Feeling the heat: the role of microclimates in buffering extreme temperatures

Climate change is a threat to global biodiversity, with predicted changes to mean temperatures and increasing frequency and intensity of extreme weather events, such as heatwaves. Heatwaves in particular pose a threat to species’ persistence, as individuals may be exposed to temperatures above their physiological tolerance. However, individuals rarely experience temperatures measured at the macroclimatic scale: instead, small changes in the environment (such as topography or vegetation) result in microclimates that differ from the macroclimate, and can provide cool refugia for individuals during heatwaves. However, to date little is known about the stability of microclimates with increasing air temperatures. In this study, we recorded microclimate temperatures across a range of different microhabitats within a calcareous grassland nature reserve in Bedfordshire, UK, in 2018, 2019 and 2022. During this time, six heatwave events occurred, including the highest air temperatures ever recorded in the UK. We investigated whether the microclimatic refugia provided by microhabitat structures changed with increasing macroclimatic temperature. The ability of microhabitats to buffer increasing air temperatures varied with topographic aspect, slope, amount of bare ground, shelter, vegetation height, and vegetation type, with encroaching scrub, high levels of shelter, tall vegetation, and north-facing slopes showing the strongest abilities to maintain relatively stable microclimate temperatures across increasing air temperatures. However, no environmental structures were able to maintain microclimates below the macroclimate temperatures during heatwaves. Microclimate temperatures were amplified close to the ground, whereas at 50 cm height temperatures were more stable and similar to the macroclimate temperature, implying that ground-dwelling species may be particularly vulnerable to extreme heat. We identified a breakdown in the ability of microhabitats to maintain cool refugia above 7°C, implying that microclimates may be unable to maintain cool refugia under extreme heat. Our results suggest that microhabitats and their associated microclimates alone are unlikely to protect individuals from extreme temperatures, with many microclimates amplifying the effects of climate change rather than mitigating them.