The role of livestock grazing and big-game browsing in the decline of aspen (Populus tremuloides), in the Intermountain West, has long been questioned. Kay and Bartos measured all known aspen exclosures on the Dixie and Fishlike National Forests in south-central Utah to determine aspen stem dynamics, successional status, and understory species composition. Aspen within all total-exclusion plots successfully regenerated and developed multi-aged stems without the influence of fire or other disturbance. Aspen subjected to herbivory, mainly mule deer (Odocoileus hemionus), either failed to regenerate successfully or regenerated at stem densities significantly lower than that on total-exclusion plots. Aspen did successfully regenerate on ungulate-use plots only when deer numbers were low. The addition of livestock grazing tended to reduce native grasses while promoting introduced species and bare soil. Kay and Bartos conclude that aspen stands in the Rocky Mountain west dominated by old or single-age trees are most likely a product of excessive ungulate browsing.
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