The recent expansion of juniper into sagebrush steppe communities throughou
t the semiarid Intermountain West is most frequently attributed to the redu
ced role of fire, introduction and overstocking of domestic livestock in th
e late 1800s, and mild and wet climate conditions around the turn of the ce
ntury. This hypothesis has, however, limited quantitative support. There ar
e few studies of fire history in the sagebrush steppe and none that examine
the chronosequence of changes in mean fire intervals, introduction of live
stock, and coincident climatic conditions with the initiation of postsettle
ment juniper expansion. This study was undertaken to test the hypothesis th
at the postsettlement expansion of juniper was synchronous with the introdu
ction of domestic livestock, reduction in fire frequency, and optimal clima
te 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 a
rea. Fire history was constructed from several small clusters of presettlem
ent 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 fir
e history of each tree sampled within the cluster. Fire events in low sageb
rush (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 suppre
ssion were summarized. Western juniper expansion began between 1875 and 188
5, with peak expansion rates occurring between 1905 and 1925. The fire reco
rd spans 1601 to 1996. Before 1897, mean fire intervals within individual c
lusters ranged from 12 to 15 years with years between fires varying between
3 to 28. Nearly one third of the fires in the basin mere large and usually
proceeded by one year of above-average tree ring growth. Two fire events w
ere recorded in the sparsely vegetated low sagebrush site, 1717 and 1855. T
he last large fire occurred in the study area in 1870 and the last small fi
re in 1897. The time sequence of wet climatic conditions between 1870 and 1
915, introduction of livestock, and the reduced role of fire support the hy
pothesis that these factors contributed to the postsettlement expansion of
western juniper.