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The responses of pubescent birch (Betula pubescens Ehrh.) leaves to elevated levels of carbon dioxide and ozone were investigated. The micropropagated birch clones used in this research were divided into coastal (Kempele 64°N °25E) and inland (Ylikiiminki 65°N 26°E) clones according to the provenance of the parent tree. The seedlings were exposed to elevated levels of CO 2 and CO 2 +O 3 in open-top chambers over four growing seasons (1993)(1994)(1995)(1996). The leaves used in this investigation were gathered in 1995 and 1996. Leaf characteristics seemed to be dependent on leaf age and weather conditions. Stomatal density was greater in plants grown in open-top chambers relative to unchambered controls. The CO 2 and CO 2 +O 3 treatments did not affect the stomatal density of the birches. However, the mean stomatal size of the coastal clone increased significantly (13%) in the CO 2 treatment compared with the filtered and ambient controls in 1995. In 1996, the mean stomatal sizes of the inland clones were also significantly higher in the CO 2 (10%), CO 2 +O 3 (11%) and non-filtered (12%) treatments compared with the filtered control. The distribution of wax deposits on the abaxial leaf surface increased in the inland clone under elevated levels of CO 2 . The growth environment affected the development of the epicuticular waxes: non-filtered air, CO 2 and CO 2 +O 3 treatments seemed to stimulate the formation of wax deposits compared with the charcoal filtered control in which wax formation was the weakest. The combined effects of elevated CO 2 and O 3 levels on the stomatal size and epicuticular wax deposits were similar in comparison to the effects of elevated CO 2 levels alone.

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The long-term effects of elevated CO 2 and CO 2 +O 3 concentrations on the growth allocation in northern provenances of Norway spruce [Picea abies (L.) Karst.], Scots pine [Pinus sylvestris (L.)] and pubescent birch clones (Betula pubescens Ehrh.) were examined in open-top chambers after a 4-year-long experiment. The total biomass responses of the tree seedlings to increased CO 2 and CO 2 +O 3 concentrations were not statistically significant and varied between the provenances and species. The seedlings of northern origin were the least sensitive in their response to treatments. The total biomass of the Norway spruce seedlings slightly decreased in response to CO 2 in three provenances. Scots pine from the local provenance had a slight biomass increase after elevated CO 2 +O 3 treatment. The slowergrowing birch clone seemed to benefit from elevated CO 2 , whereas in the faster-growing clone, reductions in biomass accumulation were seen. The combined CO 2 +O 3 treatment reduced the positive effects of elevated CO 2 , especially in the slower-growing birches. Observations of significant effects were limited to a few parameters. Carbon dioxide treatment decreased needle dry weight of Norway spruce in one northern provenance. The needle and wood dry weight increased (CO 2 + O 3 ) in local Scots pine. Significant birch response was limited to increased fine root density (O 3 + CO 2 ) in the inland clone. The diverse effects of elevated CO 2 and CO 2 +O 3 on seedling growth and biomass provide evidence that exposure of northern trees to the enhanced variable CO 2 and O 3 concentrations of the future will have varied effects on the growth of these species. The direction and magnitude of those effects will differ depending on species and origins.