Evaluating the ultraviolet protection factors of urban broadleaf and conifer trees in public spaces

Abstract

Urban trees provide natural shade and moderate human exposure to solar ultraviolet radiation. To date, most studies quantifying UV attenuation by urban tree canopies have taken place in Australia, with no studies in North America. Few studies have utilized sensors sensitive to UV-B radiation, although the shorter wavelengths are more important in determining erythemal (skin burning) UV. We collected solar UV radiation exposure data beneath 64 individuals of 16 tree species commonly planted in the City of Toronto's schoolyards and public parks, using UV electronic logging dosimeters that have a spectral response closely matching the erythemal action spectrum. Additional data were collected on canopy structure (crown radii, crown transparency, crown depth, diameter at breast height, height to live crown, and leaf-level data). UV protection factor (PF: the ratio of open-site UV to below-canopy UV) varied 2.6-fold among species, ranging from ∼1.3−3.4. Statistical models for variation among trees indicated that crown transparency (%), the ratio of crown breadth to height to live crown, and species shade tolerance were important predictors of PF. Acer platanoides ‘Crimson King’, Celtis occidentalis L., Quercus bicolor Willd., and Fagus sylvatica ‘Purpurea’ showed the highest PF values (>3), and Ginkgo biloba L. and Acer rubrum L. showed the lowest PF values (<2) among sampled species. Findings from this study can help inform tree management strategies and policies to increase UV protection in schoolyards and other public settings.