Spatial configuration and time of day impact the magnitude of urban tree canopy cooling

Abstract Tree cover is generally associated with cooler air temperatures in urban environments but the roles of canopy configuration, spatial context, and time day are not well understood. The ability to examine spatiotemporal relationships between trees climate has been hindered by lack appropriate temperature data and, perhaps, overreliance on a single ‘tree canopy’ class, obscuring mechanisms which cools. Here, we use >70 000 measurements collected car throughout Washington, DC, USA predawn (pd), afternoon (aft), evening (eve) campaigns hot summer day. We subdivided tree into ‘soft’ (over unpaved surfaces) ‘hard’ paved classes further partitioned soft distributed (narrow edges) clumped patches (edges interior cores). At each level subdivision, predicted anomalies using generalized additive models for found that all-inclusive class cooled linearly at every (pd = 0.5 °C ± 0.3 °C, aft 1.8 0.6 eve 1.7 0.4 °C), could be explained aggregate effects predominant hard cooling low high cover, respectively. Soft nonlinearly minimal effect until ?40% strongly (and linearly) across all fractions 0.7 1.1 2.0 2.9 °C). Patches times despite uneven allocation city, whereas more due increased shading. This later finding important heat island mitigation planning since it easier find planting spaces rather than forest patches.