Belowground sensors capture spatiotemporal variation in urban heat island effect

It is well established that as urbanization increases, so does temperature due to a phenomenon called the urban heat island effect (UHI). However, there has been less examination of spatial and temporal variation in the degree of the UHI in the soil which many organisms utilize as habitat. This is partially due to constraints associated with the high cost of installing sensors that accurately record soil temperatures and the inability of satellites to capture variation at small spatial scales. To overcome those constraints, we installed small, low-cost temperature sensors in the ground, just below the surface. We analyzed data from sensors in sunny and shaded locations at 27 sites along an urbanization gradient in St. Louis, Missouri, USA, to examine how the relationship between urbanization and soil temperature varies between microhabitats and times of day over the months March-August. Variation in soil temperature was explained by interactive effects of these spatial (urbanization context and microhabitat) and temporal (month and day) variables. Specifically, we detected an UHI only in shaded locations during the summer months of June and July. The UHI effect was somewhat stronger at midnight and sunrise compared to during the daytime. For studies of the UHI and its potential impacts on particular species or ecological communities, these results underscore the importance of recording temperature during the seasons, times of day, and microhabitats experienced by the organisms of interest. To facilitate future studies of microclimate in urban ecosystems, we offer methodological suggestions for improved implementation of belowground temperature monitoring.