• Productivity of long-term grazing treatments in response to seasonal precipitation

      Milchunas, D. G.; Forwood, J. R.; Lauenroth, W. K. (Society for Range Management, 1994-03-01)
      Estimates of forage production for long-term ungrazed, lightly, moderately, and heavily grazed treatments (0, 20, 40, 60% removal of annual forage production) established in 1939 in shortgrass steppe communities were subjected to multiple regression analyses to assess long-term temporal trends resulting from grazing and short-term sensitivities to abiotic factors. Average production based upon all data from 1939-1990 was 75, 71, 68, and 57 g m-2 yr-1 for ungrazed, lightly, moderately, and heavily grazed treatments, respectively. Variability in forage production was explained mostly by cool-season precipitation, and magnitude of forage production was more sensitive to annual fluctuations in precipitation than to long-term grazing treatments. Production per unit increase of precipitation was greater for cool-season than warm-season precipitation, but only when cool-season precipitation was above average. This was attributed to differences in evaporative demand of the atmosphere resulting in different utilization-efficiencies of small and large rainfall events in the 2 seasons. Based upon a regression model constructed using data from 1939 through 1962, forage production was not affected by grazing to 20 to 35% removal. For pastures of average relative productivity, grazing at 60% level of consumption for 25 years resulted in a 3% decrease in forage production in wet years and a 12% decrease in dry years. Estimates of productivity after 50 years of heavy compared to light grazing treatment were -5 and -18% for wet and average y precipitation, respectively.
    • The effect of Quercus douglasii removal on understory yield and composition

      Bartolome, J. W.; Allen-Diaz, B. H.; Tietje, W. D. (Society for Range Management, 1994-03-01)
      The canopy of Quercus douglasii H. & A. (blue oak) has been variously reported to enhance or suppress understory production. The effects of canopy removal have been reported only for the northern portion of blue oak's range. We removed all blue oaks from 6 plots in the central coast of California and found no significant change in understory biomass over 3 years. Understory herb cover averaged 32.6% on cleared plots, compared to 24.3% on uncut plots, but composition changed little with the exception of an increase in Erodium cicutarium (L.) L'Her. Clearing did not produce the distinctive species composition and forage enhancement under Q. douglasii canopy reported in other studies, an based on comparisons between unmanipulated canopy and adjacent grassland. Our results suggest that the canopy effect could instead be caused by differences in sites occupied by trees. Clearing of Q. douglasii in regions with 50 cm or less of mean annual precipitation is not recommended for increasing forage production.