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Changes in stand density. (View larger image) Credit: C. Allen, T. Swetnam and J. Betancourt |
Learning
About SW Climate > Overview of Fire Research
Ecological
Effects
Fire history research has been useful to managers in the Southwestern United States in providing the primary evidence of frequent surface fires inclined and mixed conifer forests before around 1900. These histories also indicate that large, high intensity, stand replacement fires in Southwestern ponderosa pine forests were rare. Other historical data supporting this interpretation include forest age structure and stand density reconstructions from dendrochronology, repeat photographs, historical writings, and early land survey records (Covington and Moore 1994).
The tree-ring record shows the lack of fire scars in the 20th century. Episodic surface burns that injure but do not kill mature trees have dwindled with cessation of aboriginal fires, removal of fine fuels by livestock grazing, and a vigorous program of fire suppression. Fire suppression has been partly responsible for rapid conversion of grasslands to shrublands in Arizona and New Mexico. Fire suppression has lead and could continue to lead to large-scale changes in forest structure and composition, as already evident in successional changes to more shade-tolerant trees since the turn of the century.
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Dog
Hair Thickets.
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larger image) |
Southwest fire history data also show that the end of frequent surface fire regimes was followed in subsequent decades by massive tree irruptions, leading to highly anomalous, dense stands of dog-hair thickets (Covington and Moore 1994, Fule et al. 1997) These thickets are now a key factor contributing to catastrophic, stand-replacing fires in some areas (Covington and Moore 1994). 20th-century fire statistics in the Southwest show clear trends of increasing number, size, and intensity of such catastrophic fires (Swetnam 1990, Dahm and Geils 1997, Swetnam and Betancourt 1998).
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Post-fire erosion, southeastern Arizona. (View larger image) Credit: Laboratory of Tree-Ring Research, The University of Arizona |
The next time there is an extended drought, we can expect some very severe fire behavior. What is worrisome is the scale and associated ecological effects of potential crown fires. Forests across entire mountainsides can be consumed, burning entire watersheds and resulting in severe post-fire erosion and flooding. Once such crown fires are in progress, direct fire suppression methods can seldom stop them.
References
Covington, W.W., and M.M. Moore, 1994: Southwestern ponderosa pine forest structure: changes since Euro-American settlement. Journal of Forestry 92:39-47.
Fule, P.Z., W.W. Covington, M.M. Moore, 1997: Determining reference conditions for ecosystem management of Southwestern ponderosa pine forests. Ecological Applications 7(3):895-908.
Swetnam, T.W., 1990: Fire history and climate in the southwestern United States. In J.S. Krammes, editor, Effects of Fire Management of Southwestern Natural Resources. USDA Forest Service General Technical Report RM-191:6-17.
Dahm, C.W., and B.W. Geils, 1997: An assessment of ecosystem health in the Southwest. U.S. Forest Service General Technical Report RM-GTR-295.
Swetnam, T.W., and J.L. Betancourt, 1998: Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate 11:3128-3147.
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