TreesAndSnow

Vegetation changes in maritime mountain basins contribute to shifts in snowmelt and streamflow as

much as projected climate change. Over the past 80 years, forest harvesting throughout the western U.S. and western British Columbia has resulted in a shift in the dominant forest structure to young, dense

forests. Declining trends in mountain snow water equivalent (SWE), summer river flow, and earlier snowmelt have largely been attributed to climate with little consideration of the role vegetation plays in these hydrologic processes. Because snowpack accumulation and melt rates depend on forest structure as well as climate, silvicultural manipulation has the potential to increase snow water storage, change the timing of snowmelt, and restore related ecosystem services, such as fish habitat.

The study site is in the Cedar River Municipal Watershed (CRMW) located in western Washington, which provides water to more than 1.4 million people and also supports a significant anadromous fishery. The watershed is located in the intermittent snow zone, where the snowpack is most sensitive to climate change. While overall snow accumulation is important, this study is primarily concerned with snowmelt timing, since it is crucial to watershed management in basins with limited reservoir storage, as is the case in most of the Pacific Northwest. Any delay in snowmelt from silvicultural manipulation could reduce winter flood hazards and increase summer runoff when water demand for cities, agriculture, and stream ecology is the highest.

Using a combination of distributed field measurements, combined with point and distributed hydrologic models, we propose to address three major questions:

1) What is the variability of snow interception and melt in different forest structures and classes?

2) Over multiple decades, which forest changes are of comparable magnitude to climate change in their effects on snow water storage? Which forest changes exacerbate earlier runoff, and which retain snow longer on the landscape?

3) How can silvicultural practices be used in watershed management to help offset the effects of projected climate change?