Tree Traits and Microclimatic Conditions Determine Cooling Benefits of Urban Trees

Abstract

Trees play a key role in mitigating urban heat by cooling the local environment. This study evaluated the extent to which street trees can reduce sub-canopy air temperature relative to ambient conditions (ΔT), and how ΔT relates to tree traits and microclimatic variables. Air temperature under the canopies of 10 species was recorded within residential areas in Western Sydney, Australia, during summer 2019–2020. Tree and canopy traits, namely tree height, specific leaf area, leaf dry matter content, leaf area index, crown width and the Huber value (the ratio of sapwood area to leaf area) were then measured for all species. Species differed significantly in their ΔT values, with peak cooling (maximum ΔT −3.9 °C) observed between 9–10 am and sub-canopy warming (i.e., positive ΔT values) typically occurring during afternoon and overnight. Trees with high LAI and wider canopies were associated with the greatest daytime cooling benefits and lower levels of nighttime warming. ΔT was also negatively related to windspeed and vapor pressure deficit, and positively to solar irradiance. This study provides valuable information on how tree characteristics and microclimate influence potential cooling benefits that may aid planning decisions on the use of trees to mitigate heat in urban landscapes.