Risk assessments for forest trees: The performance of the ozone flux versus the AOT concepts P.E. Karlsson a, * , S. Braun b , M. Broadmeadow c , S. Elvira d , L. Emberson e , B.S. Gimeno d , D. Le Thiec f , K. Novak g , E. Oksanen h , M. Schaub g , J. Uddling i , M. Wilkinson c a Swedish Environmental Research Institute (IVL), PO Box 5302, S-400 14, Go ¨teborg, Sweden b Institute for Applied Plant Biology, Scho ¨ nenbuch, Switzerland c Environmental Research Branch, Forest Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey, GU10 4LH, UK d Ecotoxicidad de la Contaminacio ´n Atmosfe ´rica, CIEMAT (ed 70), Avda. Complutense 22, 28040 Madrid, Spain e Stockholm Environment Institute at York, Box 373, University of York, York, YO10 5DD, UK f I.N.R.A. Centre de Recherches Forestie `res, UMR Ecologie et Ecophysiologie Forestie `res Equipe Bioclimatologie, 54280 Champenoux, France g WSL, Swiss Federal Research Institute, Zu ¨rcherstrasse 111, 8903 Birmensdorf, Switzerland h Department of Biology, University of Joensuu, POB 111, 80101 Joensuu, Finland i Department of Plant and Environmental Sciences, Go ¨teborg University, PO Box 461, SE-405 30 Go ¨ teborg, Sweden Received 27 January 2006; received in revised form 12 June 2006; accepted 15 June 2006 Ozone stomatal flux based indices were superior, as compared to AOT40, for explaining biomass reductions and leaf visible injury Abstract Published ozone exposureeresponse relationships from experimental studies with young trees performed at different sites across Europe were re-analysed in order to test the performance of ozone exposure indices based on AOTX (Accumulated exposure Over a Threshold of X nmol mol 1 ) and AF st Y (Accumulated Stomatal Flux above a threshold of Y nmol m 2 s 1 ). AF st 1.6 was superior, as compared to AOT40, for ex- plaining biomass reductions, when ozone sensitive species with differing leaf morphology were included in the analysis, while this was not the case for less sensitive species. A re-analysis of data with young black cherry trees, subject to different irrigation regimes, indicated that leaf visible injuries were more strongly related to the estimated stomatal ozone uptake, as compared to the ozone concentration in the air. Exper- imental data with different clones of silver birch indicated that leaf thickness was also an important factor influencing the development of ozone induced leaf visible injury. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Ozone; Forest trees; Ozone flux; AFstY; AOT40 1. Introduction It is generally accepted that the most severe ozone effects on plants are caused by the ozone that is taken up through the sto- mata into the leaf interior (Reich, 1987; Ashmore et al., 2004). However, differences in intrinsic ozone tolerance between species and proveniences due to various de-toxification processes are important and will also have to be taken into con- sideration to understand how ozone exposure relates to ozone impacts (e.g. Bussotti and Gerosa, 2002; Wieser et al., 2002). There is consensus within the EU and the UNECE Conven- tion on Long-Range Transboundary Air Pollution (LRTAP) that the exposure based on the rate of stomatal uptake (flux) of ozone represents the most appropriate approach for setting future ozone critical levels for forests trees (Karlsson et al., 2003a). However, uncertainties in the development and * Corresponding author. Tel.: þ46 31 7256200; fax: þ46 31 7256290. E-mail address: pererik.karlsson@ivl.se (P.E. Karlsson). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.06.012 Environmental Pollution 146 (2007) 608e616 www.elsevier.com/locate/envpol