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Abstract

Western juniper (Juniperus occidentalis) woodlands in Oregon have expanded four-fold from 600,000 ha in 1930 to > 2.6 million ha, often resulting in the reduction and fragmentation of sagebrush (Artemisia spp.) communities. We documented dynamics of western juniper across the John Day Ecological Province in central Oregon by recording size class and growth form at 178 sites. We used stratified random sampling, with strata based on vegetation association (sagebrush, juniper, other) and distance from juniper stands. Only 26 percent of sites contained pre-settlement trees (in other words, > 140 years old), and < 5 percent of the 2,254 junipers tallied were pre-settlement trees. Mean densities of pre-settlement trees by stratum ranged from 0 to 18 trees/ha, suggesting that historically, juniper was widely scattered across the landscape. Current densities of post-settlement trees ranged from 75 to 211 trees/ha in non-woodland strata to 457 trees/ha in the juniper stratum. Juniper in non-woodland strata was most abundant in sites adjacent to juniper stands and in sagebrush communities. Mean densities of post-settlement trees were greatest in the > 2.0-m tall size class (82 trees/ha), followed by the 0.3 to 1-m tall size class (52 trees/ha). These densities pose substantial risk to sagebrush communities in central Oregon. Questions remain about the extent of western juniper woodlands across the species’ range that have replaced or are expanding into sagebrush communities versus sites that historically supported woodlands. However, our findings suggest that within sagebrush communities of the John Day province, intensive management through removal of western juniper may be prudent, while retaining pre-settlement trees.

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