Identifying potential climate refugia for coastal spruce-fir forests along the Gulf of Maine

The Gulf of Maine is among the fastest-warming regions in North America, raising concerns regarding the future of its coastal forest ecosystems and how to prioritize forest conservation efforts. Spruce-fir forests exist further south of their typical range along the coast and depend on historically cool and moist conditions. In this study, we used a spatial analysis of climate data to define mesoscale climate zones in this coastal region. We compared a set of machine-learning clustering models to define these new climate zones and validated their ecological relevance using United States Forest Service Forest Inventory and Analysis data. We found that a model focused on seasonal extremes of temperature, vapor pressure deficit (VPD), and precipitation outperformed models using all climate variables or just annual averages for delineating the current distribution of spruce and fir trees. Our new climate zones clearly identified the importance of low summer temperatures and low atmospheric moisture demand (VPD), rather than annual precipitation, as key climatic drivers of spruce and fir tree persistence in this region. For cold- and moisture-dependent species, we have now identified potential climate refugia with high conservation value as well as zones of higher vulnerability. These new mesoscale climate zones provide a spatially explicit foundation that can now be used to prioritize conservation and management decisions for these highly vulnerable forest ecosystems.