During the growing seasons 1986 to 1991 ozone (O3) effects on mature spruce and larch trees were examined under field conditions at a high and a low elevation site using a twig chamber system. Twigs were exposed to ambient and above ambient O3 concentrations for one or two growing seasons under ambient climatic conditions. Only mean O3 concentrations higher than 100 ppb persisting for one vegetation period or more (AOT40 values higher than 100 ppm-h) caused a pronouncded decline in net photosynthesis. This observed lack of symptoms at AOT40 values lower than 100 ppm-h in our field experiments might be attributed to modifications in the amount of O3 entering the needles. At a given O3 concentration, the rate of O3 uptake was effectively controlled by stomatal conductance and hence by factors controlling stomatal conductance. Therefore, total O3 uptake should be used as a measure capable of eliciting a response to O3. Furthermore, the potential toxicity of ambient O3 concentration may also be modified by the capability to detoxify oxygen radicals which is also dependent on environmental factors.
