EVALUATION OF OZONE IMPACT ON MATURE SPRUCE AND LARCH IN THE FIELD

During the growing seasons 1986 to 1993 we examined the effects of ozo ne (O-3) on the gas exchange of mature Norway spruce (Picea abies) and European larch (Larix decidua) trees under field conditions at a low and a high elevation site. Twigs were enclosed in chambers and exposed to different O-3 concentrations for one or two seasons tracking ambie nt climatic conditions. After one and two fumigation periods, only mea n O-3 concentrations higher than 100 ppb caused a pronounced decline i n gas exchange, both in spruce and larch. The observed lack in symptom expression at mean O-3 concentrations lower than 100 ppb can be attri buted to modifications in the amount of O-3 entering the needles. At b oth study sites O-3 uptake (F-O3) was effectively controlled by stomat al conductance (gH(2)O) and therefore by factors such as light, humidi ty and water status, controlling gH(2)O. Water vapour pressure deficit (VPD) was the climatic factor most closely correlated with ambient O- 3 concentration. Thus, when O-3 concentrations were highest, F-O3 tend ed to be restricted by stomatal narrowing. Mitigation of potential O-3 stress by stomatal narrowing was more pronounced at the low elevation site where soil water stress and VPD were greater than at high altitu de. On the other hand, the capability to detoxify oxygen radicals is g reater in plants growing at high altitude. Therefore, we conclude that ambient O-3 concentration presently does not constitute an acute dang er for spruce and latch trees.