Aspen, elk, and fire in the Rocky Mountain national parks of North America

Trembling aspen (Populus tremuloides) forests in the Rocky Mountains are an indicator of ecological conditions because they have maintained their pres ence for thousands of years through vegetative reproduction, and these comm unities have high biodiversity. Aspen can be tied to ecosystem condition th rough a 4-level trophic model that links humans, wolves (Canis lupus), elk (Cervus elaphus), and aspen through the processes of predation, herbivory, burning, and differential wildlife responses to humans. We used a comparati ve study of research literature and historic photographs to evaluate aspen change over time in 6 Rocky Mountain national parks in Canada and the Unite d States: Jasper, Banff, Yoho, Kootenay, Yellowstone, and Rocky Mountain. A cross all parks, aspen has consistent responses to increased browsing by un gulates and decreased frequency of fire. Although aspen was historically vi gorous in all parks, today most stands are in decline. Trees are intermedia te to mature in age (60-120 years old), and elk browse off new suckers befo re they reach 2 m in height. Fire, combined with browsing, has hindered reg eneration of aspen. The exception to this pattern is northern Jasper Nation al Park where elk densities appear to have been reduced by wolves in the 19 70s, and aspen regenerated. We found a gradient of increasing human-caused ecosystem changes in Rocky Mountain national parks: (1) historic conditions with frequent fires and low elk density regulated by humans, wolves, and o ther predators (i.e., all parks); (2) current conditions of patches of high elk density, where wolves are displaced by human land use, within a matrix of moderate elk density, where wolves and other predators are present (i.e ., Banff and Jasper national parks); (3) recent conditions inside parks, wh ere wolves are absent, and very high elk density is regulated by competitio n for food (i.e., Yellowstone before 1995); and (4) potential future condit ions, where increasing human land use around parks displaces carnivores and reduces hunting, and very high elk density occurs throughout landscapes. A spen stands regenerate well in areas of low elk density and in some areas o f moderate elk density; however, in areas of high and very high elk density , aspen does not regenerate to heights >2 m, and burning accelerates clone deterioration. Our recommendations to national park managers are to restore carnivores, use fire in areas of low elk density, and control human uses t hat displace carnivores.