Tree canopy macrostructure controls heating of asphalt pavement in a moist-temperate urban forest

Tree canopies influence the urban microclimate by controlling solar radiation reaching the ground surface. Temperature gradients are easily measured between shaded and exposed asphalt pavement, but it is not clear how differences in canopy structure affect pavement heating or whether temperature variation is sufficient to affect the pavement’s structural integrity. In this study, we document pavement heating in response to solar exposure through a diurnal cycle. Pavement temperature was measured under and near thirteen street-side trees in a residential neighborhood in the Ohio Valley, USA. Temperature variation was compared with canopy coverage and structural elements including tree size (dbh), species, aspect, and canopy density. Pavement temperature in open-sky plots increased by 29–34 °C through the course of a day but only 8–15 °C in plots under tree canopies. Under-canopy plots showed intermittent heating caused by side lighting early and late in the day defined by aspect, crown geometry, and the position of adjacent trees. Observed temperature ranges suggest substantially lower tensile stress in pavement under tree canopies than in sections exposed to direct sun. Pavement temperature did not differ significantly with tree size or canopy porosity measured in hemispherical photos. We conclude that temperature in unshaded areas varies within ranges likely to reduce pavement service life, and that shading by street trees has the potential to moderate such damage. Pavement temperature is controlled by tree location and crown geometry (urban forest macrostructure); properties of individual tree canopies (microstructure) appear less important.