Western juniper (Juniperus occidentalis Hook var. occidentalis Vasek) currently occupies 5 million acres in Oregon, 3 million acres in northeastern California, a ½ million acres in Nevada and Idaho, and a few limited stands in southeastern Washington. In Oregon, western juniper is the most extensive conifer type. This species occupies a broad array of environments and soils varying from poorly drained heavy clays to excessively drained pumice sands. It is a relatively long lived species, exceeding ages of 1,000 years with the oldest living tree reported at 1600 years (Oregon's oldest tree recorded to date). On dry rocky sites, dead trees can remain standing for up to 600 years with the center growth ring dating as far back as 50 to 100 BC. However, an estimated 95% of western juniper is less than 100 years old.
Despite the large degree of variability of environments occupied by western juniper and the varying stages of woodland succession occupying today's landscapes, juniper woodlands are frequently treated generically in management, resource inventories, and wildlife habitat assessments. Our woodland expansion research program is; (1) documenting the chronology of expansion, (2) evaluating the impact of increasing tree dominance on understory plant composition across different soils and plant communities, and (3) evaluate the affects of woodland succession on abundance and diversity of avian populations.
Since the late 1800s western juniper has been actively encroaching or increasing in density in Intermountain plant communities ranging from shallow rocky heavy clay soils occupied by low sagebrush to deep clay loam or loam soils occupied by mountain big sagebrush or aspen groves. Accelerated juniper establishment began during the 1870s. The most rapid period of establishment occurred between 1885 and 1925, a period of wetter than average conditions, few fires, and intensive livestock grazing. The majority of western juniper woodlands are still in a state of change, succeeding from open juniper shrub steppe communities to closed woodlands.
As juniper dominance increases on a site the shrub understory declines. 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. Mountain mahogany, bitterbrush, and aspen 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.