This research will examine the roles of forest structure and climate change on snow accumulation and runoff. The research will combine field measurements with modeling efforts to quantify the relative importance of forest structure and climate change. Three major questions will be addressed: (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? The fine-scale spatial analysis of forest-snow interactions proposed for this study are anticipated to lead to improved hydrologic modeling of watersheds that include the intermittent snow zone of maritime mountain basins, among others. The model developed will be used in classroom education and management scenarios. Educational presentations and field trips will involve the local community and students from the Muckleshoot Indian Tribal Community College.
Background: The seasonal snowpack provides critical spring and summer water supply, and contributes to forest and ecosystem health. In forested watersheds, trees have a complex influence on snow accumulation and melting, resulting in local effects like tree wells, and in larger-scale influences on the amount and timing of spring streamflow. These phenomena have long been recognized by skiers and hikers; however, predicting the overall effect of forest on snow is challenging because the net effects of forest on snow vary in space and time. In light of recent, rapid forest change due to timber harvesting, wildfire, and/or insect outbreaks, as well as earlier seasonal snowmelt related to warming temperatures, this University of Washington investigation focused on quantifying the ways in which forests affect snow processes. Results: Four winters of field data from the western slopes of the Washington Cascades indicate that open gaps in the forest accumulate about twice as much snow as dense, second-growth forests, and that snow in the gaps lasts about two weeks longer in the spring. Snow accumulation and melt timing in thinned forests, where 30% of the trees were removed, were similar to unaltered 2nd growth forest, and snow accumulation in an old growth stand fell between the closed forest and the gap. Development and testing of a new instrument to measure the amount of snow that is captured in tree branches (where the intercepted snow is more likely to melt than the snow that makes it to the ground), resulted in new data showing that trees at this western Washington study site have the highest rates of snow capture seen worldwide. Since these results are unique to one study site in a warm, maritime climate, a subsequent synthesis of studies across the globe showed a key relationship between winter climate and snowmelt timing. In short, snow lasts longer in open areas only in regions with warm winter temperatures. In much colder regions, snow lasts about two weeks longer under the forest compared to an adjacent open area. Broader Impacts: Management Applications With changing climate and demography, western cities and states may need to change reservoir operations or water storage infrastructure, with potentially costly impacts to downstream fish populations and water supply. However, if forests can be managed to retain snow longer, some of these environmental and financial impacts may be mitigated. The results from this investigation demonstrate that in areas with warm winters, including the seaward slopes of the mountains along the U.S. west coast, forest management (by cutting gaps or openings) has the potential to offset climatic warming by increasing snow duration on the landscape and delaying spring streamflow. This allows more water to be available later in the season for hydropower, agriculture, and fish flows. However, forests in colder winter climates, such as the interior western U.S., may need to be managed to retain an optimal amount of forest cover in order to maximize snow storage in the mountains. Since snowmelt occurs in these colder regions later in the spring, when the sun is higher in the sky, trees shade the snow and slow melt rates, leading to snow lasting longer under forests in colder locations. Thus, these results provide the research foundation for making management decisions. Intellectual Advances In addition to enhancing understanding of the relationship between winter climate and snowmelt timing, these results are being integrated into watershed models that are used to predict future impacts of climate change and forest change. The observational data collected as part of this investigation represents a unique contribution to the science, and these results are actively being used to test and improve mathematical models. Lessons learned from the development of a new instrument are currently being applied to a collaborative effort to collect data quantifying snow capture in trees in several sites across the U.S., which will support increased understanding and improved model representation of this key process. Education & Communication This investigation is relevant to watershed and forest managers, students studying hydrology, and the larger community interested in understanding and adapting to future impacts. As such, the investigation included a multitude of meetings with collaborators in management roles, presentations and discussion with undergraduate and graduate courses, and communication with the greater community through press releases, videos posted to our website (http://students.washington.edu/dickers/Research.html), and presentations.
