Effects of cattle grazing on understory shrubs in Saskatchewan aspen forests
Date
1978
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Masters
Abstract
The effect of cattle grazing on understory shrub populations
in closed aspen forests on glacial till-derived soils in west-central
Saskatchewan was studied by sampling of existing fence-line contrasts.
Density of stems, stem heights, stem browse-impact classes and percent
cover were measured for each shrub species in the grazed and ungrazed
stands. Preference ratings calculated from the browse-impact class
distributions in grazed and ungrazed stands indicated that the preferences
of cattle and wild ungulates for browse species are very
similar, suggesting food competition. Effects of cattle grazing on
understory composition were studied by arranging the grazed stands on
a gradient of increasing grazing impact, their positions determined
by the average amount of change in species-abundance (measured by
height-weighted density, i.e. density with each stem weighted by its
height} from the corresponding ungrazed stands. Plots of the various
measures against this gradient showed that mean stem-height declines
under all levels of grazing, whereas the density of a species usually
increases over part of the gradient before decreasing, suggesting that
increases are by increased natality which is coincident with increased
mortality of browsed stems. In terms of height-weighted density,
Viburnum edule
decreases strongly under all levels of grazing,
Amelanchier alnifolia
shows more moderate decreases, and Rubus
strigosus,
Corylus cornuta,
Rosa acicularis,
Symphoricarpos albus,
and Lonicera dioica
increase over various portions of the gradient before declining
under heavier impacts. The interspecific variation in sensitivity to
grazing is only partly explained by differences in palatability to
cattle. The total height-weighted density of all shrub species declines
over nearly all of the gradient, and herb cover also declines with
grazing. Clipping of annual growth of twigs from independent samples
of stems indicated that twig production per stem increases nearly
linearly with stem-height, so that the changes in height-weighted
density shown by the gradient analysis were good approximations of the
corresponding changes in twig production per unit area. Twig-clipping
from browsed stems indicated that browsing reduces the twig production
per stem. Estimates of changes in above-snow twig production (from
changes in height-weighted density of stems over 50 em in height) showed
that, because of decreasing mean-heights, most species decline in production
of winter browse under all levels of grazing, and the decreases
are greater than for total browse. At the heaviest grazing impact sampled,
the abundance and twig production of shrubs (and especially the abovesnow
twig production) are nearly zero. The conclusion was that the
effect of cattle grazing on the wildlife habitat value of individual
stands is entirely negative, and the effect on cattle forage supply is
probably mostly negative as well, although there is a possibility that
production of summer browse may increase somewhat under the lightest grazing.
Description
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Citation
Degree
Master of Science (M.Sc.)
Department
Plant Ecology
Program
Plant Ecology