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The Impact of Native Ungulates and Livestock on Western Aspen Communities
Charles Kay, Utah State University, Logan, UT
Repeated browsing by mule deer, elk, cattle, or domestic sheep often prevents aspen from successfully regenerating on intermountain ranges - - defined as producing new stems greater than 2 m in height. Bark damage by elk can also hasten the decline of existing aspen trees (ramets). Excessive browsing may even eliminate entire aspen clones. In addition, ungulate use has a dramatic impact on understory species composition. Deer, elk and domestic sheep tend to remove palatable shrubs and forbs, while cattle tend to eliminate native grasses. Aspen stands heavily used by both wildlife and livestock are usually dominated by unpalatable forbs and non-native grasses, such as timothy or Kentucky bluegrass. If ungulate herbivory is excessive, fire will not successfully regenerate aspen. Instead, fire plus excessive herbivory only hastens the decline of aspen. Similarly, beaver-felled aspen will not successfully regenerate if the emerging suckers are repeatedly browsed by wildlife or livestock.
This raises the question of how aspen successfully regenerated in the past. Late 1800 photographs indicate that, historically, aspen was unbrowsed. First-person journals and archaeological data indicate that prior to European settlement, hunting by native people kept ungulate populations at very low levels. Thus, the heavily browsed aspen communities seen on many western ranges today are entirely outside the range of historical variability.