Effect of forest stand density reduction on nutrient transport at the Caspar Creek Watershed
Forest management strategies, and forest harvesting are often implicated as having adverse effects on nutrient cycling, sediment transport and hydrological processes in forested watersheds. This study is examining changes in major nutrients across sub-watersheds harvested with varying levels of stand density reduction in the South Fork of Caspar Creek. A range of treatments will be used, going from 25% reduction to a 75% reduction. Six research sub-watersheds will be harvested in 2018 at a rate of 25, 35, 45, 55, 65, and 75% reduction in leaf area, while two sub-watersheds will be established as long-term controls. The overarching objective of the third experiment is to quantify the influence of multi-aged sylvicultural systems on physical, chemical, and biological watershed processes. This experiment in the Caspar Creek watershed will result in a systematic understanding of the connection between forest canopy removal and watershed processes that can be used to develop sound management practices in similar Coastal Range watersheds in the future.
The goal of this research is to examine changes in the mass balance of major nutrients (C, N, P) and base cations/anions in four sub-watersheds of South Fork Caspar Creek in response to different levels of stand density reductions. The specific objectives are:
- Determine the changes in stream water and soil water solute concentrations and nutrient fluxes during storm flow and baseflow conditions prior- and post-harvest in the South Fork Caspar Creek watershed.
- Compare nutrient export between harvested and reference watersheds.
- Examine the linkage between nutrient flux and biotic response (using macroinvertebrate data collected as part of the bioassessment study in place).
In 2018, six of the eleven sub-watersheds within the 424 ha South Fork of Caspar Creek will be harvested across a range (i.e. 25–75%) of percent leaf area removal. Three of the eleven sub-watersheds will remain non-treated to serve as controls to determine thresholds at which hydrologic and ecological functions of the watersheds may be compromised. To address the above stated hypotheses and objectives, the following research activities to be conducted during the one-year project period.
The work will focus on a watershed-wide assessment of solute and nutrient export from the harvested and reference sub-watersheds within South Fork Caspar Creek. To characterize the flow regime and biogeochemistry of Caspar Creek at near-pristine conditions, baseline sampling of stream water has been conducted in at different nodes in the South Fork watershed since April 2016 (Fig. 2). The baseline sampling has been continued over the winter of 2016/17 to capture important data during the stormflow period of Caspar Creek.
To quantify the impact of forest thinning on stream water biogeochemistry and nutrient export, solute concentrations of key elements (calcium, magnesium, potassium, sodium, chloride, sulfate) and nutrients (carbon, nitrogen, phosphorus) will be measured in stream water samples collected during and after the harvest activities for comparison to the baseline conditions. Solute concentrations are planned to be monitored for the duration of three years after the timber harvesting to reliably determine changes and shifts in biogeochemical cycling with respect to other, influential environmental factors such as climate and year-to-year variability in streamflow.
Figure: Study sites are located in a subset of gauged sub-watersheds in the South Fork Caspar Creek, with percent reduction in stand density displayed.