Using packrat middens to assess how grazing influences vegetation change in Glen Canyon National Recreation Area, Utah

The fossil and sub-fossil plant macrofossils and pollen
grains found in packrat middens can serve as important proxies
for climate and vegetation change in the arid Southwestern
United States. A new application for packrat midden research
is in understanding post-settlement vegetation changes caused
by the grazing of domesticated animals. This work examines
a series of 27 middens from Glen Canyon National Recreation
Area (GLCA), spanning from 995 yr BP to the present, which
detail vegetation during the periods just prior to, and following,
the introduction of domesticated grazers. By comparing
middens deposited before and after the start of grazing by
domesticated sheep and cattle, the effect on the native plant
communities through time can be determined. This analysis of
change through time is augmented by measurements of change
through space by contrasting contemporaneous middens from
nearby similar grazed and ungrazed sites. These comparisons
are only made possible by the presence of inaccessible
ungrazed areas surrounded by steep cliffs.
Multivariate ordinations of the plant assemblages from
packrat middens demonstrated that even though all middens
were selected from similar geologic substrates, soils, and
vegetation type, their primary variability was site-to-site. This
suggests that selecting comparable grazed versus ungrazed
study treatments would be difficult, and that two similar sites
several kilometers apart should not be assumed to have been
the same prior to grazing without pre-grazing data. But, the
changes through time on grazed areas, as well as the differences
between grazed and ungrazed areas in the diversity of
certain taxonomic groups, both suggest that grazing by domesticated
ungulates has had a noticeable effect on the vegetation.
The changes seen through time suggested that grazing lowered
the number of taxa recorded and lessened the pre-existing
differences within sites, homogenizing the resultant plant
associations.
Late Holocene pre-settlement middens, and modern
middens from ungrazed areas, contained more native grasses, skunkbush sumac (Rhus trilobata), blackbrush (Coleogyne
ramosissima), winterfat (Krascheninnikovia lanata), Utah serviceberry
(Amelanchier utahensis), and roundleaf buffaloberry
(Shepherdia rotundifolia) than modern middens from grazed
areas. Pollen data supported the macrofossil data, recording
decreases in pollen of the goosefoot family (Chenopodiaceae),
grass family (Poaceae), and globemallow (Sphaeralcea spp.)
from pre- to post-settlement.