Mary Lou Fairweather
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A Century of Browse Impacts and the Decline and Dieback of Aspen in Arizona
Mary Lou Fairweather, Arizona Zone of Forest Health Protection, Southwestern Region, USDA Forest Service, Flagstaff, AZ
For decades, domestic and wild ungulate browse impacts on aspen regeneration were implicated, along with fire suppression, in contributing to the reduction and structural changes of aspen forests across many regions of Arizona. At present, young aspen within these areas are typically found only behind fences or on steep slopes and rocky outcrops that are more difficult for ungulates to access. The fences are termed “elk exclosures”, because they are built to keep out Rocky Mountain elk, which has become the dominant browser over the last 50 years. Although elk were not known to exist on the San Francisco Peaks in northern Arizona over the first half of the 20th Century, aspen recruitment was infrequent to nonexistent due to browse impacts by domestic livestock. However, the installation of fencing in the 1940s to prevent sheep damage to research plots resulted in a regenerated treatment block that is visibly noticeable today. The aspen here are healthier and appear to have suffered less damage from conifer succession and the severe drought of 2002-2003 than surrounding aspen forests. A nearby non-traditional aspen silviculture treatment, where trees greater than 8 inches in diameter at breast height were removed, allowed residual trees to become a stand of mature aspen that is not characteristic of the surrounding forests. These historic research projects help to demonstrate the long-term impacts of browse damage.