Aspen Ecology in Rocky Mountain National Park: Age Distribution, Genetics, and the Effects of Elk Herbivory

Zeigenfuss, Linda; Binkley, Dan; Tuskan, G

Lack of aspen (Populus tremuloides) recruitment and canopy replacement of aspen stands that grow on the edges of grasslands on the low-elevation elk (Cervus elaphus) winter range of Rocky Mountain National Park (RMNP) in Colorado has been a cause of concern for more than 70 years (Packard, 1942; Olmsted, 1979; Stevens, 1980; Hess, 1993; R.J. Monello, T.L. Johnson, and R.G. Wright, Rocky Mountain National Park, 2006, written commun.). These aspen stands are a significant resource since they are located close to the park's road system and thus are highly visible to park visitors. Aspen communities are integral to the ecological structure of montane and subalpine landscapes because they contain high native species richness of plants, birds, and butterflies (Chong and others, 2001; Simonson and others, 2001; Chong and Stohlgren, 2007). These low-elevation, winter range stands also represent a unique component of the park's plant community diversity since most (more than 95 percent) of the park's aspen stands grow in coniferous forest, often on sheltered slopes and at higher elevations, while these winter range stands are situated on the low-elevation ecotone between the winter range grasslands and some of the park's drier coniferous forests. Several studies (Olmsted, 1979; C.E. Olmsted, University of Northern Colorado, 1997, written commun.; Baker and others, 1997; Suzuki and others, 1999) have documented the notable lack of regeneration on the low-elevation, eastern elk winter range of RMNP, especially since 1970, 2 years after the park's natural regulation, or "hands off" policy, of elk management went into effect (Baker and others, 1997). Similar results have been found at other low-elevation sites in the Colorado Front Range where chronic and intensive elk browsing occurs (Kashian and others, 2007). The findings of these studies contributed to a modeling effort to determine the history and future of aspen on the low-elevation, east side winter range of RMNP (M.B. Coughenour, Colorado State University, 2002, written commun.). The results of this modeling effort led to the speculation that these winter range aspen stands may have become established during the period when elk were rare or absent on the winter range from 1880 and 1920. When large predators, such as wolves and grizzly bears, still existed in the park, they may have contributed to aspen establishment or release by limiting elk numbers and concentrations. Based on the findings of all components of this study, the low-elevation, winter range aspen stands of Rocky Mountain National Park are without doubt a unique resource, but conservation 6 efforts are necessary to protect them if park managers desire to increase their chances for continued viability. These stands appear to be highly resilient in the face of very intensive herbivory by elk and harsh environmental conditions. They have adapted through processes of natural selection to produce many genetically unique individuals, which may have a range of characteristics specifically selected to help them cope with the intense herbivory and xeric site conditions that characterize the low-elevation, east side elk winter range of RMNP. However, conservation efforts through fencing protection and reduced elk browsing pressure would enhance the continued survival of these aspen stands by providing conditions that encourage stem recruitment to the tree canopy. Monitoring of these low-elevation, winter range stands as new elk management actions are implemented and adapted would ensure that these stands are sustained on the landscape.

Aspen Ecology in Rocky Mountain National Park: Age Distribution, Genetics, and the Effects of Elk Herbivory

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