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Impacts of Forest Thinning on Ecohydrological Processes in the Soil-Plant-Atmosphere Continuum - A Plot Scale Analysis
AuthorSerpa, Benjamin Michael
AdvisorTyler, Scott W.
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Since 2010, Lassen National Forest has implemented Group Select and Diversity Thinning treatments, consisting of the removal of all but 2-4 seed trees in a <2 acre area and removing 40-50% of canopy cover, respectively, to reduce fuel loads, restore historic species composition, and re-establish a healthy, fire-adapted, and drought resilient landscape. The purpose of this study is to further our understanding of how these forest management practices influence ecohydrologic processes in the soil-tree-atmosphere continuum. Modifying forest density and canopy structure impacts local mass and energy budgets, resulting in changes to forest microclimates, precipitation partitioning, and evapotranspiration. The Ashpan monitoring site is present in Lassen National Forest in the upper Hat Creek catchment, just north of Lassen Volcanic National Park, to discern the impacts of forest management practices on forest-water interactions. This work uses micrometeorological observations, tree stem cores, and stable isotopes of water to identify the effects of forest structure modifications on understory microclimate, radial tree growth, and tree water source depth. Analyses of air temperature, shortwave radiation, wind speed, and relative humidity observations indicate that Group Select and Diversity Thin treatments increase diurnal extremes of air temperature and vapor pressure deficit compared to an adjacent untreated forest. Similarly, results demonstrate that Group Select and Diversity Thin treatments increase understory wind speeds and shortwave radiation incident at the forest floor by factors of 2 to 5. Analyses of tree stem cores reveals the thinning treatments increased radial tree growth by a factor of 2 to 3 in the Group Select and 1.5 to 2 in the Diversity Thin treatments. Although long-term tree growth exhibits significant positive correlations to winter and spring air temperature, fall and winter precipitation, and fall discharge in Hat Creek, and negative correlations to summer air temperature, our results suggest winter and spring air temperatures are the primary factor limiting tree growth in ponderosa pine at the Ashpan monitoring site. Stable isotope results suggest that mature ponderosa pine at the site source water from 50 – 100 cm depth in June and then shift to mean source depths greater than 100 cm in September in response to annual soil moisture recession. Stable isotope results show changes in forest density and structure do not influence mean source depth of root water uptake. Stable isotope results, and a subsequent catchment water balance, reveals that groundwater discharge at Big Spring can be plausibly balanced by a recharge fraction of 0.4 across the 275 km2 catchment above the spring. Current forest management practices in Lassen National Forest increase forest productivity, resilience to drought, and promote spatial diversity in microclimates. Future research should consider how changing climate could dampen or exacerbate these impacts to inform management decisions best.