Aspen age structure and stand conditions on elk winter range in the northern Yellowstone ecosystem

by Larsen, Eric J.

Aerial photographs and field sampling were used to compare aspen (Populus

tremuloides) age structure and stand conditions on elk winter range in the northern

Yellowstone ecosystem. The elk winter ranges studied were the northern range in

Yellowstone National Park (YNP) and the Gallatin National Forest and the

Sunlight/Crandall elk winter range in the Shoshone National Forest.

I found significant differences when comparing aspen stands inside and

outside of YNP borders. The aspen stands in the Gallatin and Sunlight/Crandall areas

had a greater incidence of tall aspen suckers and stems in the 1-4, 5-9, and 10-19 cm

DBH classes. Aspen stems within YNP had a significantly higher percentage of stems

with high levels of bark damage (>66% of bark surface damaged on the lowest 3 m of

stem) than aspen stems in stands in the Gallatin or Sunlight/Crandall.

An aspen age structure was developed using 598 increment cores. The aspen

age structure in YNP was significantly different than the age structures of the Gallatin

and Sunlight/Crandall elk winter ranges. The Gallatin and Sunlight/Crandall areas did

not have significant differences in their age structures. The greatest differences

between YNP and the National Forest areas was in the younger age classes, measured

as aspen stems originating between 1920-1989.

Within YNP, I found that the aspen age structure, size class distribution,

incidence of tall suckers, and the percentage of browsed suckers of the scree habitat

type was significantly different than the xeric and mesic habitat types. Scree forms a

"natural exclosure" where ungulate browsing is reduced.

Aspen stands have successfully recruited new stems into their overstories in all

habitat types from 1880-1989 on the Sunlight/Crandall elk winter range and the

Gallatin's portion of the northern range. Within YNP, aspen stands successfully

recruited new overstory stems between 1860-1929 in all habitat types. Since 1930,

YNP aspen have successfully recruited overstory stems mostly in scree habitat type

stands and other areas of reduced browsing pressure. I discussed several potential

ecological factors impacting aspen overstory recruitment and conclude that changes in

ungulate browsing patterns best explains the spatial and temporal pattern I observed.