The effects of gaseous air pollutants, acidic deposition and mineral deficiency on the gas exchange of trees are reviewed, and published data which illustrate important responses are described in detail. Data are presented which show that mixtures of SO2 plus NO2 can decrease photosynthesis in Betula pendula, that ambient pollutant concentrations (especially O3) tend to increase stomatal conductance of Populus deltoides X Populus nigra in the early part of the day for young growing leaves and throughout the day for mature leaves, and that simulated acid rain (pH 2.8) can cause temporary increases of stomatal conductance. Data for forest sites indicate that ambient concentrations of gaseous air pollution are sufficient to alter the gas exchange of woody plants (Fagus sylvatica, Populus deltoides X Populus nigra, Picea sitchensis and Picea abies). Both inhibitions and stimulations of stomatal conductance have been observed, often associated with no effect or small depressions of photosynthesis. The link between stomatal response to gaseous pollutants and growth effects remains unclear. Initial exposure to wet deposited sulphates and nitrates often leads to stomatal opening while longterm exposure results in closure. Soil-mediated effects may be cumulative and operate via effects on nutrient supply and thus mesophyll characteristics and phloem loading
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