PS 48-231 - Differential effects of urban heat on street tree leaf characteristics

Abstract: In urban areas, spatial heterogeneity of land cover results in highly variable temperatures. Because unvegetated areas are correlated with higher temperatures, a commonly proposed solution for ameliorating elevated urban heat is to plant more trees. Many studies have investigated how trees affect temperature via cooling through shading and transpiration. However, fewer studies have provided insights into the inverse - how heat affects urban trees and the ecosystem services they provide.



The goal of this study was to determine how urban street trees respond to heat. We hypothesized that leaf characteristics would exhibit phenotypic plasticity in response to differences in surface temperature. Specifically, in hotter areas we expected leaves to be smaller and more dissected for more efficient leaf convective cooling.



We measured leaf characteristics of trees growing in relatively hot and cool areas of Sacramento, California. We studied two common street tree species, London plane (Platanus acerifolia, a non-native hybrid) and valley oak (Quercus lobata, a planted native species). Using land surface temperature data from satellite imagery and Sacramento’s public tree inventory, we located trees in hotter and cooler areas and collected their leaves. As response variables, we measured leaf area, leaf mass per area, and leaf perimeter per area – all parameters which have been shown to respond to temperature. We also noted the presence of irrigation, and measured tree height and diameter at breast height as covariates.



We found that valley oak leaves were smaller and more dissected in hotter urban areas, supporting our hypothesis. London plane leaves showed no variation between hotter and cooler areas. These results suggest that the greater phenotypic plasticity of valley oak leaves may allow them to acclimate to elevated heat, while London plane leaves appear less equipped to do so.



Urban tree responses such as the ones we measured might be used to inform urban tree species selection as the climate warms; specifically, species that show phenotypic plasticity may be better candidates for future plantings. Subsequent studies could explore whether leaf characteristics represent holistic tree responses to heat, and whether these responses translate into ecosystem services.