Ozone sensitivity of silver birch ( BETULA PENDULA Roth) has been thoroughly investigated since early 1990's in Finland. In our long-term open-field experiments the annual percentage reduction in basal diameter and stem volume increment were the best non-destructive growth indicators for ozone impact when plotted against AOTX. Remarkable differences in defence strategies, stomatal conductance, and defence compounds (phenolics), clearly indicate that external exposure indices are ineffective for accurate risk assessment for birch. For flux-based approaches, site-specific values for G(max) and G(dark) are necessary, and determinants for detoxification capacity, ageing of leaves, and cumulative ozone impact would be needed for further model development. Increasing CO(2) seems to counteract negative ozone responses in birch, whereas exposure to spring time frost may seriously exacerbate ozone damage in northern conditions. Therefore, we need to proceed towards incorporating the most important climate change factors in any attempts for ozone risk assessment.