Reactions of Norway spruce and beech trees to 2 years of ozone exposure and episodic drought

Nine-year-old Norway spruce from two clones and beech grown in open-top chambers were exposed to both drought and elevated (ambient + 0.05 mu l l(-1)) ozone. Beech was less damaged from the effects of ozone by drought. Compared to trees exposed to elevated ozone and well watered, beech trees grown under elevated ozone and exposed to drought had lower CO2 compensation points and were not affected by foliar necroses observed in well watered trees. Both Norway spruce clones were more resistant than beech to ozone alone, but the Istebna clone grown in drought and elevated ozone showed an increased stomatal conductance. Therefore these trees have significantly more negative pre-dawn leaf water potentials than droughted trees grown in filtered air (without ozone). The increased stomatal conductance implies a greater ozone flux to the needles. In association with this increased ozone uptake, 1-year-old needles became discoloured and were prematurely shed. (C) 1998 Elsevier Science B.V. All rights reserved.