HISTORIC EXPANSION OF JUNIPERUS-OCCIDENTALIS (WESTERN JUNIPER) IN SOUTHEASTERN OREGON

The chronology of Juniperus occidentalis (western juniper) expansion i n eastern Oregon, the effect of plant canopy and interspace on J. occi dentalis seedling establishment and growth rates, and the age of J. oc cidentalis maximum reproductive potential were determined. Measurement s were recorded in twenty-two 0.4-ha plots established in sagebrush-gr assland communities and six 0.1-ha plots in Populus tremuloides (quaki ng aspen) communities. J. occidentalis began increasing during the 188 0s in stands containing trees >130 yr old. Relatively steady establish ment ensued into the 1950s and then began to progress at a geometric r ate in the 1960s. J. occidentalis encroachment into aspen stands began between 1910 and 1920. The largest proportion of juvenile trees estab lished 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 sc attered 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 establi shment, reduced fire-return intervals, and heavy livestock grazing. Th e 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.