Biotic Communities of the Colorado Plateau

Quaking Aspen Forest

Old-growth aspen forest, south slope Boulder Mountain, Aquarius Plateau. Photo © 1999 Ray Wheeler

The widespread distribution of quaking aspen (Populus tremuloides) forests on the region’s high plateaus and mountain ranges and their importance to many wildlife species make these forests a significant biotic community on the Colorado Plateau. Large, nearly pure stands of aspen can be seen on the Markagunt, Aquarius, Pansaugunt, and Wasatch Plateaus of central and southern Utah and in the La Sal Mountains on the eastern border of the state. Grand Mesa and the Uncompaghre Plateau in far western Colorado also support extensive aspen forests. Further south, aspen is abundant in the White Mountains, on the Kaibab Plateau, and on the San Francisco Peaks in Arizona. Aspen is most commonly found between 7500 and about 10,500 feet on the Colorado Plateau, particularly on well-watered south-facing slopes.

In the La Sal Mountains of southeastern Utah, a zone of quaking aspen, interspersed with grass and forb meadows, occupies an elevational belt from 8500 to 10,000 feet. These pure aspen forests may actually represent "climax" vegetation, as Douglas-fir, white fir, and blue spruce are absent. It may be too dry to support these conifers, but it appears that soil type may be a more important factor. The aspens grow on soil derived from shale, and the conifers may have difficulty competing with aspen on that soil type.

The understory of most aspen communities is luxuriant when compared with those of associated coniferous forests. The combination of more abundant sunlight and favorable moisture conditions in many stands often leads to a rich forest floor of grasses, forbs, and shrubs. Drier groves are characerized by an herbaceous layer consiting mostly of grasses, with some wildflowers and medium-sized shrubs such as ninebark or cinquefoil. Wetter forests have an abundance of wildflowers, including meadowrue, arnica, lupine, and paintbrush, and sometimes a thicker shrublayer consisting commonly of snowberry or wild rose.

Aspen grow in distinct clones, which often can be distinguished in early summer when the trees are leafing out or during the fall when clones change different colors or at different times. Aspen stands are usually moist and do not readily burn, but the tree has thin bark and is easily killed by a light fire. However, after a fire aspen can readily re-sprout or sucker from shallow lateral roots. This is an important advantage over competing conifers which may have to reseed.

Although aspen is considered a climax species on some sites, it is usually seral to conifers. This replacement is gradual and can take from 100 to 200 or more years. If an aspen stand is within a mixed conifer forest, conifers can become established within a single decade. Because aspen stands are so different from conifer stands, they are very important for landscape diversity and wildlife habitat. Although aspen stems are short lived and snags do not stand long, the wood is soft, often decayed, and therefore useful to cavity-dependent species. Young sprouts are heavily browsed by elk and deer.

Late successional aspen forest. Photo by Keith Pohs

Aspen stands are in decline on the Colorado Plateau. The U.S. Forest Service extimates that between 1962 and 1986, aspen stands have declined by 46% in Arizona and New Mexico. The combination of modern fire suppression and a steady increase in elk herbivory has prevented aspen regeneration in many forests; conifer understories are now widely overtopping aspen stands. Aspen clones are able to persist in a suppressed state in the understories of conifers for many years, but without major fires aspen stands will continue to decline. The high probability of intense fires in southwestern conifer forests in the coming decades suggests that new aspen stands will develop again soon, changing their status from declining to increasing.

Research:

Changed Southwestern Forests: Resource effects and management remedies. Over 150 years of occupancy by northern Europeans has markedly changed vegetative conditions in the Southwest. Less fire due to grazing and fire suppression triggered a shift to forests with very high tree densities, which in turn contributed to destructive forest fires. Options to deal with these changes include prescribed fire, thinning and timber harvest to mimic natural disturbances and conditions. However, there are barriers to implementing these activities on a scale large enough to have a significant benefit. Adapted from a published journal article by Marlin Johnson.

Resources:

Allen, C. 1996. Fire effects in southwestern forests: Proceedings of the Second La Mesa Fire Symposium, Los Alamos, New Mexico, March 29-31, 1994.General Technical Report RM-GTR-286. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, CO.

Bartos, D., Ward, F. R. and Innis, G. S. 1983. Aspen succession in the intermountain West: a deterministic model. General Technical Report INT–153. USDA Forest Service, Intermountain Research Station, Ogden, UT, 60 pp.

Fitzhugh, E. L., Moir, W. H., Ludwig, J. A. and Ronco, F., Jr. 1987. Forest habitat types in the Apache, Gila, and part of the Cibola National Forests, Arizona and New Mexico. General Technical Report RM-145. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 116 pp.

Jones, J. R. and DeByle, N. V. 1985. Fire. Pp. 77–81 In: DeByle, N. V. and Winokur, R. P., editors. Aspen: ecology and management in the western United States. Report RM–119. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Mac, M. J., Opler, P. A. and Puckett, C. E. Status and trends of the nation's biological resources-grasslands. <http://159.189.96.215/resource/2000/grlands/grlands.htm> 7/24/00.

Mac, M. J., Opler, P. A., Haecker, C. E. P. and Doran, P. D., editors. 1998. Status and Trends of the Nation's Biological Resources. United States Geological Survey, Biological Resources Division, 986 pp.

Moir, W. H. 1993. Alpine tundra and coniferous forest. Pp. 47-84 In: Dick-Peddie, W. A., editor. New Mexico vegetation: past, present, and future. University of New Mexico Press, Albuquerque.

Potter, L. D. and Krenetsky, J. C. 1967. Plant succession with released grazing on New Mexico range lands. Journal of Range Management 20: 145-51.

USDA Forest Service. 1993. Changing conditions in southwestern forests and implications on land stewardship. U.S. Forest Service, Southwest Region, Albuquerque, N.M., 8 pp. {main}