Agropyron smithii populations exposed to 3 controlled SO2 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 influential in determining growth and tillering responses. Defoliating 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 biomass 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. smithii in terms of biomass and tiller numbers and forage sulfur concentration. Decreased plant growth in response to SO2 plus defoliation was dependent on defoliation 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 shortand long-term compensatory growth potential of a system simultaneously exposed to grazing and air pollution and the potential effect on consumers. During the first 4 years of a field experiment designed to investigate the response of a native North American grassland to controlled low concentrations of sulfur dioxide, it was observed that exposures during the growing season had subtle, but potentially important effects upon the system (Lauenroth and Heasley 1980). For example, exposure of Agropyron smithii Rydb. to S02 reduced chlorophyll concentration (Lauenroth and Dodd 1981), increased sulfur concentration (Lauenroth et al. 1979, Milchunas et al. 198 la), decreased the functional lives of leaves (Heitschmidt et al. 1978, Milchunas et al. 198 lb), and decreased the amount of labile carbon stored in the rhizomes (Lauenroth and Heasley 1980). Regrowth of A. smithii in the spring or after defoliation is thought to be dependent upon carbohydrates stored in rhizomes and roots (Bokhari 1977). Before the SO2 treatments were begun the area had been grazed by cattle. Following exclusion of cattle, a significant year to year increment in rhizome biomass was observed on the control plot. This was interpreted as an indication of recovery from grazing. B3y contrast, rhizome biomass failed to recover on the 170 ,Ag . mSO2 treatment. This suggested that, although the mechanisms by which SO2 exposure and grazing effect plants are different, the response of the plants to SO2 or grazing may be similar and/or additive. Populations under stress are often considered to be more susceptible to damage caused by additional perturbations. Because of the importance of these grasslands to the livestock economy of the northern Great Plains and the high probability that coal combustion for electric power production will increase in this area, a field experiment was conducted to examine the potential interactions between defoliation and SO2 exposure on the native vegetation. The hypothesis was that defoliation would be more detrimental to Authors are associate professor, Range Science Department, and Natural Resource Ecology Laboratory; associate director, Natural Resource Ecology Laboratory, and associate professor, Range Science Department; research associate, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins; and professor, Range Management Division, University of Wyoming, Laramie. Manuscript accepted April 30, 1984. regrowth of A. smithii populations which had been exposed to S02 for 4 years than to populations not exposed to SO2.
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