• Changes in reproductive habitat of gray partridge after burning

      Novoa, C.; Dumas, S.; Prodon, R. (Society for Range Management, 1998-11-01)
      We investigated the effects of winter controlled burning on the breeding habitat of the Pyrenean gray partridge (Perdix perdix hispaniensis Reich.). Floristic composition and vegetation structure were sampled on 198 sites, including 64 recently used by hens for nesting or rearing of broods, 90 within partridge habitat burned under dry conditions, and 44 burned under wet conditions. During the early breeding season, birds selected shrublands of broom (Cytisus purgans (L.) Boiss.) with an average canopy coverage of 60% and an average height of 0.50 m. Birds avoided sites where shrub cover was more than 80% or less than 20%. The most critical effect of burning on gray partridge brood habitat was the reduction of the cover in the 2 vegetation layers providing protection against predators (0.05-0.25 m, and 0.25-0.50 m). In the case of "dry burning", the recovery of suitable habitat took more than 8 years for nesting hens and flightless chicks, but only 5-6 years for broods older than 3 weeks. Data obtained by radio-tracking eight broods indicated that "wet burns" (mean size = 4 ha) were better than "dry burns" (mean size = 15 ha) for maintaining good brood habitat. For the "dry burns", we recommend that burned patches be equal to or less than 5 ha and separated by unburned patches of 10-15 ha. In both cases, the frequency of fires should not exceed once every 15-20 years.
    • Estimating ashe juniper leaf area from tree and stem characteristics

      Hicks, R. A.; Dugas, W. A. (Society for Range Management, 1998-11-01)
      Ashe juniper (Juniperus ashei Buchh.) is increasing in density and areal coverage on the Edwards Plateau of Texas. This is causing changes in community leaf area that could impact the ecosystem water balance through increased transpiration and interception of precipitation. Our objective was to estimate leaf area of selected trees and shoots in a range of size and age classes using nondestructive methods. We harvested all leaf material from 9 trees ranging in height from 0.8 to 4.8 m and recorded tree height and canopy diameter. We divided each tree into 6 sections based on 3 horizontal strata and 2 vertical hemispheres. Projected leaf area of subsamples, collected from each section, was multiplied by pi to give full cylinder leaf area which was used to calculate specific leaf area (cm2 g-1). Dried leaf biomass for each stratum, hemisphere, and tree was multiplied by the specific leaf area to determine the leaf area. We harvested leaf biomass from shoots and measured in situ stem diameters, dried the leaf biomass, and multiplied it by the specfic leaf area to determine shoot leaf area. There was no significant effect of stratum or hemisphere on specific leaf area or of hemisphere on leaf area. The middle stratum had a significantly greater percentage of total leaf area (52%). Total tree leaf area was best predicted (r2 = 0.97) by canopy area. Shoot leaf area was best predicted (r2 = 0.93) by stem area. Canopy and stem area measurements are rapid, nondestructive means of accurately estimating Ashe juniper tree and shoot leaf area, respectively.
    • Heterogeneity in tall fescue pastures created and sustained by cattle grazing

      Cid, M. S.; Brizuela, M. A. (Society for Range Management, 1998-11-01)
      In continuous grazing systems the amount of herbage and its quality is a matter of primary concern. However, at moderate stocking, cattle grazing usually leads to the generation of patches differing in forage quality and quantity even in virtually monospecific pastures. This patchiness influences subsequent vegetation and animal responses. We analyzed the heterogeneity created and sustained by cattle grazing in a tall fescue (Festuca arundinacea Schreb.) pasture at stocking densities ranging from 1.8 to 4.1 animals ha-1 over 2 years. Cattle grazing created and maintained a mosaic of areas with different degrees of utilization. Heavily utilized patches had less biomass per unit surface, but their live biomass was more dense and had a higher nitrogen concentration. Patch boundaries fluctuated throughout the year at all stocking densities. Patch locations were more stable at the lower stocking densities, where cattle repeatedly returned to heavily utilized patches even though they represented less than 30% of the total surface. This reinforces the idea that, at low and moderate stocking densities, cattle can obtain a nutritional benefit by patch grazing. The percentage of heavily utilized patches reached a maximum value at an instantaneous grazing pressure of approximately 0.0016 animal units kg forage-l. When this threshold is passed, animal selection between patches could be conditioned by the presence of feces or thistles, and pasture condition affected by overgrazing of the heavily utilized patches.