Effects of cattle grazing on understory shrubs in Saskatchewan aspen forests
Thorpe, Jeffrey Peter
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.