Biologically effective UV-B exposures of an oak-hickory forest understory during leaf-out

Abstract

Although it is known that many plant species that are sensitive to solar UV radiation exist in low radiation environments, little is known about the sensitivity of young tree seedlings and herbaceous forest vegetation that inhabits the low radiation environment of the forest understory. Furthermore, little is known about the UV radiation environment of this vegetation during the deciduous forest leaf-out period when solar radiation levels are the highest. The development of the forest canopy and the penetration of solar UV radiation through the canopy to the understory of a deciduous upland forest was studied between 15 April and 1 June of 2004. During the early part of the leaf-out period, the mean canopy transmittance in the PAR exceeded that in the UV-A or UV-B. By 20 days into the leaf-out period, the PAR transmittance was nearly equal to that of the UV. As the leaves in the canopy reached full expansion, the PAR, UV-A and UV-B penetration was essentially equal and small. Canopy transmittance during leaf-out was not dependent on cloud cover. Greater penetration of solar radiation in longer wavelengths within the UV was probably a result of scattering of direct beam radiation off tree bark in the canopy. The effective canopy transmittance from 15 March through the canopy leaf-out was 47%. For an expected period of UV exposure for the herbaceous layer of 15 March through 15 May, the tree seedlings and herbaceous plants received 41 kJ m −2 of biologically effective UV-B radiation. It is projected that the herbaceous layer vegetation will receive approximately 27% more UVB BE radiation on clear days with a decreased stratospheric ozone of 20 DU and approximately 3% more UVB BE radiation with increased temperatures causing leaf-out to begin 10 days earlier than 2004. More research is needed to understand the potential interactions of these and other climate change components on the deciduous forest.