A climate-vulnerable species uses cooler forest microclimates during heat waves

While climate change will have significant impacts on terrestrial organisms, microclimates offer potential refugia for vulnerable species when regional conditions become unsuitable and temperatures exceed physiological limits. The efficacy of microclimates as buffers to extreme events is determined not only by their availability, but also by the behavioral flexibility of individuals to access these unique environments during stressful periods. We characterized (i) how forest structure and topography shaped microclimates at roost sites used by California spotted owls (Strix occidentalis occidentalis), a climate-vulnerable species, and (ii) how owls outfitted with temperature sensors used microclimates during warm temperature events. Higher elevations, taller canopies, and greater canopy cover promoted cooler maximum temperatures in stands used for roosting. Within those stands, individuals roosted in cooler sites, especially when roosts were in forest stands on warmer slopes. Tall canopies created relatively cooler microclimates when temperatures were hot, and individuals actively used roost sites that minimized increases in operative temperature, suggesting spotted owls have an adaptive capacity to use cooler microclimates. However, roosts at low elevations consistently exceeded physiological thresholds when temperatures were warm, potentially explaining vacancies in low elevation territories with more open forest. For spotted owls to persist in warming environments, conserving tall, closed-canopy forests that promote cooler microclimates for roosting is critical. While a future climate promises heat events that approach and exceed thermal extremes for a suite of canopy-dwelling species, cooler microclimates provided by forest structure and topography could constitute important refugia in a rapidly changing system.