Effects of forest stand density reduction on nutrient transport at the Caspar Creek Watershed

Healthy headwater streams are critical to protecting downstream water quality for many natural ecosystems and societal water uses. Attention to the critical role that headwater watersheds play in providing clean surface water for ecosystems and human consumption has been magnified in recent years as threats to their sustainability continue to intensify through land use change, urbanization, timber harvesting, and climate change induced extreme events and wildfires. Many of the headwater catchments in the United States and across the world are forested, with forests providing several functions to their aquatic ecosystems such as shade, nutrients, soil moisture storage, erosion and flood control, and buffers for extreme events. Yet, many forests also provide food, fuel and fiber for humans, hence, timber harvest is a frequent management activity and ecological disturbance event in many headwater watersheds.

Decades of research on the impact of timber harvest or deforestation on downstream aquatic ecosystems have shown that timber harvest in general increases total water yield (Fulton and West, 2002). However, there is less agreement on how timber harvest affects biogeochemical processes, nutrient export, and stream water chemistry. Previous studies have shown that biogeochemical trends observed after timber harvest are often confounded by specific watershed characteristics, such as watershed area, forest and vegetation types, soil types, slope, aspect, and light availability, to name a few (Lamontagne et al., 2000; Bernhardt et al., 2003). Determining the relationship between stand density reduction, streamflow, and fluxes of major nutrients and water chemistry parameters across a range of harvest intensities allowed identifying relationships and thresholds at which reducing stand density may begin to affect biogeochemical processes to the extent that nutrient export and stream chemistry are significantly affected.

The goal of this study was to quantify the effect of different levels of stand density reduction on the mass balance of major nutrients (carbon, nitrogen, and phosphorus) and base cations and anions in the South Fork Caspar Creek. Our main hypothesis was that stand density reduction will increase export of total N, total P, nitrate, and dissolved organic carbon from the treated watersheds immediately following the timber harvest, with greater effects observed with greater stand density reduction. The increased export of nutrients and base cations/anions is further hypothesized to be facilitated by rapid flow pathways and increased hydrologic connectivity associated with macropore flow and fast subsurface stormflow above the clay-rich, argillic soil horizon present in the watershed. This study is addressing the following specific research questions:

1. What are the temporal (annual, seasonal) variations and patterns of nutrient and base cation/anion fluxes from coast redwood forests?
2. How do different stand density reductions change the patterns, concentrations and fluxes of nutrients and base cations and anions compared to pre-harvest conditions?