Age distribution of aspen in Rocky Mountain National Park, USA

The age structure of aspen in Rocky Mountain National Park was characterized to determine if the number of aspen established in recent decades was substantially lower than would be expected from long-term trends. At lower elevations on the eastside of the Park, aspen density averaged 1250 trees/km2, compared with 550 trees/km2 at higher elevations and on the westside. Most aspen in the Park were relatively young, with trees less than 80 years old accounting for 90% of aspen trees and 50% of aspen basal area. The number of trees in each decadal age class increased exponentially from 1855 through 1965, with no decade showing significantly higher or lower aspen numbers than expected from the long-term trend. The number of aspen dating to 1975–1995 at lower elevations on the eastside fell 80–95% below the level expected from the long-term trend. No decline was evident in the rest of the Park. A variety of factors influences the rates of aspen mortality and recruitment, including major disturbances, climate patterns, conifer in-growth, and browsing. Severe browsing by elk appears to be the main factor accounting for low aspen numbers at low elevations on the eastside.