INFLUENCE OF NITROGEN SUPPLY ON THE RESPONSE OF CLONES OF BIRCH (BETULA-PENDULA ROTH) TO OZONE

Clonal birch (Betula pendula Roth.) seedlings were exposed to slightly higher than ambient levels of ozone and different levels of nitrogen supply under open-field conditions in three experiments: (1) three 2-y ear-old clones exposed to ozone and 74/150 kg N ha(-1) y(-1) for two g rowing seasons, (2) three 2-year-old clones under a corresponding expo sure regime for one growing season and (3) one 2-year-old clone expose d to ozone and 37/74/150 kg N ha(-1) y(-1) for one growing season. The cumulative ozone exposures were 1.5 x (1992) and 1.7 x (1993) higher than ambient levels. In these experiments no significant differences b etween the clones were found in their response to each treatment. Ozon e exposure reduced the mean leaf size and the leaf area, accelerated a utumn yellowing and loss of leaves, and increased the palisade layer t hickness. High nitrogen availability increased the height growth, the mean leaf size, the leaf area, the shoot/root ratio, the leaf water co ntent and the stem dry weight, but decreased the number of leaves and the leaf dry mass per area, and retarded autumn senescence of leaves. It also reduced the leaf and palisade layer thickness and increased th e intercellular space of the palisade layer. Significant interactions between ozone and nitrogen supply were found as an increase in leaf bi omass production, root dry weight and spongy intercellular space, and as a decrease in leaf loss, yellowing of leaves and palisade layer thi ckness. The results suggest that sufficient nitrogen supply can confer the birch with greater resistance to ozone.