• Fire history of the Rochelle Hills Thunder Basin National Grasslands

      Perryman, B. L.; Laycock, W. A. (Society for Range Management, 2000-11-01)
      A fire scar chronology was constructed from ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and Rocky Mountain juniper (Juniperus scopulorum Sarg.) trees within the 70 km2 Rochelle Hills Area of the Thunder Basin National Grasslands, in north-east Wyoming. A total of 65 fire scars occurred in 48 crossdated samples, and a master fire chronology was constructed for the period 1565 to 1988. No trees recorded more than 3 fires and most (26 of 42) recorded only one. For this reason, fire frequency intervals were considered as fire-free intervals in the Rochelle Hills Area. The Weibull Median Probability Interval (WMPI) for the entire period of record was 7.4; 7.9 for the non-suppression period (1565 to 1939); and 6.7 for the suppression period (1940 to 1988). Infrequent occurrence of multiple scars, rough topography, and low potential substrates suggest that understory fuel loads were limited in amount and spatial consistency during most fire years. Position of scars within annual growth rings suggests that most fires (80%) occurred during the latter stages of the growing season or during the dormant period.
    • Impacts of western juniper on plant community composition and structure

      Miller, R. F.; Svejcar, T. J.; Rose, J. A. (Society for Range Management, 2000-11-01)
      Western juniper (Juniperus occidentalis Hook.) has been actively invading shrub steppe communities during the past 120 years. The majority of these stands are still in transition, from early open juniper shrub steppe communities to closed juniper woodlands. In addition, juniper expansion has been occurring across a broad array of soils and topographic positions. Despite the high degree of spatial and developmental heterogeneity, juniper woodlands are frequently treated generically in resource inventories, management, and wildlife habitat assessments. Our goal was to evaluate the impact of western juniper encroachment and dominance on plant community composition and structure across several plant associations. This study was conducted in southeastern Oregon and northeastern California on low sage-brush (Artemisia arbuscula Nutt.), mountain big sagebrush (A . tridentata spp. vaseyana (RYBD. )Beetle), and aspen (Populus tremuloides Michx.) alliances. Stages of woodland development across plant associations were categorized into 1 of 4 successional phases (early, mid, late, and closed) based on tree growth and stand structural characteristics. Plant cover by species group, species diversity and richness, bareground cover, soil characteristics, elevation, aspect, and slope were measured in 108, 60 x 46m macroplots. Twinspan was used to sort plant communities. Regression analysis was used to evaluate the relationship of tree canopy cover to shrub and herbaceous cover. Herbaceous and bareground cover were compared between early and closed stands within plant communities. Woodland structure at stand closure was different among associations varying from 19% cover and 64 trees ha-1 in a low sagebrush community to 90% cover and 1,731 trees ha-1 in an aspen community. Increase in juniper dominance had little impact on low sagebrush and an inconsistent effect on bitterbrush (Purshia tridentata Pursh.). In the mountain big sagebrush alliance, sagebrush cover declined to approximately 80% of maximum potential as juniper increased to about 50% of maximum canopy cover. Aspen (Populus tremuloides Michx.) also declined as juniper dominance increased. Herbaceous cover and species diversity declined and bare ground increased with increasing juniper dominance in the mountain big sagebrush/Thurber needlegrass association. However, herbaceous cover on the deeper soils characterized by Idaho fescue did not decrease with increasing juniper dominance. To determine the effect of juniper dominance or woodland management on community composition and structure, plant community and stage of stand development should be identified.