Urban air temperature modification by tree canopy and impervious surface in eastern (USA) cities

Keywords: urban heat, urban tree canopy, gam, air temperature
Session Type: Virtual Paper Abstract
Day: Friday
Session Start / End Time: 2/25/2022 09:40 AM (Eastern Time (US & Canada)) - 2/25/2022 11:00 AM (Eastern Time (US & Canada))
Room: Virtual 39

Abstract

Tree cover is generally associated with cooler air temperatures in urban environments but the roles of canopy configuration, spatial context, and time of day are not well understood. The ability to examine spatiotemporal relationships between trees and urban climate have been hindered by lack of appropriate air temperature data and, perhaps, by overreliance on a single “tree canopy” class, obscuring the mechanisms by which canopy cools. Our first study leveraged >70,000 air temperature measurements collected by car throughout Washington, DC, USA in predawn, afternoon, and evening campaigns on a hot summer day. We subdivided tree canopy into “soft” (over unpaved surfaces) and “hard” (over paved surfaces) canopy classes and further partitioned soft canopy into distributed (narrow edges) and clumped patches (edges with interior cores). At each subdivision, we predicted air temperature anomalies using generalized additive models (GAM) for each time of day. Soft canopy cooled nonlinearly in the afternoon with minimal effect until ~40% cover but linearly across all cover fractions in the evening. Patches cooled at all times of day, whereas distributed canopy cooled strongly in the evening due to increased shading. This latter finding is important for mitigation of urban heat since it is easier to plant individual trees than new patches.

Air temperature data collected using identical methods is now available in eastern US cities, typified by deciduous-mixed forest. In this work, we expand our DC analyses geographically and examine whether the relationships established above are consistent across climate zones, urban development patterns, and tree species assemblages.