| Enhancement of ultraviolet radiation was considered in relationship to its effect on tree foliar chemistry, and to cascading effects exhibited at a higher trophic level. The specific objectives of this study were: (1) investigate foliar phenolic response to enhanced ultraviolet-B radiation (UV-B) in several woody species; (2) establish a link between that phenolic response and the success of an herbivore dependent upon that foliage.; Pinus ponderosa, Populus trichocarpa, Pseudotsuga menziesii, and Quercus rubra are important North American forest tree species with contrasting leaf morphologies. They were exposed to enhanced (2X), ambient (1X) and/or subambient (0X) levels of biologically effective UV-B radiation for a period ranging from three months to three years. Chromatographic analyses of treated foliage illustrated the age-, species-, and morphological-specific effects of UV-B radiation on both quantity and composition of foliar UV-B absorbing phenolics. These phenolics protect the plant from excessive, damaging UV-B radiation, but are also biologically active in plant-plant, plant-herbivore, and herbivore-predator interactions. Phenolics are toxic compounds that generally reduce consumption efficiency of herbivores; however, some phenolics are feeding stimulants. The influence of enhanced UV-B radiation across two-trophic levels was tested by analyzing the behavior of Chrysomela scripta (an important co-evolved herbivore of Populus species) that was fed foliage exposed to each of the UV-B radiation treatments. Foliage exposed to enhanced UV-B radiation contained higher concentrations of phenolics, including increases in a known feeding stimulant: salicortin. Insects developing on foliage exposed to enhanced UV-B radiation had reduced growth efficiencies, but tended to prefer feeding on that tissue. Thus a link was established between the direct effects of UV-B radiation on foliar chemistry, and the indirect effects displayed at the ecosystem level.; Results suggest that enhanced UV-B radiation due to stratospheric ozone depletion will alter foliar phenolic profiles in forest tree species, and that the effects will be species-specific. In addition, the direct effects exhibited at the molecular-phytochemical level will elicit significant responses at higher trophic levels, which may ultimately affect forest canopy structure, plant competitive interactions, and ecosystem level processes. Consequently, enhanced solar UV-B radiation may significantly alter trophic structure in some ecosystems. |
