What environmental features give rise to thermal refuges? A systematic review

Thermal refuges – thermally buffered locations where organisms avoid exposure to unfavorable temperatures – are emerging as one the largest hopes for the persistence of populations during climate warming. The body of literature on the causes and consequences of thermal refuges has grown rapidly in the past decade; however, no synthetic effort has addressed the current status of research regarding what environmental features (referred to here as ‘determinants') moderate high temperatures at biologically relevant spatial scales. Resolving the determinants of thermal refuges for different taxa will contribute to global conservation by aiding the identification of thermally suitable habitat under climate change. We perform a systematic literature review and assemble a database of 751 publications focused on contemporary, cold thermal refuges to 1) assess how commonly studies have identified determinants of thermal refuges, 2) determine patterns in the definitions, focal organisms, biomes, and geographic regions of thermal refuge research, and 3) identify the major features that promote cold thermal refuges across ecosystems. We find that research on the determinants of thermal refuges is recent (median publication year 2018) and has been a minority focus (17%) of the overall literature on thermal refuges. Terrestrial environments (50%, led by temperate forests) and freshwater environments (30%, dominated by rivers and streams) were most represented. Determinant studies generally identified thermal refuges based on abiotic features (73%) rather than species occurrence or physiological thresholds. The studies that focused on determinants described 42 unique features that moderate high temperatures, representing both abiotic features (led by groundwater inflows and topography) and biological features (primarily vegetation cover). Many determinants can be actively managed to enhance an environment's ability to provide thermal refuges during global change. We conclude by recommending future directions for understanding what gives rise to thermal refuges.