Growth and stomatal responses of birch (Betula pendula Roth.) clones to ozone in open-air and chamber fumigations.

It is evident that even outside the classical high ozone areas some plant species are at risk. In this study seedlings of five birch clones (Betula pendula Roth.) were exposed to a slightly elevated ozone level in an open-field experiment over two growing seasons. The ozone doses were 1.2 × (1991) and 15 × (1992) higher than the ambient. The clones were also fumigated with 150 nl I ozone for 24 d (12 h d ) in a chamber experiment. They were found to differ in their susceptibility to ozone: ozone fumigation decreased the height growth and leaf biomass, and increased the amount of advanced visible injury in leaves of the most sensitive clone, but did not affect the height growth or amount of visible injury of the most tolerant clone. The other three clones were intermediate with respect to their sensitivity to ozone. In all clones, elevated O concentrations increased diffusive resistance to water vapour but increased the stomatal density in the emerging leaves. The appearance of autumn colouring was accelerated in the field experiments. The results support the conclusion that birch populations in Finland can be negatively affected by ozone. The effect is most likely in sensitive birch individuals during ozone episodes.