Differential Stomatal Responses to Surface Permeability by Sympatric Urban Tree Species Advance Novel Mitigation Strategy for Urban Heat Islands

As urbanization draws more people to metropolitan areas, a steadfast increase in impervious surfaces ultimately contributes to a pronounced urban heat island effect. While city greening strategies to mitigate urban thermal effects often tout street-tree cover expansion, many plant species are susceptible to heat stress, limiting survivorship, primary productivity, and ecosystem services. Our research objective was to characterize how urban imperviousness impacted the photosynthetic traits of four sympatric tree species in Old Town La Verne, California. We found that while Camphor trees ( Camphora officinarum ) and Carrotwood trees ( Cupaniopsis anacardioides ) did not differ significantly in photosynthetic traits at sites with impervious and pervious surfaces, both Coast Live Oak trees ( Quercus agrifolia ) and Olive trees ( Olea europaea ) showed significant differences in leaf stomatal length and density. Our findings suggest that the photosynthetic traits of some exotic tree species may be less susceptible to surface permeability than either native or floristically indigenous tree species. We propose that urban greening initiatives adopt a temporal strategy for mitigating urban heat island effects, starting with an urban canopy composed of exotic trees more resilient to impervious surfaces and later transitioning to a recombinant canopy ecology of floristically relevant tree species suited for the soil permeability native to southern California.