Aspen Age Structure in the Northern Yellowstone Ecosystem

Age-structure analysis of aspen was conducted on Rocky Mountain elk winter range in the northern Yellowstone area by collecting increment cores from aspen trees in Yellowstone National Park, the Gallatin National Forest, and the Sunlight/Crandall area of the Shoshone National Forest. Our goal was to compare aspen age structure for elk winter range in the park with age structures developed for elk winter range in the national forests. We collected increment cores from aspen in three diameter size classes and three aspen habitat types (xeric, mesic, and scree). A special effort was made to collect increment cores from the relatively rare scree habitat type, since scree forms a "natural exclosure" where browsing pressure on aspen is reduced. The age structure of aspen in Yellowstone was significantly different from the age structures of aspen in either of the national forest areas. The Gallatin and Sunlight/Crandall age structures were not significantly different. Only 6% of aspen stands in Yellowstone contained stems that originated from 1920–1989, while 87% and 84% of the stands in the Gallatin and Sunlight/Crandall areas, respectively, contained stems from that period. Within Yellowstone, the age structure of aspen in the scree habitat type differed significantly from the mesic and xeric sites that were available for browsing. Aspen stems originating after 1920 dominated the scree stands, while trees originating between 1870 and 1920 dominated the non-scree stands. Aspen stands have successfully recruited new stems into their overstories in all habitat types from 1880–1989 in elk winter range on national forest areas surrounding the park. Within the park, aspen stands recruited new overstory stems between 1860 and 1929 in all habitat types. Since 1930, Yellowstone aspen have recruited overstory stems mostly in scree habitat type stands and other areas of reduced browsing pressure.