Risk assessments for forest trees: The performance of the ozone flux versus the AOT concepts
Introduction
It is generally accepted that the most severe ozone effects on plants are caused by the ozone that is taken up through the stomata 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 consideration 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 Convention 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 application of flux-based approaches, as they are available at their current stage, have been regarded as too large to justify their application as a standard risk assessment method for trees at a European scale. Therefore AOT40 (Accumulated exposure Over a Threshold of 40 nmol mol−1) has been retained as the basis for the ozone critical levels for trees (Karlsson et al., 2003a). However, at a recent LRTAP workshop in Obergurgl, Austria (Wieser and Tausz, 2006) it was suggested that an ozone flux based index, AFst1.6 (Accumulated Flux through stomata above a threshold of 1.6 nmol m−2 s−1), should be used for risk assessments for forests on the European scale for the integrated assessment modelling within the LRTAP convention.
In a previous analysis of the relationships between biomass reductions and ozone exposures using experimental data with young trees from different sites across Europe, AFst1.6 was not found to provide better correlations with biomass reductions, as compared to AOT40 (Karlsson et al., 2004b). However, in that study the aim was to develop new ozone critical levels for the protection of the most sensitive receptor with the highest possible accuracy. Therefore broadleaved and coniferous tree species were analysed separately, each divided into two sensitivity categories.
The aim of this study was to further analyse published ozone exposure–response data in order to test the hypothesis that ozone impacts on young trees under experimental conditions are better explained by indices that are based on stomatal ozone flux, as compared to indices based on the ozone concentrations in the surrounding air.
Section snippets
Description of the data sets
The experimental data sets that were used in the present study for the statistical analyses of exposure–response relationships for ozone impacts on biomass were identical to those described in Karlsson et al. (2004b), with the exception of an additional data set for silver birch (Betula pendula) from Birmensdorf, Switzerland. This latter dataset has previously been described by Uddling et al. (2004). Some information about the different data sets is presented in Table 1. In brief, the
Comparison of AFst1.6 and AOT40-based exposure indices for explaining biomass reductions in young trees
The results from the linear regression analyses of the exposure–response data sets with broadleaved and coniferous tree species, which were rated as ozone sensitive and less sensitive, respectively (Table 1; Karlsson et al., 2004b), are shown in Table 2 and Fig. 1, Fig. 2, Fig. 3, Fig. 4. Both regression methods, outlined in Section 2, indicated that AFst1.6 was substantially better than AOT40 in explaining the reductions in biomass for sensitive broadleaved and sensitive coniferous tree
Discussion
When an analysis was made including several different broadleaved and coniferous tree species that were all rated as ozone sensitive, but with considerably different leaf morphology, then the stomatal AFst Y concept was indeed superior to the concentration based AOTX concept. The results were similar for both linear regression analysis methods that were applied. It is evident that this result depended to a large extent on the differences in the values for maximum conductance used when
Conclusions
The results presented in this study support the hypothesis that impacts on biomass, as well as on leaf visible injuries, could be better explained by the accumulated stomatal ozone flux, AFst1.6, as compared to the concentration based index, i.e. AOT40, for young trees of species that were considered as ozone sensitive and that were exposed under experimental conditions. Regarding young trees of species, which are considered to be less ozone sensitive, the flux concept was not superior to AOT40
Acknowledgements
This research was funded by The Foundation for Strategic Environmental Research, through the programme “Abatement Strategies for Transboundary Air Pollution”. We are grateful to Christian Schindler, Institute for Social and Preventive Medicine, University of Basel for statistical advice.
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