Volume 22, Issue 4, August 2014, Pages 293–301
Abstract
The American Bison (Bison bison
Linnaeus) in the Henry Mountains are one of the last free-roaming,
genetically pure herds of bison remaining in North America. Anecdotal
evidence indicates that this herd is utilising a cattle winter range
during the summer and fall, creating a conflict between the state agency
that manages the bison, and the Bureau of Land Management (BLM) and
local ranchers. In theory, the addition of bison grazing pressure could
reduce forage availability in the short term and lead to undesired
changes in the plant community in the long term. Our objective was to
determine whether bison have altered the plant species composition of
the cattle winter range. We characterised plant species composition,
percent cover, and grazing intensity on three adjacent,
geomorphologically similar mesas. Grazing regimes were different on the
three mesas, one with bison and cattle present, one with cattle only
present, and the third with neither cattle nor bison present. Vegetation
surveys were accompanied by a 28-year remote sensing time series to
test for temporal shifts in an index of primary productivity. We found a
higher grazing intensity on two dominant forage species on the bison
plus cattle grazed mesa in fall, before the cattle were turned out to
winter pasture. Despite this difference in grazing intensity, we found
few differences in species composition, percent cover, or NDVI across
the three grazing regimes. Our results suggest that high intensity
summer bison grazing, while likely creating short-term reductions in
forage availability, has not caused differences in plant community
composition or productive potential. Shifts in community composition can
take years to unfold and just as long to correct; therefore, continued
monitoring of the combined effects of cattle and bison is needed.
Keywords
- Grazing;
- NDVI;
- Plant community composition;
- Plant–herbivore interactions;
- Rangeland management
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