Microclimate buffering by epiphytes reduces climate overlap across elevations

Abstract

Thermal variation over space and time shapes global patterns of biodiversity and responses of species to climate change. Vertical microclimate gradients from the understory to the canopy mediate biogeographic patterns of arboreal species across larger spatial scales. Yet, the magnitude to which microhabitats in the canopy, such as large epiphytes, alter the exposure of arboreal species to hot and variable canopy conditions across biogeographic gradients is not well known. We first examine the extent to which maximum temperatures and temperature variation in epiphytic bird's nest ferns (BNFs) and the understory are lower than those in the canopy across an elevation gradient in the Philippines and the environmental factors influencing these differences. We find that differences in maximum temperatures (i.e., offset) and temperature variation (i.e., buffering) between the canopy and BNFs are greater than differences between the canopy and understory. Offset and buffering are largest when canopy temperatures are high and are positively impacted by precipitation. Furthermore, we address the theory that exposure to higher thermal variation in the canopy may increase the range sizes of arboreal species. We find that lower thermal variation over time in BNFs reduces the thermal overlap between low and high elevations relative to canopy temperatures. Arboreal species that shelter in BNFs during the day may therefore have narrow physiological tolerances and consequently smaller elevational ranges. By providing thermal regimes more similar to the understory than the canopy, BNFs may therefore effectively function as “arboreal soil”, mediating impacts of arboreality on patterns of biodiversity and trait variation.