|Abstract or Summary
- Evidence suggests that western juniper
(Juniperus occidentalis) in Central Oregon affects
understory production and composition. As trees
increase in size and density, understory production is
reduced and composition changes. This study was
designed to identify the relationship between
production and composition of understory vegetation
and various tree canopy sizes of western juniper, and
to describe the responses of understory vegetation to
removal of western juniper. Understory production was
sampled in 1983 and 1984 by clipping plots on an
individual trees basis from areas with trees present and areas from which trees had been removed in 1982.
Biomass production was determined from trees within
three canopy diameter size classes and from two zones,
beneath the canopy and in the interspace. Production
was examined on two sites, a lower slope, shallow soil
site and an upper slope, moderately deep soil site.
Some year-to-year differences in production
of individual species and specific tree sizes may be
explained by variation in precipitation during
critical growth periods the two years of the study.
Sandberg bluegrass and, possibly, Idaho fescue
production may have been greater in 1983 than 1984 due
to variation in late spring precipitation, especially
on shallow soils associated with small trees.
Differences in patterns of production of
individual species were most clearly developed
associated with large, rather than small and
intermediate trees. Cheatgrass, squirreltail,
bluebunch wheatgrass, and Idaho fescue were common
beneath the canopy, while Sandberg bluegrass was
common in the interspaces. Cheatgrass and perennial
forb production beneath the canopy increased with tree
size. Production of other species, such as Sandberg
bluegrass, was apparently not affected by tree size. Canopy removal resulted in species-specific
increases in biomass production both years. Production
increases were greater beneath the canopy than in the
interspaces. Perennial grasses provided small variable
production increases beneath the canopy of large
trees. Annual grasses, primarily cheatgrass, and
annual forbs, primarily Epilobium paniculatum,
contributed most to elevated productivity the first
two years following juniper removal. Cheatgrass
response was mainly associated with large trees, while
annual forb response was independent of tree size.
Sandberg bluegrass production seemed more closely tied
to growing season precipitation than to canopy
removal, regardless of tree size.