Biologically-effective UV-B exposures in understories of forest canopies: potential impacts of climate change

The herbaceous layer of deciduous canopies consists of many plant species that develop prior to the leafing out of the overstory. Most of the solar UV exposure of the understory herbaceous layer is received during the over-canopy leaf-out. The penetration of UVB radiation into the understory of a deciduous forest canopy diminishes as the canopy develops overhead. During the leafing-out period in mid-latitudes there are many changes in ozone column and cloud cover associated with weather systems resulting in a highly variable solar radiation period. To what degree does the above-canopy variability influence the UVB exposure of the herbaceous layer?

The solar UV radiation that reached the herbaceous layer was measured in spring 2004 using a portable ultraviolet multi-filter rotating shadow band radiometer (UV-MFRSR) and compared to the UV-MFRSR at the West Lafayette UVB Climate Monitoring Station to determine the temporal penetration of UVB through the canopy to the understory. Canopy view factors were determined from hemispherical photography. Ozone column depth was determined from the UV-MFRSR measurements. The UV-MFRSR measures the diffuse and global solar irradiance in seven narrow (2 nm nominal FWHM bandwidth) wavebands at nominal center wavelengths of 300-, 305-, 311-, 317-, 325-, 332-, and 368- nm. Direct beam irradiance is calculated by subtraction of diffuse from the global irradiance. Irradiance measurements in the understory were recorded for periods of approximately 2 hours centered on solar noon on various days during the leafing out of the deciduous forest. The diffuse and direct beam UVB irradiance in sunlit and shaded regions of the understory were modeled for solar zenith angles between 15o and 60o and sky conditions from cloud-free to overcast using a three dimensional geometric model. Regression equations were developed to estimate the shade and sunlit relative irradiances across the entire understory domain for each cloud cover fraction as functions of the solar zenith angle and canopy cover. These equations were then used to predict the spatial variation in irradiance received by understory vegetation of a canopy leafing out in the spring under the measured conditions. The measured diffuse and direct beam irradiance was compared to modeled irradiance of a three-dimensional canopy radiation transport model that accounted for cloud cover, ozone column depth, and canopy density to evaluate the effect of changing cloud cover and ozone column depth on biologically effective UVB exposure under the canopy. Changes in penetration of UVB radiation at mid-latitudes due to changes in ozone column depths and cloud cover were then evaluated to determine if the herbaceous layer would be likely to receive more or less UVB exposure in the future.