Great Basin Naturalist


The chronology of Juniperus occidentalis (western juniper) expansion in eastern Oregon, the effect of plant canopy and interspace on J. occidentalis seedling establishment and growth rates, and the age of J. occidentalis maximum reproductive potential were determined. Measurements were recorded in twenty-two 0.4-ha plots established in sagebrush-grassland communities and six 0.1-ha plots in Populus tremuloides (quaking aspen) communities. J. occidentalis began increasing during the 1880s in stands containing > 130 yr old. Relatively steady establishment ensued into the 1950s and then began to progress at a geometric rate in the 19602. J. occidentalis encroachment into aspen stands began between 1910 and 1920. The largest proportion of juvenile trees established beneath Artemisia species in sagebrush-grassland communities. J. occidentalis trees appeared to reach full reproductive potential at > 50 yr of age. The ratio of male:female trees increased from 1.7 in scattered J. occidentalis stands to 3.8 in closed stands. The initiation of J. occidentalis encroachment during the late 1800s coincides with optimal climatic conditions for Juniperus berry production and establishment, reduced fire-return intervals, and heavy livestock grazing. The accelerated increase in J. occidentalis expansion since 1960 may be due to the continued absence of fire, abundant woody plant cover, and the large increase in J. occidentalis seed production.