Impacts of timber harvest treatments on stream biogeochemistry in a northern California redwood forest

Timber harvesting has long been a key aspect of forestry management in the redwood forests on the northern California coast. Over recent years, best management practices for harvesting have evolved to consider the importance of forested stream and riparian ecosystems. Previous research has indicated that nutrient export from forested headwater streams is often biologically controlled, and has highlighted the importance of in-stream metabolic processes in response to forest disturbance and timber harvest. In this study we use a paired-watershed approach in the South Fork Caspar Creek Experimental Watershed to study the effects of timber harvest treatments (0, 35, 55 and 75% basal area reductions) on the dominant hydrologic and biogeochemical processes over a 3-year study period. Nutrient analysis of pre and post-harvest streamwater samples focused on carbon, nitrogen and phosphorous fluxes in these watersheds through analysis of dissolved organic carbon, inorganic and organic nitrogen species (NO^3-, NH₄⁺ , TN and DON) and both dissolved reactive, and total phosphorous. Preliminary results show greatest nutrient flux events occurring during the onset of the winter/wet season and in early spring. Spring events suggest significant in-stream metabolic activity and biomass turnover before spring leaf-out of riparian vegetation (alder and understory plant species), while summer nutrient concentrations remain generally low. In addition to the paired watershed studies, two nutrient addition experiments were performed in July 2019 in order to estimate stream nutrient uptake, using nitrogen and phosphate additions alongside a bromide conservative tracer. Results include continuous dissolved oxygen, chlorophyll-a, and dissolved organic matter concentrations, which we will discuss with respect to the importance of photosynthetically active radiation levels in relation to stream nutrient uptake capacity.