Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2000
Publication Date: N/A
Interpretive Summary: Western juniper has been actively invading sagebrush/bunchgrass rangelands in the Great Basin for the past 120 years. It has been documented that juniper can have negative effects on the quality of habitat, forage production, and soil stability during the advanced stages of invasion. Despite the fact that western juniper is expanding into a wide variety of habitats, there has been no research which identifies the stages of invasion or documents the effects of juniper invasion in different habitats. Our study involved detailed measurements in 108 juniper stands in a two state area. We were able to identify 4 distinct phases of juniper invasion, and found that not all existing plant communities are affected by juniper invasion in the same manner. The results will allow land managers to identify stages of juniper invasion and sites where impacts are likely to be greatest, which will improve the planning process for juniper management.
Technical Abstract: This study was conducted in southeastern Oregon and northeastern California on low sagebrush (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 one of four 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 46 m macroplots. 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 in a low sagebrush community to 90% cover and 1731 trees/ha in an aspen community. Increase in juniper dominance had little impact on low sagebrush and an inconsistent affect 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.