THE IMPACT OF UV-B RADIATION AND OZONE ON TERRESTRIAL VEGETATION

Although terrestrial vegetation has been exposed to UV-B radiation and ozone over the course of evolutionary history, it is essential to vie w the effects on vegetation of changing levels of these factors in the context of other features of climate change, such as increasing CO2 l evels and changes in temperature and precipitation patterns. Much of o ur understanding of the impacts of increased UV-B and ozone levels has come from studies of the effects of each individual factor. While suc h information may be relevant to a wider understanding of the roles th at these factors may play in climate change, experience has shown that the interactions of environmental stresses on vegetation are rarely p redictable. A further limitation on the applicability of such informat ion results from the methodologies used for exposing plants to either factor. Much of our information comes from growth chamber, greenhouse or field studies using experimental protocols that made little or no p rovision for the stochastic nature of the changes in UV-B and ozone le vels at the earth's surface, and hence excluded the roles of repair me chanisms. As a result, our knowledge of dose-response relationships un der true field conditions is both limited and fragmentary, given the w ide range of sensitivities among species and cultivars. Adverse effect s of increased levels of either factor on vegetation are qualitatively well established, but the quantitative relationships are far from cle ar. In both cases, sensitivity varies with stage of plant development. At the population and community levels, differential responses of spe cies to either factor has been shown to result in changes in competiti veness and community structure. At the mechanistic level, ozone genera lly inhibits photosynthetic gas exchange under both controlled and fie ld conditions, and although UV-B is also inhibitory in some species un der controlled conditions, others appear to be indifferent, particular ly in the field. Both factors affect metabolism; a common response is increased secondary metabolism leading to the accumulation of phenolic compounds that, in the case of UV-B, offer the leaf cell some protect ion from radiation. Virtually no information is available about the ef fects of simultaneous or sequential exposures. Since both increased su rface UV-B and ozone exposures have spatial and temporal components, i t is important to evaluate the different scenarios that may occur, bea ring in mind that elevated daytime ozone levels will attenuate the UV- B reaching the surface to some extent. The experimentation needed to a cquire unequivocal effects data that are relevant to field situations must therefore be carried out using technologies and protocols that fo cus on quantification of the interactions of UV-B and ozone themselves and their interactions with other environmental factors.