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.