Canopy traits and climatic conditions determine cooling benefits of urban trees

Mahmuda Sharmin, Manuel Esperon-Rodriguez, Mark G. Tjoelker, Paul Rymer and Sally A Power, Hawkesbury Institute for the Environment, Western Sydney University, Richmond, NSW, Australia, Sebastian Pfautsch, Western Sydney University, Parramatta, NSW, Australia

Background/Question/Methods
Trees play a key role in mitigating urban heat by cooling the local environment. However, the extent to which trees can provide such temperature benefits is influenced by differences in species’ functional traits, canopy architecture and prevailing climatic conditions. This study aimed to 1) evaluate the extent to which different urban tree species can reduce sub-canopy air temperatures, relative to ambient conditions (ΔT) and 2) identify how ΔT relates to tree canopy traits and local climatic variables. To measure these, we undertook high-resolution monitoring of air temperature using sensors installed at the base of tree crowns (i.e. subcanopy; 3-4 m height above ground) of the ten most abundant street tree species in Western Sydney, Australia, during summer (December 2019 to February 2020). Ten replicates per species were selected within low-rise residential areas comprising a mix of impervious and green surfaces; measurements included tree height, specific leaf area, leaf area, leaf area index (LAI), canopy width and Huber value (the ratio between sap wood area and leaf area). Data from the nearest meteorological stations were sourced to calculate ΔT (subcanopy-ambient air temperature) and extract climatic variables e.g. vapor pressure deficit (VPD), radiation and days since it last rained.
Results/Conclusions
The magnitude of difference between subcanopy and ambient air temperatures (ΔT) was found to depend on climatic variables and tree characteristics (Rc2= 0.40, p<0.001). During daytime, sub-canopy temperatures were lower than ambient, including during periods of high solar radiation. The magnitude of cooling was however reduced at high VPD. Sub-canopy night-time temperatures were generally warmer than ambient air, especially under conditions of low VPD. Trees with wide and dense canopies (i.e. high LAI) were associated with the highest ΔT, while those with higher Huber values had lower ΔT values. ΔT differed among tree species with the biggest daytime cooling observed for Platanus × acerifolia (ΔT = -1.0 °C) and the lowest for Jacaranda mimosifolia (ΔT = 0.0°C). Our findings provide new information on tree characteristics related to potential cooling benefits that may aid planning on the use of trees in urban streetscapes to mitigate heat.