Agropyron smitlrii populations exposed to 3 controlled SOa concentrations were defoliated either once or twice during the growing season at a light and a heavy intensity. The intensity and frequency of defoliation were most iduentiai in determining growth and tillering responses. Defoliatig twice, at either intensity, had a large negative impact on plant growth whereas compensatory growth occurred after defoliating once at either intensity. Sulfur dioxide alone had no significant effect on

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... ss or the number of tillers, even though sulfur accumulated approximately in proportion to exposure concentration. Sulfur dioxide exposure with the additional influence of defoliation affected both the regrowth of A. smifhii in terms of biomass and tiller numbers and forage sulfur concentration. Decreased plant growth in response to SOZ plus defoliation was dependent on defolhtion frequency, whereas the effect of So2 plus defoliation on plant sulfur concentration was positive and negative and depended on a complex interaction of SO2 concentration and defoliation frequency and intensity. The results are discussed in relation to the short-and long-term compensatory growth potential of a system simultaneously exposed to grazing and air pollution and the potential effect on consumers.