• 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.
    • Some Growth Characteristics of Four Old World Bluestems

      Coyne, P. I.; Bradford, J. A. (Society for Range Management, 1985-01-01)
      The growth dynamics of 4 Old World bluestem (Bothriochloa spp.) accessions were monitored in the field by periodic harvests of the aboveground biomass components (leaf blades, leaf sheaths plus enclosed stems, exerted stems plus inflorescences) during 2 growth cycles from April to September 1982. The first cycle extended from spring growth initiation (15 March) through flowering (6 July). The second cycle was initiated following the completion of the first by mowing the plants to a 50-mm stubble height and ended at flowering stage (20 September). Growth conditions during the first cycle were considered near optimum when precipitation was 1.71 of the long term mean and soil moisture averaged about 0.29 by volume. Temperatures during the second cycle were approximately 10 degrees C higher than during the first and precipitation was only 0.18 of normal. Soil moisture declined linearly throughout the second cycle and all accessions exhibited visual symptoms of drought stress. There were significant differences among accessions for most of the experimental parameters and accession ranking changed with the imposition of drought stress during the second cycle. Accessions with high relative growth rates were least tolerant of drought stress. Severe defoliation by clipping at the start of cycle 2 revealed 2 regrowth patterns which suggested potential differences in grazing tolerance. Two accessions tended to optimize canopy development by maximizing leaf area index while minimizing biomass and nitrogen investment per unit leaf area. The remaining 2 accessions produced fewer leaves with more investment per leaf. Lower relative growth rates of biomass and the ability to optimize canopy development following defoliation may result in a more stable forage source through time. Plants with these characteristics may not be top producers during periods of favorable growing conditions, but will likely maintain a level of performance under stress which compares more favorably with pre-stress performance.