water relationships under various management practices. This work uses distributed temperature sensing (DTS) technology, sap flow and soil moisture observations, and the Structure for Unifying Multiple Modeling Alternatives (SUMMA) model to examine the impacts of forest structure modification (changes in forest density, canopy cover, LAI) on snow cover duration, sap flow, and transpiration. With DTS technology, we found that management practices, such as Group Selection and Diversity Thinning, promote greater snow accumulation and snow duration compared to the dense, untreated Control site. Sap flow and soil moisture observations indicate that individual trees in the Group Select and Diversity Thinning reached soil moisture levels that limited transpiration not only during major drought but also during years of normal precipitation. Modeling results show that changes in LAI do not impact water stress during major drought, but impact transpiration response to water stress during annual soil dry-out. Thus, thinned forests with lower LAI will likely be more resilient to drought, disease, and fire. Current management practices in Lassen National Forest promote healthy and resilient forests and future research should consider how changing climate may alter or intensify these impacts in order to best guide management decisions.