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 pl ant 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 x (1991) and 1.5 x (1992) higher than the ambient. The clone s were also fumigated with 150 nl l-1 ozone for 24 d (12 h d-1) in a c hamber experiment. They were found to differ in their susceptibility t o 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 w ere intermediate with respect to their sensitivity to ozone. In all cl ones, elevated 03 concentrations increased diffusive resistance to wat er vapour but increased the stomatal density in the emerging leaves. T he appearance of autumn colouring was accelerated in the field experim ents. The results support the conclusion that birch populations in Fin land can be negatively affected by ozone. The effect is most likely in sensitive birch individuals during ozone episodes.