Air pollution and forest health: toward new monitoring concepts K.E. Percy a, *, M. Ferretti b a Natural Resources Canada, Canadian Forest Service-Atlantic Forestry Centre, PO Box 4000, Fredericton, New Brunswick, Canada E3B 5P7 b LINNAEA ambiente Srl, Via G. Sirtori 37, I-50137 Firenze, Italy Received 30 June 2003; accepted 17 October 2003 ‘‘Capsule’’: There is a need for reappraisal of monitoring methods used to assess air pollution effects on forest health. Abstract It is estimated that 49% of forests (17 million km 2 ) will be exposed to damaging concentrations of tropospheric O 3 by 2100. Global forest area at risk from S deposition may reach 5.9 million km 2 by 2050, despite SO 2 emission reductions of 48% in North America and 25% in Europe. Although SO 2 levels have decreased, emissions of NO x are little changed, or have increased slightly. In some regions, the molar SO 4 /NO 3 ratio in precipitation has switched from 2/1 to near 1/1 during the past two decades. Coin- cidentally, pattern shifts in precipitation and temperature are evident. A number of reports suggest that forests are being affected by air pollution. Yet, the extent to which such effects occur is uncertain, despite the efforts dedicated to monitoring forests. Routine monitoring programmes provide a huge amount of data. Yet in many cases, these data do not fit the conceptual and statistical requirements for detecting status and trends of forest health, nor for cause–effect research. There is a clear need for a re-thinking of monitoring strategies. Crown Copyright # 2004 Published by Elsevier Ltd. All rights reserved. Keywords: Air pollution; Forest health; Monitoring 1. Introduction Air pollutants affecting forest health at national and multinational scales include increasing tropospheric, or surface level, ozone (O 3 ) concentrations, increasing atmospheric carbon dioxide (CO 2 ) concentrations, and acidic precipitation. At a regional to local scale, emis- sions of sulphur dioxide (SO 2 ), oxides of nitrogen (NO x ) and a number of other pollutants such as amonia (NH 3 ) emissions from animal feeding operations affect forests downwind of point or urban sources, particularly in rapidly industrializing regions of the world. Increasing levels of ultraviolet-B radiation from stratospheric O 3 depletion at a global scale are possibly a threat to forest health (Percy and Gordon, 1998). There is evidence as well for an increasing emission of persistent organic pollutants (POPs) (EMEP, 2000). Coincidentally, the world’s climate is changing and anthropogenic influ- ences are strongly implicated (Houghton et al., 2001). Air pollution and climate change are two key factors comprising the global change threat to forest health and sustainability. Considerable scientific effort in northern hemisphere countries has been devoted to the enhance- ment of our understanding of forest responses to global change at the process, organ, system, stand and ecosys- tem levels (see reviews by McLaughlin and Percy 1999; Innes and Oleksyn, 2000; Percy et al., 2000; Innes and Haron, 2001). Here we draw upon recent reports and retrospective analyses in order to provide a short sum- mary of trends in air pollution and forest health as it is routinely assessed. 2. Air pollutant trends 2.1. Ozone Surface level O 3 is a secondary air pollutant formed in the atmosphere under bright sunlight from the oxid- ation of the primary pollutants NO x and volatile organic compounds (VOC). Ozone is the most prominent of secondary pollutants formed and is well understood 0269-7491/$ - see front matter Crown Copyright # 2004 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2003.10.034 Environmental Pollution 130 (2004) 113–126 www.elsevier.com/locate/envpol * Corresponding author. E-mail address: kpercy@NRCan.gc.ca (K.E. Percy).