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

Abstract: Premise of the study- The unique geography of the Olympic Peninsula has created a flora with exceptional endemism that may face high rates of extinction due to climate change. Conserving endemic taxa requires a deeper understanding of general and individual species' responses to warming in topographically complex environments. The Olympic Mountains provide an unparalleled opportunity to test predicted responses and inform conservation plans for threatened alpine taxa. Methods- I developed Species Distribution Models (SDMs) for five endemic alpine plants of the Olympic Peninsula to estimate the potential impact of climate change on these rare taxa. First, I created high-resolution raster surfaces on the Olympic Peninsula with climatic and topographic variables for the current period and the year 2080. I used Principal Components Analysis to visualize changes in climate niche-space in the Olympic alpine and the exposure of the five endemics to changing conditions. I then constructed climate and topographic-based SDMs, to estimate changes in the distribution of habitat for each taxon. Finally, I identified potential thermal refugia as areas where suitable habitat conditions would remain by the year 2080. Key Results- The Olympic alpine will experience novel environmental conditions in the future, with a pronounced decrease in winter snow and an increase in growing season moisture stress. Suitable habitat will be greatly reduced for all five focal taxa, with thermal refugia remaining only on the highest peaks of the eastern Olympics. Topographic SDMs suggested further decreases in the total amount of projected suitable habitat. Furthermore, I identified differences in the proportion of suitable micro-topography within each taxon's climate envelope that greatly affect the species-specific climate change prognosis. Conclusions- The Olympic alpine endemics are stranded on ever shrinking habitat islands, and are projected to lose 85-99% of their suitable habitat by 2080. Their survival depends on their ability to persist in isolated thermal refugia on the highest peaks. Within these refugia, the distribution of micro-topography may be a critical factor in determining the long term survival prospects of threatened taxa. The Olympic Mountains and their endemic taxa are a model system illustrating the biogeographic characteristics that underpin vulnerability to climate change. Globally, taxa with narrow distributions that are already occupying the coldest locally available habitats should be considered extremely vulnerable, and may be at the forefront of climate-driven extinctions.