Submitted to: Oregon Agriculture Experiment Station Special Report
Publication Type: Experiment Station
Publication Acceptance Date: 6/1/1999
Publication Date: N/A
Interpretive Summary: During the past 120 years western juniper has been expanding into sagebrush steppe communities in the northern Great Basin. In many cases the expansion of western juniper results in a loss of understory species, which reduces forage production and increases erosion potential. However, there are also situations where western juniper appears to have minimal impact on understory plants. We sampled 108 juniper stands in southeastern Oregon and northeastern California, to determine which community types or topographic sites were most likely to be impacted by western juniper invasion. We found that shallow soils and/or south-facing slopes were most vulnerable to juniper impacts. The results will help land managers focus their juniper control treatments on sites that are most likely to be impacted.
Technical Abstract: Western juniper (Juniperus occidentalis) has been actively invading shrub steppe communities in the Pacific Northwest during the past 120 years. The majority of these stands are still in a state of transition from shrub steppe to juniper woodland. In addition to different stages of stand development, juniper expansion occurs in different plant communities occupying different soils and topographic postions. Despite this high degree of variability, juniper woodlands are frequently treated generically in resource inventories, management, and wildlife habitat assessments. The goal of this study was to evaluate the influence of juniper dominance on plant community dynamics across several plant communities commonly impacted by western juniper encroachment in southeast Oregon and northeast California. The increase in juniper dominance had little impact on low sagebrush and an inconsistent affect on bitterbrush. However, as juniper dominance increased to about 50% of maximum canopy cover, mountain big sagebrush declined by nearly 80% of its maximum potential. Aspen also significantly declined as juniper dominance increased. Herbaceous cover and species diversity declined and bareground increased with increasing juniper dominance in the mountain big sagebrush/Thurber needlegrass community. However, herbaceous cover on the deeper soils characterized by Idaho fescue and Columbia needlegrass did not decrease with increasing juniper dominance.