• Impact of SO2 Exposure on the Response of Agropyron smithii to Defoliation

      Lauenroth, W. K.; Detling, J. K.; Milchunas, D. G.; Dodd, J. L. (Society for Range Management, 1985-01-01)
      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 short- and long-term compensatory growth potential of a system simultaneously exposed to grazing and air pollution and the potential effect on consumers.
    • Influence of Season and Intensity of Defoliation on Bluebunch Wheatgrass Survival and Vigor in Southern British Columbia

      McLean, A.; Wikeem, S. (Society for Range Management, 1985-01-01)
      Bluebunch wheatgrass (Agropyron spicatum Pursh) Scribn. & Smith) response to various clipping regimes which incorporated different times, frequencies, and intensities of defoliation was examined in southern British Columbia. The experiment was repeated for 3 consecutive years at a low (296 m) and a high (1,112 m) elevation site. Plant survival and vigor was evaluated the summer following defoliation. Greatest injury was incurred by treatments involving defoliation to a 5-cm stubble height from mid April to the end of May or from early May to mid June at the low and high elevation sites, respectively. Reduced injury occurred from treatments which left 10 or 15-cm stubble heights or which ceased defoliation earlier in the season. No appreciable damage was incurred by fall clipping to 5 cm or by season-long defoliation to 20 cm. Injury resulting from spring plus fall as compared to spring only defoliation was inconsistent. Greatly reduced injury for many treatments at the low elevation site in one year was attributed to unusually warm spring temperatures and attendant rapid spring growth.