Individual- and scattered-tree influences on ultraviolet irradiance

Many of the potential effects of ultraviolet radiation (UVR)—damage to materials, altered herbivory of insects and activity of microbes, modified growth of vegetation, and adverse or beneficial effects on human health—are modified by the presence of trees that influence UVR exposure to various degrees. Though tree effects on total solar irradiance have been investigated often by measurements and modeling, tree influences on UVR, particularly in the ultraviolet B (UVB, 320–280 nm), differ substantially from tree influences on the rest of the solar spectrum, and thus the ratio of UVB to photosynthetically active radiation (PAR) is altered. Trees greatly reduce both UVB and PAR irradiance in their shade when they obscure both the sun and sky. Beneath dense forest canopies, relative irradiance (Ir=irradiancebeneath trees/above-canopy irradiance) for both UVB and PAR radiation may be 0.01–0.02. In the shade of a single tree, Ir on the horizontal in the PAR and total shortwave (SW) was about 0.1, whereas in the UVB and ultraviolet A (UVA, 320–400 nm), Ir was about 0.4. Conversely, where direct beam radiation came through gaps between crowns in a group of deciduous trees in winter, PAR Ir values averaged 0.95 and UVB Ir averaged only 0.41. In comparisons of minimum values of Ir on horizontal and the south-facing vertical surfaces in tree shade for UVB, UVA, SW, and PAR, where UVB Ir on the horizontal ranged from 0.22 to 0.62, depending on solar zenith angle, UVB Ir on the vertical ranged from 0.05 to 0.27. UVB Ir consistently exceeded UVA Ir on both the horizontal and vertical surfaces. PAR and SW Ir differed little between horizontal and vertical surfaces in tree shade. Modeled average Ir on the horizontal below a regular grid of ellipsoidal tree crowns was given byIp=1−m−(θ0.711/5.05)sin(πm), where m is fraction of area covered by tree crowns and θ is solar zenith angle. The tree influences will vary with pollutants in the boundary layer, which affect scattering of UVR.