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Biota of the Colorado Plateau

Biotic Communities

Alpine Tundra
Subalpine Conifer Forest
Quaking Aspen Forest
Mixed Conifer Forest
Ponderosa Pine Forest
Montane Chaparral/Scrub
Pinyon-Juniper Woodland
Mountain Grasslands
Semi-arid Grasslands
Mountain Wetlands
Riparian Areas
Elevational Range
Merriam's Life Zones

Changes in the Biota

Endangered Species
California Condor
Endangered Fish
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Megafaunal Extinction
Invasive/Exotic Species
Forest Composition
Species Range Expansion
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Status and Trends of Plants
Riparian Degradation
Loss of Beaver
Wildfire History and Ecology
Ponderosa Fire Ecology
Tamarisk Invasion

Agents of Biotic Change

biotaBiotic Communities of the Colorado Plateau

Pinyon-Juniper Woodland

Pinyon-juniper forest

Pinyon-juniper woodland on the west face of Mt. Pennell in the Henry Mtns.
Photo © 1999 Ray Wheeler

Pinyon-pine and juniper woodlands are widespread on the Colorado Plateau between about 5000 feet to 7000 feet in elevation. 11.4% of New Mexico, or about 3.6 million hectares, consists of pinyon-juniper habitat. In Arizona there are over 5 million hectares of "P-J," as this "elfin" or "pygmy" woodland is often called.

While the U.S. Forest Service distinguishes 32 pinyon and 23 juniper plant communities, Colorado pinyon pine (Pinus edulis) is the most common pine species in this woodland type, and Utah juniper (Juniperus osteosperma) is the most common juniper. One-seed (J. monosperma), Rocky Mountain (J. scopulorum), and alligator (J. deppeana) junipers can be abundant in different areas of the Plateau.

Annual precipitation is typically from 10 to about 15 inches in pinyon-juniper woodlands, and tree species in these communities have evolved both drought and cold resistance. Pinyons dominate at higher elevations, and tend to form more closed-canopied stands that exhibit forestlike dynamics and species composition, commonly including a significant shrub component of oaks and alderleaf, mountain mahogany and limited grasses. Juniper tends to grow at lower elevations and in more arid areas as its scaled foliage allows it to conserve water more effectively than pinyon pine. Juniper-dominated woodlands tend to include open savannas of scattered trees without a significant shrub component, except in areas where big sagebrush has become dominant as a consequence of overgrazing. There are relatively few vertebrates endemic to pinyon-juniper woodlands, but there are significant levels of biodiversity in less prominent organisms such as herbaceous vegetation and soil organisms.

Juniper and big sagebrush

Utah Juniper (the tree in the right foreground) with big sagebrush (gray-green shrubs on the left). Photo courtesy USGS/BRD.

The long history of livestock grazing in many pinyon-juniper woodlands on the Colorado Plateau has both diminished and altered herbaceous vegetation, leading to widespread desertification of understory conditions. Although there is little firm documentation of woodland fire histories, many researchers believe that year-round grazing since the late 1800s suppressed former fire regimes; surface fires could no longer spread though the bare interspaces between the trees. Accelerated precipitation runoff and soil erosion commonly occur in these areas, leading to significant, permanent losses of site productivity and erosive watershed conditions. Major vegetative changes include decreases in cool-season grasses, and increases in grazing-resistant plants such as snakeweed and big sagebrush.

Woodland communities have expanded considerably over the course of this century in many parts of the Colorado Plateau. Tree densities have increased, and junipers and pinyon pines have expanded upslope into ponderosa pine forests and downslope into grass and shrub communities. Densities have increased in some areas to the point that larger proportions of pinyon-juniper woodland can now support crown fires. For a discussion of the causes of this type of woodland expansion, see the research essay, Where Have All the Grasslands Gone?

Chaining of pinyon-juniper woodland

Snowcovered chaining project - Dark Canyon Plateau, Utah. Photo © 1999 Ray Wheeler.

"Chaining," or mechanical removal of woodlands by land management agencies, has been commonly used in an effort to convert woodlands to grasslands for livestock. For example, about 600,000 hectares of pinyon-juniper woodlands were mechanically treated in Arizona during the 1950s and early 1960s, as were about 223,000 hectares of woodland on U.S. National Forests alone between 1950 and 1985. This process, though effective in uprooting and killing the trees, is damaging to other community species and may be less beneficial than other management strategies. Its use has been greatly reduced since the 1970s. Harvests for fuelwood also peaked in the 1970s, when harvest levels proved unsustainable in some areas. Current management policy by the U.S. Forest Service focuses more attention on active management of woodlands to restore less erosive watershed conditions.


Where have all the grasslands gone? Numerous ecological studies across the Southwest have documented the decline in herbaceous vegetation (grasses and non-woody flowering plants) while forests thicken and brush invades. Documenting the changes in the Jemez Mountains of northern New Mexico, ecologist Craig Allen considers the evidence that these patterns are tied to changes in land use history, primarily livestock grazing and fire suppression.

Packrat Midden Research in the Grand Canyon. On the Colorado Plateau the ice age (Pleistocene) vegetation of the Grand Canyon has been determined through the analysis of plant fossils preserved in caves and fossil packrat middens.  Large changes occurred as the most recent ice age ended and the Holocene era began. Adapted by Kenneth L. Cole from his journal article.


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Archer, S. 1994. Woody plant encroachment into southwestern grasslands and savannas: rates, patterns and proximate causes. Pp. 13-68 In: Vavra, M., Laycock, W. A. and Pieper, R. D., editors. Ecological implications of livestock herbivory in the west. Society for Range Management, Denver, CO.

Arnold, J. F., Jameson, D. A. and Reid, E. H. 1964. The pinyon-juniper type of Arizona--Effects of grazing, fire and tree control. Forest Service Production Research Report 84. U.S. Department of Agriculture, 28 pp.

Barney, M. A. and Frischknecht, N. C. 1974. Vegetation changes following fire in the pinyon-juniper type of west-central Utah. Journal of Range Management 27: 91-96.

Betancourt, J. L. 1987. Paleoecology of pinyon-juniper woodlands: Summary. Pp. 129-139 In: Proceedings of the Pinyon-Juniper Conference. General Technical Report 215. USDA.

Betancourt, J. L., Pierson, E. A., Aasen-Rylander, K., Fairchild-Parks, J. A. and Dean, J. S. 1993. Influence of history and climate on New Mexico pinyon-juniper woodlands. Pp. 42-62 In: Aldron, E. F. and D.W.Shaw, editors. Managing pinyon-juniper ecosystems for sustainability and social needs: proceedings of the symposium: Santa Fe, NM, April 26-30. General Technical Report RM-236. USDA Forest Service.

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Branson, F. A. 1985. Vegetation changes on western rangelands. Range Monograph No. 2. Society for Range Management, Denver, CO.

Brown, D. E. 1994. Biotic communities of the Southwestern United States and northwestern Mexico. University of Utah Press, Salt Lake City, 342 pp.

Cartledge, T. R. and Propper, J. G. 1993. Pinon–juniper ecosystems throughout time: information and insights from the past. Report RM-236. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, Santa Fe, NM.

Christensen, E. M. and Johnson, H. B. 1963. Presettlement vegetation and vegetational changes in three valleys in central Utah. Brigham Young University Science Bulletin Biological Series 4: 1-16.

Cottam, W. P, and G. Stewart. 1940. Plant succession as a result of grazing and of meadow desiccation by erosion since settlement in 1862. Journal of Forestry 38:613­626.

Davenport, D. W., Breashears, D. D., Wilcox, B. P. and Allen, C. D. 1998. Viewpoint: Sustainability of Pinon-juniper Ecosystems -- A unifying perspective of soil erosion thresholds. Journal of Range Management 51: 229-238.

Davis, O. K. 1987. Palynological evidence for historic juniper invasion in central Arizona: a late-Quaternary perspective. Pp. 120-124 In: The Pinyon-Juniper Ecosystem, A symposium: 1987. Utah State University, Logan, UT.

Despain, D. W. 1987. History and results of prescribed burning of pinyon-juniper woodland on the Hualapai Indian reservation in Arizona. Pp. 145-151 In: Everett, R. L., editor Pinyon-juniper conference. General Technical Report INT-215. USDA Forest Service, Intermountain Research Station, Ogden, UT.

Despain, D. W. and Mosley, J. C. 1990. Fire history and stand structure of a pinyon-juniper woodland at Walnut Canyon National Monument, Arizona. Technical Report. Cooperative National Park Resources Studies Unit, University of Arizona, Tucson, 27 pp.

Dick-Peddie, W. A. 1993. New Mexico vegetation: Past, present and future. University of New Mexico Press, Albuquerque, 244 pp.

Dwyer, D. D. and Pieper, R. D. 1967. Fire effects on blue grama-pinyon-juniper rangeland in New Mexico. Journal of Range Management 20: 359-362.

Evans, R. A. 1988. Management of pinyon­juniper woodlands. U.S. Forest Service General Technical Report INT-249, 34 pp.

Everett, R. L. 1987. Proceedings--Pinyon­Juniper conference. General Technical Report INT-215. U.S. Forest Service Intermountain Research Station, Ogden. UT, 581 pp.

Gottfried, G. J., Swetnam, T. W., Allen, C. D., Betancourt, J. L. and Chung-MacCoubrey, A. L. 1995. Pinyon-Juniper Woodlands. Pp. 95-132 In: Finch, D. M. and Tainter, J. A., editors. Ecology, diversity, and sustainability of the Middle Rio Grande Basin. General Technical Report RM-GTR-268. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.

Grissino-Mayer, H., Swetnam, T. W. and Adams, R. K. 1997. The rare, old-aged conifers of El Malpais: Their role in understanding climatic change in the American Southwest. Pp. 155-161 In: Mabery, K., editor. Natural History of El Malpais National Monument. Bulletin 156. New Mexico Bureau of Mines and Mineral Resources, Socorro, NM.

Hull, A. C. and Hull, M. K. 1974. Presettlement vegetation of Cache Valley, Utah and Idaho. Journal of Range Management. 27: 27-29.

Jacobs, B. F. and Gatewod, R. G. 1999. Restoration studies in degraded pinon-juniper woodlands of north-central New Mexico. Pp. 294-298 In: Monsen, S. B., Stevens, R., Tausch, R. J., Miller, R. and Goodrich, S., editors. Proceedings: Ecology and Management of Pinyon-Juniper Communities Within the Interior West. Proc. RMRS-P-9. USDA Forest Service, Ogden, UT.

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Johnsen, T. N., Jr. and Elson, J. W. 1979. Sixty years of change on a central Arizona grassland-juniper woodland ecotone. Agricultural Reviews and Manuals ARM-W-7. U.S. Department of Agriculture Science and Education Administration, 28 pp.

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