Abstract
Freshwater ecosystems in mountain landscapes are threatened by climate change. Accumulated heat can result in lethal short-term heat exposure, while velocity of change governs severity and rates of long-term heat exposure. Here, we novelly integrate heat accumulation and velocity of change approaches to classify climate-vulnerable USA mountain watersheds. We combine watershed position and air temperature data to calculate degree-days. We then calculate the current velocity of this change and used discriminant function analyses to classify watershed vulnerability through 2100. Our results demonstrate how rates of heat accumulation are increasing across mountain landscapes. We estimate 19% of watersheds are at greatest vulnerability to accumulated heat, and this will increase to 33% by 2100. Further, mean killing degree days (i.e., region-specific mean number of days above 90th temperature percentile) are projected to increase 215-254% (mean = 236%) over this same time frame. Together, results indicate heat accumulation will increase substantially over the next 75 years; changes are projected to be most severe in lower elevation landscapes and those with greatest historical velocity of change. These changes will likely restructure species' distributions. Decision-makers can use these classifications to better understand landscapes, species' needs, and ecosystem services, thereby enabling effective allocation of conservation resources.