Succession
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Late successional white fir beneath ponderosa pine trees. Photo courtesy of Rocky Mountain Research Station, Flagstaff, AZ. |
A naturally occurring agent of change in the biotic communities of the Colorado Plateau is ecological succession. Following any disturbance, either natural or human-caused, communities undergo a somewhat orderly process of recolonization termed succession. Disturbances can include avalanches, wildfires, blowdowns, insect infestations,disease outbreaks, grazing, and logging.
Ecologists have traditionally defined plant communities based on climax vegetation. The climax vegetation of an area is that which will outcompete other species over time and eventually dominate a site for a prolonged period, perhaps several hundred years or more, barring any new disturbance. Communities that are considered disclimax are thought to be recently disturbed and may include several species which, though they may be abundant at this time, will eventually be excluded as the climax species takes over the site.
Succession that occurs in an area that was previously vegetated is termed secondary succession. This differs from primary succession, when species colonize a site that was previously unvegetated, such as a talus slope, a recent mudslide, or a recent glacial moraine.
Plants that initially invade sites are considered pioneer species. These species recolonize the area and essentially prepare it for the invasion of later successional species. These later seral species may occupy the site for several hundred years until finally the climax species take over.
In forested communities, most climax tree species are tolerant of shade and establish themselves in the understory of an early seral stand. They eventually crowd out the less shade-tolerant seral species, forming a new climax forest. The photograph above shows white fir, a climax tree in some areas currently dominated by ponderosa pine, establishing new seedlings beneath the seral pine. Eventually these fir may take over this site.
Some seral species, such as ponderosa pine, are long-lived and can attain great heights. This allows them to remain part of a climax forest for an extended period of time, despite their intolerance of shade. They simply grow above the cover of the climax species and thus receive enough sunlight to sustain themselves.
Research:
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.
The Social and Ecological Consequences of Early Cattle Ranching in the Little Colorado River Basin. Examines the early development of cattle ranching in the Little Colorado River Basin, the various factors which contributed to overgrazing in the region, and the pervasive effects that early commercial cattle ranching had on the local environment. Adapted from a published journal article by William S. Abruzzi.
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. Adapted from a journal article by Thomas W. Swetnam and Julio L. Betancourt.
Resources:
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.
Cottam, W. P. and Stewart, G. 1940. Plant succession as a result of grazing and meadow desiccation by erosion since settlement in 1862. Journal of Forestry 38: 613-626.
Hanley, D. P., Schmidt, W. C. and Blake, G. M. 1975. Stand structure and successional status of two spruce-fir forests in southern Utah. Research Paper INT-176. USDA Forest Service, Intermountain Research Station, 16 pp.
Irvine, J. R. and West, N. E. 1979. Riparian tree species distribution and succession along the lower Escalante River, Utah. Southwestern Naturalist 24: 331-346.
Kleiner, E. F. 1983. Successional trends in an ungrazed, arid grassland over a decade. Journal of Range Management 36: 114-118.
Moir, W. H. and Dieterich, J. H. 1988. Old-growth ponderosa pine from succession in pine-bunchgrass forests in Arizona and New Mexico. Natural Areas Journal 8: 17-24.
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.
West, N. E. and Pelt, N. S. V. 1987. Successional patterns in pinyonÂjuniper woodlands. In: R. L. Everett, c., editor. Proceedings--PinyonÂJuniper conference. U.S. Forest Service General Technical Report INT-215.