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Published ozone exposure–response 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 AFstY (Accumulated Stomatal Flux above a threshold of Y nmol m−2 s−1). AFst1. 6 was superior, as compared to AOT40, for explaining 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. Experimental 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. [Copyright &y& Elsevier]

A model has been developed to estimate stomatal ozone flux across Europe for a number of important species. An initial application of this model is illustrated for two species, wheat and beech. The model calculates ozone flux using European Monitoring and Evaluation Programme (EMEP) model ozone concentrations in combination with estimates of the atmospheric, boundary layer and stomatal resistances to ozone transfer. The model simulates the effect of phenology, irradiance, temperature, vapour pressure deficit and soil moisture deficit on stomatalconductance. These species-specific microclimatic parameters are derived from meteorological data provided by the Norwegian Meteorological Institute (DNMI), together with detailed land-use and soil type maps assembled at the Stockholm Environment Institute (SEI). Modelled fluxes are presented as mean monthly flux maps and compared with maps describing equivalent values of AOT40 (accumulated exposure over threshold of 40 ppb or nl l-1), highlighting the spatial differences between these two indices. In many cases high ozone fluxes weremodelled in association with only moderate AOT40 values. The factorsmost important in limiting ozone uptake under the model assumptions were vapour pressure deficit (VPD), soil moisture deficit (for Mediterranean regions in particular) and phenology. The limiting effect of VPD on ozone uptake was especially apparent, since high VPDs resulting in stomatal closure tended to co-occur with high ozone concentrations. Although further work is needed to link the ozone uptake and deposition model components, and to validate the model with field measurements, the present results give a clear indication of the possible implications of adopting a flux-based approach for future policy evaluation. [ABSTRACT FROM AUTHOR]

It has been proposed that stomatal flux of ozone would provide a more reliable basis than ozone exposure indices for the assessment of the risk of ozone damage to vegetation across Europe. However, implementation of this approach requires the development of appropriate models which need to be rigorously tested against actual data collected under field conditions. This paper describes such an assessment of the stomatal component of the model described by Emberson et al. (2000. Modelling stomatal ozone flux across Europe. Environmental Pollution 110). Model predictions are compared with field measurements of both stomatal conductance (gs) and calculated ozone flux for shoots of mature Norway spruce (Picea abies) growing in the Tyrol Mountains in Austria. The model has been developed to calculate gs as a function of leaf phenology and four environmental variables: photosynthetic flux density (PFD), temperature, vapour pressure deficit (VPD) and soil moisture deficit (SMD). The model was run using climate data measured on site, although the SMD component was omitted since the necessary data were not available. The model parameterizationfor Norway spruce had previously been collected from the scientific literature and therefore established independently from the measurement study. Overall, strong associations were found between model predictions and measured values of stomatal conductance to ozone (GO3) and calculated stomatal ozone flux (FO3). Average diurnal profiles of GO3 and FO3 showed good agreement between the field data and modelled values except during the morning period of 1990. The diurnal pattern of ozone flux was determined primarily by PFD and VPD, as there was little diurnal variation inozone concentration. In general, the model predicted instances of high ozone flux satisfactorily, indicating its potential applicability in identifying areas of high ozone risk for this species. [ABSTRACT FROM AUTHOR]

Leaf or needle ozone uptake was estimated for young trees at seven experimental sites across Europe using a stomatal conductance simulation model. Dose–response relationships based on cumulative leaf uptake of ozone (CUO) were calculated using different hourly ozone flux thresholds and these were compared to dose–response relationships based on daylight AOT40, which is currently used within the UNECE Convention on Long-Range Transboundary Air Pollution (CLRTAP). Regression analysis showed that the CUO–biomass response relationships were highly significant for both coniferous and broadleaf trees, and independent of which ozone flux threshold was applied. On the basis of this regressions analysis, an hourly flux threshold of 1.6 nmol m−2 s−1 (CUO>1.6) is proposed as the most appropriate for all species categories in deriving dose–response relationships. The analysis indicated that the current critical level for ozone impacts on European forests of AOT40 10 ppm h may not protect the most sensitive receptors and that critical levels for AOT40 and CUO>1.6 of 5 ppm h and 4 mmol m−2, respectively, are more appropriate. The research identified weaker dose–response relationships for the CUO exposure index compared with AOT40. Distinguishing between sensitive and less sensitive species substantially improved the CUO–biomass response relationships although, still, to a lesser extent than when exposure was expressed as AOT40. [Copyright &y& Elsevier]