Urban trees for ultraviolet protection. A 440-tree, multi-species field study in Hanover, Germany

This study investigates solar ultraviolet (UV) reduction by urban trees to mitigate the risks posed by increasingly harmful UV radiation in urban areas. To date, there is a lack of comprehensive comparisons in terms of UV reduction with different species. In this study 440 individuals from 52 genera were systematically measured to quantify UV-A and UV-B reduction. All trees were planted simultaneously in 1998 at the same age, share identical site and tree-pit conditions, at the same city location in Hanover, Germany and have received uniform arboricultural care. Additional data (crown shape, leaf shape, deciduous/coniferous, canopy openness, luminance [luma], origin, vitality) were collected and analysed for each individual. Tree canopies reduced UV by up to 96.5%. Tilia has one of the highest levels of UV reduction at the genus, species, and variety levels. Even when compared with climate-resilient trees, Tilia trees were among the highest-performing. This work identifies criteria to inform new tree plantings. Healthy trees with low canopy openness generally provide greater UV reduction. Cordate leaves and round crowns were also associated with higher UV reduction. New plantings in urban spaces should be selected not only for their design qualities and site requirements, but also for their UV-reduction potential. Additionally, appropriate maintenance to preserve tree health is essential to sustain high vitality and canopy density, thereby maximising UV reduction. Findings of this study give indications for more specific tree selection that minimise UV exposure and promote public health.