Subalpine Conifer Forest
At elevations of 10,000 feet to about 11,500 feet on the Colorado Plateau, annual precipitation can be relatively high for this semi-arid region, from 30 to 35 inches a year. Much of this falls as snow during the cooler months, but a significant portion falls as heavy rains during the summer monsoon season, especially along the southern margin of the Plateau. Where lingering snowpacks keep the forest floor moist for much of the year, subalpine conifer forests dominated by Engelmann spruce and subalpine fir occur commonly as small, isolated mountaintop stands. Spruce-fir forests grade into bristlecone pine stands on some treeline sites, particularly on limestone substrates and drier south-facing slopes, and into mixed-conifer forests at lower elevations. Significant stands of quaking aspen occur in subalpine forests, particularly after fires.
In contrast to mixed-conifer and ponderosa pine forests found at lower elevations, natural fire is relatively uncommon in subalpine conifer forests. Some spruce-fir stands experience mixed fire regimes, with patchy crown fires occurring about every several hundred years, and more frequent surface fires occuring every 15 to 30 years. Overall, subalpine forests have probably been less altered by modern fire suppression than have the lower elevation forests. Livestock grazing has also had less impact, due to the natural lack of herbage in these forests.
Subalpine conifers are adapted to the strong winds and frigid temperatures atop the high peaks and tablelands of the region. The branches of the spruce and fir trees are short and brittle, so they grow close together to buffer the wind; both of these species have narrow, pointed crowns, which help shed snow. In contrast, the subalpine pines, limber and bristlecone, have more open crowns with flexible branches which help them withstand heavy snowfalls and blustery winds with less damage. Nevertheless, the uprooting and blowdown of subalpine trees by wind (windthrow) is a major natural disturbance factor. Windthrow is exacerbated where partial cutting of spruce-fir forest exposes remaining old trees to new wind stresses. Logging operations also contribute to outbreaks of spruce beetles, since these insects prefer downed trees. In areas where spruce-fir stands have been clearcut, regeneration has been a problem. Projected climate changes might eliminate some subalpine forests from isolated mountain ranges.
Lack of aspen regeneration has also been a consequence of modern fire suppression, and conifer understories are now widely overtopping aspen stands. Elk herbivory on aspen sprouts also retards regeneration on small burns or clear-cuts. Between 1962 and 1986, the area of aspen stands declined by 46% in Arizona and New Mexico. Many aspen forests in the Southwest are now composed of trees more than 100 years old. These trees are particularly susceptible to increased insect and disease problems. Without major fires, aspen stands will continue to decline, although aspen clones are able to persist in a suppressed state in the understories of conifer forests for many years. 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.
Paleobotany and Paleoclimate of the Southern Colorado Plateau. The biota of the Colorado Plateau during the middle (50,000-27,500 B.P.) and late (27,500-14,000 B.P.) Wisconsin time periods was dramatically different from that seen today. Differences were primarily a result of major climate changes associated with the last major glacial period. This site examines the environment of the southern plateau during this time. Adapted by R. Scott Anderson from his journal article.
Late Holocene Environmental Change in the Upper Gunnison Basin, Colorado. The Upper Gunnison Basin is a high elevation (3100 to 3600 m) region on the edge of the Colorado Plateau in southwestern Colorado. Its unusual ecological characteristics include an absence of plant and animal taxa that should occur here. Fossil and archaeological evidence indicates that many of the missing species existed in the Basin during the late Pleistocene to middle Holocene. Authored by Steve Emslie.
References and other Resources:
Alexander, R. R. 1987. Ecology, silviculture, and management of the Engelmann sprucesubalpine fir type in the central and southern Rocky Mountains. U.S. Department of Agriculture, Agriculture Handbook 659, Washington, D.C. 144 pp.
Allen, C. D., technical editor. 1996. Fire effects in southwestern forests: proceedings of the second La Mesa fire symposium. U.S. Forest Service General Technical Report RM-GTR-286. 216 pp.
Anderson, R. S., Hasbargen, J., Koehler, P. A. and Feiler, E. J. 1999. Late Wisconsin and Holocene subalpine forests on the Markagunt Plateau of Utah, southwestern Colorado Plateau, U.S.A. Arctic, Antarctic & Alpine Research 31: 366-378.
Aplet, G. H., Laven, R. D. and Smith, F. W. 1988. Patterns of community dynamics in Colorado Engelmann spruce-subalpine fir forests. Ecology 62: 312-319.
Fall, P. L. 1985. Holocene dynamics of the subalpine forest in central Colorado. Pp. 31-46 In: Jacobs, B. F., Fall, P. L. and Davis, O. K., editors. Late Quaternary vegetation and climates of the American Southwest. Contributions Series Number 16. American Association of Stratigraphic Palynolologists Foundation, Houston, TX.
Gosz, J. R. 1992. Gradient analysis of ecological change in time and space: implications for forest management. Ecological Applications 2:248261.
Moir, W. H. 1993. Alpine tundra and coniferous forest. Pages 4784 in W. A. Dick-Peddie. New Mexico vegetation: past, present, and future. University of New Mexico Press, Albuquerque. 244 pp.
Moir, W. H., and L. S. Huckaby. 1994. Displacement ecology of trees near upper timberline. International Conference for Bear Research and Management 9(1):3542.
Savage, M., Reid, M. and Veblen, T. T. 1992. Diversity and disturbance in a Colorado subalpine forest. Physical Geography 13: 240.
Touchan, R., Allen, C. D. and Swetnam, T. W. 1996. Fire history and climatic pattens in ponderosa pine and mixed-conifer forests of the Jemez Mountains, northern New Mexico. Pp. 179-195 In: Allen, C. D., editor. Fire Effects in Southwestern forests: Proceedings of the Second La Mesa Fire Symposium. General Technical Report RM-286. USDA Forest Service, Fort Collins, CO.
USDA Forest Service. 1993. Changing conditions in southwestern forests and implications on land stewardship. U.S. Forest Service, Southwest Region, Albuquerque, N. Mex. 8 pp.
Veblen, T. T. and Hadley, K. S. R., M.S. 1991. Disturbance and stand development of a Colorado subalpine forest. Journal of Biogeography 18: 707-716.