Impacts of western juniper on plant community composition and structure

Western juniper (Juniperus occidentalis Hook.) has been actively invading s hrub steppe communities during the past 120 years. The majority of these st ands are still in transition, from early open juniper shrub steppe communit ies to closed juniper woodlands. In addition, juniper expansion has been oc curring across a broad array of soils and topographic positions. Despite th e high degree of spatial and developmental heterogeneity, juniper woodlands are frequently treated generically in resource inventories, management, an d wildlife habitat assessments. Our goal was to evaluate the impact of west ern juniper encroachment and dominance on plant community composition and s tructure across several plant associations. This study was conducted in sou theastern Oregon and northeastern California on low sagebrush (Artemisia ar buscula Nutt.), mountain big sagebrush (A. tridentata spp. vaseyana (RYBD.) Beetle), and aspen (Populus tremuloides Michx.) alliances. Stages of woodla nd development across plant associations were categorized into 1 of 4 succe ssional phases (early, mid, late, and closed) based on tree growth and stan d structural characteristics. Plant cover by species group, species diversi ty and richness, bareground cover, soil characteristics, elevation, aspect, and slope were measured in 108, 60 x 46 m macroplots. Twinspan was used to sort plant communities. Regression analysis was used to evaluate the relat ionship 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 assoc iations varying from 19% cover and 64 trees ha(-1) in a low sagebrush commu nity 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 e ffect on bitterbrush (Purshia tridentata Pursh,). In the mountain big sageb rush alliance, sagebrush cover declined to approximately 80% of maximum pot ential as juniper increased to about 50% of maximum canopy cover. Aspen (Po pulus tremuloides Michx.) also declined as juniper dominance increased. Her baceous cover and species diversity declined and bare ground increased with increasing juniper dominance in the mountain big sagebrush/Thurber needleg rass association. However, herbaceous cover on the deeper soils characteriz ed by Idaho fescue did not decrease with increasing juniper dominance. To d etermine the effect of juniper dominance or woodland management on communit y composition and structure, plant community and stage of stand development should be identified.