The Lasting Effects of Prescribed Fire and Harvesting on Forest Floor and Soil Nutrients in the Eastern Sierra Nevada Mountains
AuthorRoaldson, Lauren Marie
AdvisorJohnson, Dale W
Environmental and Natural Resource Sciences
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Silviculture thinning and prescribed fire are common management tools used to eliminate thick fuel loads that could otherwise cause a harmful wildfire. Although these mechanical harvesting techniques are becoming more popular, little is known about their long-term effects on soil fertility, forest health, and water quality. The objective of this study was to quantify the lasting effects of prescribed fire and thinning on forest floor and soil nutrients approximately eight to nine years after a burn occurred within two study sites in the Eastern Sierra Nevada Mountains. The first study site is located outside of Truckee, CA and includes a prescribed fire following various harvest and understory removal treatments: whole-tree (WT) thinning, cut-to-length (CTL) thinning, and no harvest. The second study site is located within the North Lake Tahoe area and also includes a prescribed fire following a CTL harvesting system in conjunction with an on-site slash chipping. At both study sites, data was collected before, immediately after, and eight to nine years following the prescribed burns. All forest floor and soil samples were analyzed for nutrients (total N, P, K, Ca, Mg; mineral N, ortho-P, exchangeable cations, cation exchange capacity, iron, manganese, copper, zinc, and δ15N (N. Tahoe only)). Resin lysimeters were installed at the Truckee site and resin capsules and Plant Root Simulator (PRS) probes were installed at the North Lake Tahoe site in order to assess soil leaching. For the Truckee site, fuel reductions were still evident nine years after the burning in the burned plots. The most significant finding was a reduction in mineral N in the burned plots nine years post-burn. On the other hand, in the unburned CTL treatment, soil total N and mineral N has increased due to the incorporation of decomposed slash material into mineral soil. At the N. Lake Tahoe site, it was found that thinning in combination with fire was successful in reducing fuel loads for at least eight years. The initial pulse in NH4+ that was seen post-burn had disappeared eight years later. At this point, chipping does not appear to immobilize available nutrients in the mineral soil; however, the chips were showing signs of decomposition in that O horizon mass had decreased and C: N ratios had declined in mineral soil in the thinned, unburned plots. This site also experienced an influx of post-fire nitrogen fixing vegetation; most abundant is Ceanonthus velutinus Dougl. The data showed a strong correlation between dense clusters of ceanothus and reduced mineral soil δ15N to nearer atmospheric levels. However, soil mineral N has not yet increased in mineral soil and is therefore not reflecting nitrogen fixation.