Possible impacts of changes in UV-B radiation on North American trees and forests Joe H. Sullivan * Department of Natural Resource Sciences and Landscape Architecture, University of Maryland, College Park, MD, USA Received 10 December 2004; accepted 31 January 2005 Projected increases in UV-B are not likely to have a direct impact on trees, but indirect responses could impact planteherbivore interactions and nutrient cycling in forests. Abstract Approximately 35 species representing 14 tree genera have been evaluated for responses to UV-B radiation in North America. The best representation has been in the conifers where some 20 species representing three genera have been studied. Overall, about 1/3 of these have demonstrated some deleterious response to UV-B. However, most negative impacts have been observed under controlled environment conditions where sensitivity may be enhanced. Therefore, it seems unlikely that expected levels of ozone depletion will result in direct losses in productivity. However, the role that ambient or enhanced levels of UV-B may play in forest ecosystem processes is more difficult to access. One possible indirect response of forests to changes in UV-B radiation levels could be via alterations in plant secondary metabolites. Increases in phenolics and flavonoids that enhance epidermal UV-screening effectiveness may also influence leaf development, water relations or ecosystem processes such as planteherbivore interactions or decomposition. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: UV-B radiation; Forest ecosystems; Global change; Flavonoids 1. Introduction and background information Increases in solar ultraviolet-B radiation (UV be- tween 280 and 320 nm) due to continuing depletion of stratospheric ozone have been well documented over the past several decades (e.g. WMO, 2003). However, the magnitude, longevity and spatial variation of these changes remain a matter of study and poorly un- derstood. For example, although chlorine levels in the stratosphere may be near their peaks and are expected to decline over the next several decades, bromine levels continue to increase and future levels of stratospheric chlorine are dependent upon worldwide compliance of emission restrictions (WMO, 2003). Also a linkage between ozone depletion and global warming due to other atmospheric trace gasses such as CO 2 and CH 4 had been proposed because of stratospheric cooling which favors ozone depletion reactions (Aldhous, 2000; Hartmann et al., 2000). Therefore, the issue of the consequences of future changes in ozone concentrations and incident levels of UV-B on the surface of the Earth should not be considered closed. Although UV-B radiation received little attention prior to the mid to late 1970s (but see Caldwell, 1971 for an example of earlier interest in this subject), rather extensive studies have been conducted on the response of * Tel.: C1 301 405 1626; fax: C1 301 314 9308. E-mail address: js128@umail.umd.edu 0269-7491/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2005.01.029 Environmental Pollution 137 (2005) 380e389 www.elsevier.com/locate/envpol