DO FOREST-SNOW FEEDBACKS MITIGATE OR EXACERBATE A MOUNTAIN’S HYDROLOGIC RESPONSE TO A WARMING WORLD?

In mountains subject to relatively warm winters (such as the Pacific Northwest and California), more snow accumulates and snow lasts longer in openings than beneath the forest canopy. This effect is even more pronounced in warmer winters. For example, in the record warm year of 2015, snow cover at one site in the Washington Cascades lasted 96 days longer in an opening than under an adjacent forest. Model simulations suggest that with warming temperatures, snow-fed seasonal streamflow will come earlier and drop to lower flows in a fully forest-covered basin compared to an identical basin without trees. In other words, forest cover exacerbates the hydrologic impact of climate change. However, an increased frequency of warm and dry winters would also result in massive forest die offs, which have recently been observed worldwide due to fires, insect infestations, and drought. Such forest removal could lead to more forest gaps and openings and relatively better snow accumulation and retention, effectively mitigating a basin’s hydrologic response to warming. Here we review distributed observations and modeling, exploring the timescales and feedbacks between forest change, snow change, climatic change, and runoff in mountains of the Western United States, with a discussion of where the scientific community should focus to provide meaningful predictions in such a dynamically changing and inter-related landscape.