Whether lightning-caused or started by native peoples, wildfires were once common occurences throughout the grasslands and forests of the Colorado Plateau. Frequent fires maintened an open forest structure in the region's middle-elevation forests, prevented tree encroachment into mountain meadows and grasslands, and in some areas replaced forested land with grassland or savannah.
Prior to white settlement, fires likely burned through the Plateau's extensive pinyon-juniper woodlands every 1030 years, through the region's ponderosa pine communities every 210 years, and through mixed-conifer forests every 525 years. The much wetter and cooler spruce-fir forests atop the highest mountains and plateaus of the region probably went 150 years or more between fires.
The historic fire regimes changed dramatically with the arrival and settlement of Anglo-Americans. Livestock grazing removed much of the grassy fuels that carried frequent, surface fires; roads and trails broke up the continuity of forest fuels and further contributed to reductions in fire frequency and size. Because settlers saw fire as a threat, they actively suppressed it whenever they could. Initially, fire suppression was very successful because of low fuel loadings; but without fires to consume them, large fuel loads have accumulated over time.
The continuing threat of fire led to an approximately 60-year history of fire suppression policies by the United States Forest Service and other land management agencies. These efforts have resulted in far less frequent fires, disrupting the natural cycles of the region's forests and resulting in many damaging ecological effects. Forests with historically frequent, low-intensity fires were those initially most affected. Pinyon-juniper woodlands, ponderosa pine forests, and drier mixed conifer forests shifted from a fire regime of frequent, surface fires to one of stand-replacing, high-intensity fires.
Fire suppression has contributed to the buildup of organic materials (fuels) on the forest floor. Logging has added heavy fuels in the form of limbs, tree tops, and cull logs. In some areas, these heavy fuels have been removed by slash disposal (fuel treatment), prescribed fire, or firewood collection.
By the early 1900s, fire exclusion began altering forest composition and structure. The disruption of natural fire regimes has decreased the diversity of forested areas across the landscape. Frequent fires once killed conifer seedlings encroaching into forest meadows, maintaining numerous open parks in the region's highlands. Fire exclusion permits this encroachment, and meadow acreage has decreased significantly. Establishment of young trees in older stands has provided a fuel ladder for carrying fires into the canopy. With more stand-replacing fires, average stand age is reduced; the diversity inherent in old stands is lost.
Because of heavy fuel accumulations, fires that occur now are more intense and more difficult to contain. Certainly there are more larger fires and more catastrophic crown fires today than historically. On Southwestern forests, the number of fires burning more than 10 acres has increased each decade since the 1930s. The average size of fires since the 1970s has ranged from 14 to 16 acres per fire, double the average size of fires in the earlier decades of the 1940s to 1960s.
Fire-Southern Oscillation relations in the Southwestern United States. A close linkage between fire and climate could diminish the importance of local processes in the long-term dynamics of fire-prone ecosystems. The structure and diversity of communities regulated by fire may have nonequilibrial properties associated with variations in global climate. Successful prediction of vegetation change hinges on a better understanding of climatically driven disturbance regimes and the relative contributions of regional versus local processes to community dynamics.
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.
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.
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.
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.
Medany, M.H. and N.E. West. 1983. Livestock grazing - fire regime intervections within montane forests of Zion National Park, Utah. Ecology 64: 661-667.
Savage, M., and T. W. Swetnam. 1990. Early and persistent fire decline in a Navajo ponderosa pine forest. Ecology 70:23742378.
Stewart, Omer C. 1951. Burning and Natural Vegetation in the United States. Geographical Review 41: 317-320.
Swetnam, T.W. and J.L. Betancourt. 1990. Fire-Southern Oscillation relations in the southwestern United States. Science 24: 1017-1021.
Tausch, R.J. and N.E. West. 1988. Differential establishment of pinyon and juniper following fire. American Midland Naturalist 119: 174-184.
Touchan, R., T. W. Swetnam, and H. Grissino-Mayer. 1995. Effects of livestock grazing on pre-settlement fire regimes in New Mexico. Pages 268272 in J. K. Brown, R. W. Mutch, C. W. Spoon, and R. H. Wakimoto, technical coordinators. Symposium on fire in wilderness and park management. Missoula, Montana, 30 March to 1 April, 1993. U.S. Forest Service General Technical Report INT-320.
Weaver, H. 1974. Effects of Fire on Temporate Forests: Western United States. Pp. 279-319 In: T.T. Kozlowski and C.E. Ahlgren, editors. Fire and Ecosystems. Academic Press: New York, NY.