Moisture, thermal inertia, and the spatial distributions of near-surface soil and air temperatures: Understanding factors that promote microrefugia

Abstract

Climate change poses significant threats to biodiversity, but some species may be able to escape its effects in small locations with unusual and stable climates (microrefugia). However, there are still great uncertainties about where microrefugia are located, and the exact role that moisture plays in buffering extreme temperatures. In this study we quantified the effects of moisture on the distribution and variability of near-surface soil and air temperatures. We collected hourly 1 cm soil and 5 cm air temperatures and humidities at 111 sites from May 2011 to March 2012. Sites were diverse in terms of elevation (2–1428 m), distance from coast (180 m–403 km), canopy cover (0–100%), topographic exposure, and susceptibility to cold air drainage. We found that variability (diurnal range) of both soil and air temperatures decreased under moister conditions. While air temperatures were related more strongly to humidity, soil temperatures were related more to vapour pressure deficit (VPD). That is, both high temperature and low humidity were required before the VPD was sufficient to dry out the soil and allow soil temperatures to vary. We then used a regional regression approach to model the spatial distribution of minimum and maximum air and soil temperatures for each day over the 10 months in terms of latitude, elevation, canopy cover, distance to coast, cold air drainage potential, and topographic exposure to the south and northwest. We found that elevation was the dominant factor explaining the distribution of soil and air temperatures under moist conditions. Other factors, such as canopy cover and topographic exposure, had a stronger influence on air temperatures whenever humidity was low. However, these factors only affected soil temperatures at times when higher temperatures combined with low humidity to produce higher VPD. Our results provide new insights into how moisture influences the spatial distribution of near-surface soil and air temperatures. Microrefugia will be more apparent under drier conditions, but climate change may affect refugia for soil and air temperatures differently. Higher temperatures will cause VPD to increase more than would be expected by any change in humidity, and refugia in terms of soil temperatures may therefore become increasingly apparent.