Retreat to refugia: Severe habitat contraction projected for endemic alpine plants of the Olympic Peninsula

Abstract

The unique geography of the Olympic Peninsula in Washington State, USA has promoted a flora with exceptional endemism that may face high rates of extinction due to climate change. The Olympic Mountains provide an unparalleled opportunity to test predicted responses in topographically complex environments and inform conservation plans for threatened alpine taxa. We estimated the potential impact of climate change for five endemic alpine plants of the Olympic Peninsula. We used Principal Components Analysis (PCA) to visualize changes in climate niche-space in the alpine zone and the exposure of the endemics to changing conditions. We then constructed climate and topographic-based Species Distribution Models (SDMs) to estimate changes in the distribution of habitat for each taxon. The Olympic alpine zone will experience novel environmental conditions in the future, with a pronounced decrease in winter snow and an increase in growing-season moisture stress. When topography was taken into account, the losses were more dramatic than the impact of climate alone. Indeed, 85-99% of the suitable habitat will be lost for each of the five focal taxa by 2080, with thermal refugia remaining only on the highest peaks of the eastern Olympics. The Olympic alpine endemics are stranded on ever-shrinking habitat islands. Within a few, isolated thermal refugia, the distribution of micro-topography may be a critical factor in determining long-term survival. The Olympic Mountains and their alpine taxa are a model system for endemics worldwide, illustrating the biogeographic characteristics that underpin vulnerability to climate change.