Impacts of Western Juniper on Plant Community Composition and Structure

Abstract

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 sagebrush (Artemisia arbuscula Nutt.), mountain big sagebrush (A. t r i d e n t a t a spp. vaseyana (RYBD.)Beetle), and aspen (P o p u l u s 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 46 m 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.

Sagebrush steppe recovery after fire varies by development phase of Juniperus occidentalis woodland

“…Twelve Phase 2 and nine Phase 3, 0.63-ha plots were established in May 2003. Criteria for determining woodland phase (cover of herbaceous, shrub and tree life forms) were taken from Miller et al (2000Miller et al ( , 2005. Phase 2 and Phase 3 woodlands were intermixed within an area of 15 km 2 and were independent of each other (Fig.…”

A Process-Based Application of State-and-Transition Models: A Case Study of Western Juniper (Juniperus occidentalis) Encroachment

“…The ecohydrologic impacts of tree encroachment on soil water dynamics have been described and quantified (Miller et al, 2005;Mollnau et al, 2014;Roundy et al, 2014b). Junipers have competitive advantages over intercanopy herbaceous species in terms of soil water extraction, including extensive root systems capable of soil water redistribution (Breshears et al, 1997(Breshears et al, , 1998Miller et al, 2000Miller et al, , 2005Newman et al, 2010). Most of the woodland encroachment research dealing with impacts on soil water dynamics have focused on changes following juniper removal (Bates et al, 2000;Young et al, 2013;Mollnau et al, 2014;Roundy et al, 2014b), which may be influenced by the short-term flush in herbaceous vegetation immediately following tree removal (Miller et al, 2014;Roundy et al, 2014a).…”

Journal of Range Management, Volume 54, Number 6 (November 2001)

“…As aspen canopy cover decreased, juniper canopy cover increased (p = .0001). Others have reported a significant negative correlation between juniper canopy cover with aspen canopy cover and density (Miller et al 2000). Direct observation of juniper growth rings supports this competitive relationship.…”