Analysis of Aspen Stand Structure and Composition in the Western U.S.: Implications for Management

Aspen communities in the western U.S. are considered at risk because of low levels of disturbance (primarily fire) and high levels of herbivory by wild and domestic ungulates. Although estimates of the affected area vary, there appears to be a trend toward the loss of aspen-dominated stands West-wide. In some cases the loss is caused by succession, with shade-tolerant conifers becoming dominant. On dry sites where aspen is considered to be the climax, deterioration of aspen stands and loss of regeneration may lead to a conversion to sagebrush or grassland. As a result, regeneration efforts have tended to classify aspen stands according to two “classic” models: aspen as seral to conifer communities, and pure aspen as a climax community. However, there is evidence that other stand dynamics models exist in stands having an aspen component. The discovery of very old aspen (275-300 years) and multiple aspen age classes in spruce-fir forests suggests that aspen may persist in these stands through gap-phase replacement. In stands where aspen and lodgepole pine are only tree species, dominance may shift in favor of either species depending on the type and intensity of disturbance. Aspen is also a common associate of ponderosa pine, and the two species may coexist over time under a “maintenance” fire regime. Data from 3371 Forest Inventory and Analysis plots, located in stands with an aspen component and covering the range of aspen west of the 103rd meridian in the lower 48 United States, are being used to classify stands according to composition and structure. The results of this analysis should aid management decisions by providing a method for classifying stands according to their successional status, and may suggest alternative stand dynamics models by which aspen may be maintained on the landscape.