Pa. Knapp et Pt. Soule, RECENT JUNIPERUS-OCCIDENTALIS (WESTERN JUNIPER) EXPANSION ON A PROTECTED SITE IN CENTRAL OREGON, Global change biology, 4(3), 1998, pp. 347-357
The expansion of Juniperus occidentalis (western juniper) has been ext
ensive in the last century, and increases in density and cover have be
en linked with the indirect effects of domestic livestock grazing (i.e
. cessation of periodic fires, increases of nurse-plant sites), and mo
re favourable climatic conditions. In this study, we document changes
in vegetation (including J. occidentalis) in central Oregon over a 23-
year period and relate these changes to their probable causes. In June
1995 we returned to the Horse Ridge Research Natural Area (HRRNA), a
site that has a history of minimal anthropogenic impacts, to replicate
a 1972 vegetation survey. Using the canopy-intercept method, line int
ercept method, and aerial photography analysis to measure herbaceous c
over, shrub cover and tree cover, respectively, we found significant c
hanges had occurred in the 23-year period between studies. Relative ch
anges of tree, shrub, and perennial herbaceous cover were 59%, 7%, and
-38%, respectively. Relative increases in J. occidentalis density, as
measured by the number of clumps and the number of stems, were 37% an
d 53%, respectively. Mean maximum height of J. occidentalis had increa
sed by 10%. We examined the role of potentially confounding influences
(e.g. fire, grazing, pathogens, climatic variability) and found that
none of the traditional mechanisms implicated in J. occidentalis expan
sion adequately explained the observed changes. We suggest that the ro
le of biological inertia of both anthropogenic and natural means may h
ave had a profound effect on the J. occidentalis ecology of HRRNA.