Quantifying the direct effects of SO2 and O3 on forest growth

The direct effects of sulphur dioxide (SO2) and ozone (O3) on the growth of two Douglas-fir stands are quantified from continuous measurements of the concentration of air pollution above the canopy combined with a quantitative description of the direct effects of gaseous uptake of pollutants on photosynthesis and respiration. Effects at the leaf level are scaled up to the stand level, using a deterministic model of forest growth that uses time steps of 1 day. This approach enables total uptake of gaseous air pollutants and their effects on canopy assimilation and growth to be quantified, taking ambient weather conditions and stand structure into account. By using a description of water balance in the model, the influence of limitations in soil moisture availability can also be incorporated, and the combined effects of air pollution and water shortage can be accounted for. Under Dutch conditions, with average daily concentrations of 10 μg m−3 for SO2 and 50 μg m−3 for O3, the short-term effects on tree growth appear to be minor, except near local sources, and during episodes of high concentrations under stable weather conditions. The possible magnitude of long-term effects of exposure to O3 is assessed using a hypothetical relationship between dose (exposure multiplied by duration) and foliage loss. From this it is concluded that long-term effects may be significant in stands with a low leaf area index.