Species' ranges can both expand and contract over time. For example, ponderosa pine rapidly expanded onto the Colorado Plateau over thousands of years following the end of the Pleistocene. The range of the pine may have contracted locally over the last 50 years due to the ecologic effects of fire suppression.
A common instance of species expansion on the Colorado Plateau over the last century has been the advancement of non-palatable woody shrubs and trees such as sagebrush and juniper into the region's grasslands. Heavy grazing reduced community diversity and plant competition; as a result there were no fine fuels to carry surface and ground fires which were once common in the area's grasslands. Grazing also reduced competition from herbaceous species, allowed rapid growth of pinyon, juniper, oaks, and sagebrush into adjacent grassland communities (see photographs below). In the 1960s and 1970s juniper expansion blamed on fire suppression and livestock grazing justified aggressive programs of chaining and burning pinyon-juniper woodlands to improve forage and water yield.
Several authors have suggested that pinyon-juniper expansion may at least in part represent recovery from prehistoric fuel harvesting, at least in those areas that were heavily populated within the last 1000 years (Samuels and Betancourt 1982; Kohler 1988). Evidence also strongly suggests that there were cycles of increase and decrease in juniper woodlands during prehistoric times (Miller, 1994 p.145).
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.
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.
Archer, S., Schimel, D. S. and Holland, E. A. 1995. Mechanism of shrubland expansion: Land use, climate or CO2? Climate Change 28: 91-99.
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. 1995. Long and short-term climatic influences on southwestern shrublands. Pp. 5-9 In: Barrow, J. R., MacArthur, E. D., Sosebee, R. E. and Tausch, R. J., editors. Proceedings: Symposium on Shrubland Ecosystem Dynamics in a Changing Climate, Las Cruces, NM, 1995 May 23-25. General Technical Report INT-GTR-338. USDA Forest Service, Intermountain Research Station.
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.
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. Technical Report RM-GTR-268. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO.
Idso, S. B. 1992. Shrubland expansion in the American Southwest. Climate Change 22: 85-86.
Johnsen, T. N., Jr. 1962. One-seed juniper invasion of northern Arizona grasslands. Ecological Monographs 32: 187-207.
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.
Kohler, T. A. 1988. Long-term Anasazi land use and forest reduction: A case study from southwest Colorado. American Antiquity 53: 537-564.
Miller, R. F. and Wigand, P. E. 1994. Holocene changes in semiarid pinyon-juniper woodlands. Bioscience 44: 465-474.
Moore, M. M. 1994. Tree encroachment on meadows of the North Rim of Grand Canyon National Park. Report #CA B000-B-0002. National Park Service, 86 pp.
Samuels, M. L. and Betancourt, J. L. 1982. Modeling the long-term effects of fuelwood harvest on pinon-juniper woodlands. Environmental Management 6: 505-515.
West, N. E., Rea, K. H. and Tausch, R. J. 1975. Basic synecological relationships in pinyon-juniper woodlands. In: The Pinyon-Juniper Ecosystem: A symposium, Logan, UT. Utah State University.