Fire history and western juniper encroachment in sagebrush steppe

Abstract

The recent expansion of juniper into sagebrush steppe communities throughout the semiarid Intermountain West is most frequently attributed to the reduced role of fire, introduction and overstocking of domestic livestock in the late 1800s, and mild and wet climate conditions around the turn of the century. This hypothesis has, however, limited quantitative support. There are few studies of fire history in the sagebrush steppe and none that examine the chronosequence of changes in mean fire intervals, introduction of livestock, and coincident climatic conditions with the initiation of post-settlement juniper expansion. This study was undertaken to test the hypothesis that the postsettlement expansion of juniper was synchronous with the introduction of domestic livestock, reduction in fire frequency, and optimal climate conditions for plant growth. We documented the fire history and western juniper (Juniperus occidentalis Hook). woodland chronology for a sagebrush steppe in a 5,000 ha watershed in south central Oregon. Regional tree ring data were used as proxy data for presettlement climatic conditions. Western juniper age distribution was determined by coring trees across the study area. Fire history was constructed from several small clusters of presettlement ponderosa pine (Pinus ponderosa Laws.) scattered across the study area. Samples were crossdated to determine fire occurrence to the calendar year. Mean fire intervals were computed for each cluster based on cumulative fire history of each tree sampled within the cluster. Fire events in low sagebrush (Artemisia arbuscula Nutt.) were documented by determining death dates of fire-killed western juniper trees. Records dating the introduction and buildup of livestock during the late 1800s and dates of initial fire suppression were summarized. Western juniper expansion began between 1875 and 1885, with peak expansion rates occurring between 1905 and 1925. The fire record spans 1601 to 1996. Before 1897, mean fire intervals within individual clusters ranged from 12 to 15 years with years between fires varying between 3 to 28. Nearly one third of the fires in the basin were large and usually proceeded by one year of above-average tree ring growth. Two fire events were recorded in the sparsely vegetated low sagebrush site, 1717 and 1855. The last large fire occurred in the study area in 1870 and the last small fire in 1897. The time sequence of wet climatic conditions between 1870 and 1915, introduction of livestock, and the reduced role of fire support the hypothesis that these factors contributed to the postsettlement expansions of western juniper.