Biotic Communities of the Colorado Plateau

Ponderosa Pine Forest

Restored ponderosa pine forest near Flagstaff, Arizona. Photo by Keith Pohs.

Forests of ponderosa pine cover many of the higher mesas and mountains of the Colorado Plateau from 6000 feet to about 8000 feet in elevation. On higher terrain above 6500 feet in the southern part of the region, including the Kaibab Plateau and atop the Mogollon Rim, ponderosa pine often forms nearly pure stands covering tens of thousands of acres. The forest stretching from near Flagstaff along the rim to the White Mountains region is the largest ponderosa pine forest on the continent.

The predominant form of the pine throughout the Colorado Plateau is the three-needled, Rocky Mountain ponderosa pine (Pinus ponderosa var. scopulorum). Gambel oak (Quercus gambelii) is a common associate of ponderosa pine at lower elevations in the forest along with New Mexico locust (Robina neomexicana). At higher elevations, associates include southwestern white pine (Pinus strobiformis), Rocky Mountain Douglas-fir, (Pseudotsuga menziesii var. glauca), Rocky Mountain white fir (Abies concolor var. concolor), and quaking aspen (Populus tremuloides). Common understory plants include grasses such as Arizona fescue and mountain muhly and forbs such as lupine. Buckbrush, cliffrose, currant, and apache plume can be seen growing beneath the tall, spreading crowns of the pines as well.

Ponderosa pine forests have a remarkably dynamic history on the Plateau. Evidence found in packrat middens, alluvial and cave sites, and in ancient pollen samples collected from lake, bog, and wetland sites throughout the region reveal almost no ponderosa pine during the middle (50,000-27,500 B.P.) and late (27,500-14,000 B.P.) Wisconsin time periods. Instead, areas that are today vast forests of ponderosa were thickly-forested with a mixed assortment of different conifers, including subalpine species such as Engelmann spruce which today grow only at the highest elevations, thousands of feet above their former range. Differences are primarily a result of major climate changes associated with the end of the last major glacial period. Studies indicate that during the middle-Wisconsin temperatures on the Colorado Plateau were approximately 3-4 degrees Celsius cooler than they are today, and perhaps 5 degrees cooler during the late-Wisconsin.

Even during the Holocene of the last 11,000 years B.P., evidence suggests that ponderosa pines were more widespread at lowel elevation during the early to middle Holocene, only to contract their ranges in the late Holocene. Fire is a critical factor in the ecology of ponderosa pine, so relatively subtle changes in climate such as the timing of seasonal rains and their associated lightning strikes, may have a larger impact on ponderosa pine than on some other western conifers.

Today, the typical climate of a ponderosa pine forest includes an adequate, annual amount of moisture for good vegetative growth and conditions favorable for frequent early summer fires. Winters are relatively mild (average slightly above 30° F) and precipitation as snow saturates the soil. The spring dry season is accompanied by increasing air temperatures, low humidity, and persistent winds. The drought is broken in early to mid-July with the development of almost daily thunder and lightning storms, especially along the southern edge of the Colorado Plateau in areas such as the Mogollon Rim and White Mountains. July and August are the wettest, warmest months. A second dry season occurs in the fall. This climatic pattern is particularly conducive for development of a pine-grass savanna maintained by frequent surface fires.

Before European settlement, widespread surface fires that occurred every 2-15 years favored grasses and limited pine densities. Early explorers described majestic, open stands with rich grasses and occasional shrubs beneath, as young ponderosa seedlings were often killed by the low-intensity fires while mature pines, with their thick yellowish red bark, were only scarred. The effects of grazing and fire suppression since the late 1880s on ponderosa pine forests have been profound, including a shift to forests with very high tree densities, which in turn has contributed to destructive forest fires. Separate web pages give more detail on ponderosa pine fire ecology and reintroduction of fire to forest ecosystems. Two longer research essays, Changed Southwestern Forests: Resource effects and management remedies, and Restoring Ecosystem Health in Ponderosa Pine Forests of the Southwest are also available in this website.

Photograph courtesy of W. Sylvester Allred, Northern Arizona University

The tassel-eared Abert's squirrel is a distinctive inhabitant of ponderosa pine forests on the central and southern Colorado Plateau. Few mammal species are so closely tied to a particular tree as this squirrel is to ponderosa pine. The bushy-tailed squirrel uses the tree for nesting, shelter, and food, feeding on the ponderosa's seeds and the tree's cambium layer. Abert's squirrels can be quite acrobatic, sometimes jumping 40 feet or more to the ground unharmed.

Mule deer and rocky mountain elk also live among the pines along with smaller mammals including numerous chipmunks and voles. Several bird species are commonly seen in this pine habitat, including Steller's jay, brown creeper, white and red-breasted nuthatches, juncos, red-shafted flicker, and the colorful western tanager, a summer resident from the tropics.

Follow these links to:
Ponderosa Pine Fire Ecology
Reintroduction of Fire to Forest Ecosystems

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.

Restoring Ecosystem Health in Ponderosa Pine Forests of the Southwest. Restoration of ecosystem structure and reintroduction of fire are necessary for restoring rates of decomposition, nutrient cycling, and net primary production to natural, presettlement levels. The rates of these processes will be higher in an ecosystem that approximates the natural structure and disturbance regime. Adapted from a published journal article by W. Wallace Covington et al.

Allen, C. D. and Breshears, D. 1998. Drought-induced shift of a forest-woodland ecotone: Rapid landscape response to climate variation. Proceedings of the National Academy of Sciences 95: 14839-14842.

Allen, C. D. In review. Ecological patterns and environmental change in the Bandelier landscape. In: Kohler, T. A., editor Village Formation on the Pajarito Plateau, New Mexico: Archaeology of Bandelier National Monument. University of New Mexico Press, Albuquerque.

Allen, C. D., Betancourt, J. L. and Swetnam, T. W. 1998. Landscape changes in the southwestern United States: Techniques, long-term data sets and trends. Pp. 71-84 In: Sisk, T. D., editor Perspectives on the Land Use History of North America: A context for understanding our changing environment. Biological Science Report USGS/BRD/BSR-1998-0003. U.S. Geological Survey, Biological Resources Division, Reston, VA.

Anderson, R. S. 1989. Development of the southwestern ponderosa pine forests: what do we really know? Pp. 15-22 In: Multiresource Management of Ponderosa Pine Forests. General Technical Report RM-185. USDA Forest Service.

Avery, C. C., Larson, F. R. and Schubert, G. A. 1976. Fifty-year records of virgin stand development in southwestern ponderosa pine. General Technical Report RM-22. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 71 pp.

Belsky, A. J. and Blumenthal, D. M. 1996. Effects of livestock grazing on stand dynamics and soils in upland forests of the interior west. Conservation Biology 11: 315-327.

Betancourt, J. L. 1990. Late Quaternary biogeography of the Colorado Plateau. Pages 259­292 in J. L. Betancourt, T. R. Van Devender, and P. S. Martin, editors. Packrat middens: the last 40,000 years of biotic change. University of Arizona Press, Tucson. 468 pp.

Brown, D. E. 1982. Biotic communities of the American Southwest - United States and Mexico. Desert Plants 4: 1-341.

Cooper, C. F. 1960. Changes in vegetation, structure, and growth of southwestern pine forests since white settlement. Ecological Monographs 30: 129-164.

Covington, W. W. and Moore, M. M. 1994. Southwestern ponderosa forest structure and resource conditions: changes since Euro-American settlement. Journal of Forestry 92: 39-47.

Covington, W. W., Everett, R. L., Steele, R. W., Irwin, L. I., Daer, T. A. and Auclair, A. N. D. 1994. Historical and anticipated changes in forest ecosystems of the inland west of the United States. Journal of Sustainable Forestry 2: 13-63.

Covington, W. W., Fulé, P. Z., Moore, M. M., Hart, S. C., Kolb, T. E., Mast, J. N., Sackett, S. S. and Wagner, M. R. 1997. Restoring ecosystem health in ponderosa pine forests of the Southwest. Journal of Forestry 95: 23-29.

Ffolliott, P. F. and Gottfried, G. J. 1991. Natural tree regeneration after clearcutting in Arizona's ponderosa pine forests: two long-term case studies. Research Note RM-507. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station.

Harrington, M. G. and Sackett, S. S. 1992. Past and present fire effects on southwestern ponderosa pine old growth. Pp. 44–50 In: Proceedings of a workshop; Old-growth forests of the Southwest and Rocky Mountain Regions. RM-213. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Harrod, R. J., McRae, B. H. and Hartl, W. E. 1999. Historical stand reconstruction in ponderosa pine forests to guide silvicultural prescriptions. Forest Ecology and Management 114: 433-446.

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Pearson, G. A. 1950. Management of ponderosa pine in the southwest. U.S. Dept. of Agriculture Monograph 6: 218.

Savage, M., Brown, P. M. and Feddema, J. 1996. The role of climate in a pine forest regeneration pulse in the southwestern United States. Ecoscience 3: 310-318.

Stein, S. J. 1988. Explanations of the imbalanced age structure and scattered distribution of ponderosa pine within a high-elevation mixed coniferous forest. Forest Ecology and Management 25: 139-153.

Swetnam, T. W. 1990. Fire history and climate in the southwestern United States. Pages 6­17 in J. S. Krammes, technical coordinator. Effects of fire management of southwestern natural resources: proceedings of the symposium, November 15­17, 1988, Tucson, Arizona. U.S. Forest Service General Technical Report RM-191. 293 pp.

Swetnam, T. W., and C. H. Baisan. 1996. Historical fire regime patterns in southwestern United States since A.D. 1700. Pages 11­32 in C. D. Allen, technical editor. Fire effects in southwestern forests: proceedings of the second La Mesa fire symposium. U.S. Forest Service General Technical Report RM-GTR-286.

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: C. D. Allen, 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.

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