In three experiments young trees were exposed to SO2 and NO2, singly and in mixtures, at concentrations of 110 nl l−1 or 100 nl l−1 for 5 d each week, and their growth was compared with that in clean air.

Exposure of Populus nigra during the summer and autumn (11/6/80 to 12/11/80) showed that greater than additive and additive growth inhibitions occurred in SO2 and NO2 during the growing season. Exposure to SO2 during dormancy (29/1/81 to 9/4/81) delayed leaf growth in the following spring, but the presence of NO2 did not influence this effect. Detailed observations of a few plants of six species during summer and winter fumigation (23/3/81 to 17/5/82) confirmed that the responses were influenced by season, and showed that pollutant effects can change during continuous exposures. In SO2 the growth effects did not occur to Tilia cordata, Malus domestica, Betula pendula and P. nigra until the second year of exposure, but in Betula pubescens and Alnus incana decreases of dry weight occurred in the first year. Nitrogen dioxide alone initially stimulated the shoot growth of T. cordata, B. pendula and A. incana, but these effects were lost for T. cordate and B. pendula in the second season of exposure. Inhibitory effects on a number of parameters developed more rapidly in SO2+ NO2 mixtures than in SO2 alone. In most instances the nature of these responses altered after overwintering, with the damage done by SO2 becoming more marked and the beneficial effects of NO2 being lost.

The responses to SO2 and NO2, were variable, depending on species, on the duration of exposure and the time of year. Sequential observations of the shoots suggested that the physiology of growth influenced pollution sensitivity. Foliar blemish, senescence and abscission were the main symptoms of injury in SO2 and SO2+ NO2, and the severity of these effects was well correlated with the magnitude of the decreases in dry weight.