Forest Fire in the American Southwest The Problem header
The problemAnalysisSolutionsResourcesHot topicsLatest news
   
Ecology
Fire regimes
Anglo settlement
Lightning strikes
Climate cycles
Indian burning
Land use management
Livestock grazing
Fire suppression
Logging
Fuel loads

Bark beetles

 
Home
Index
Site map
Search this website

Tree Densities

Today's dense green ponderosa pine forests of the American Southwest bear little resemblance to the forests of centuries past. If H.G. Well's time machine whisked you back over a hundred years to a time prior to Anglo settlement, you would likely wonder if you had accidentally landed in a completely different forest.

Historical records from around the year 1900 show that the density of trees measuring 12 inches or greater in diameter at breast height (dbh) ranged from 8 to 51 trees per acre (Woolsey 1911). The forests were characterized as open and park-like, with diverse grasses, forbes, and shrubs in the understory. Numbers of trees in different age classes were evenly distributed with approximately equal numbers of young, middle-aged and old trees.

Today's forests are crowded with small-diameter ponderosa pines, leaving little room for the diversity of plant species that once flourished in the understory. Total tree densities now often exceed 1000 trees per acre (Allan 1998). The majority of the trees are young 50 to 100 year old trees with diameters of 3 to 6 inches dbh, fewer trees in the 6 to 9 inch class and even less in the 9 to 12 inch dbh class. Fire resistant trees, over 12 inches dbh, are relatively uncommon (see figure below).

Trees per acre graph

High densities of small diameter trees with low crown heights serve as ladder fuels for wild fires, carrying flames into the treetops of larger trees with devastating effects. Historically unprecedented fuel loads now cause fires to burn hotter, increasing their destructive potential.

J.G.

References

Allen, C. D. 1998. A ponderosa pine natural area reveals its secrets. Pages 551–552 in M. J. Mac, P. A. Opler, C. E. Puckett Haecker, and P. D. Doran, editors. Status and trends
of the nation’s biological resources. Two volumes. U.S. Department of Interior, U.S. Geological Survey, Reston, Virginia, USA. Available online at http://biology.usgs.gov/s+t/SNT/noframe/sw153.htm 4/24/03.

Allen, Craig D. et al. 2002. Ecological Restoration of Southwestern Ponderosa Pine Ecosystems: A Broad Perspective. Ecological Applications 12(5): 1418 – 1433. Available online at http://www.swfa.org/doc%20files/Allen_SWRestoration.pdf 4/24/03.

Conner, R.C., J.D. Born, A.W. Green, and R.A. O'Brien. 1990. Forest resources of Arizona. USDA Forest Service Resource Bulletin INT-69.

Cooper, Charles F. 1960. Changes in vegetation, structure, and growth of southwestern pine forests since white settlement. Ecological Monographs 30(2): 129-164.

Covington, W. W., R. L. Everett, R. Steele, L. L. Irwin, T. A. Daer, and A. N. D. Auclair. 1994. Historical and anticipated changes in forest ecosystems of the Inland West of the United States. Journal of Sustainable Forestry 2:13–63.

Covington, W. Wallace and M.M. Moore. 1994. Southwestern ponderosa pine forest structure: changes since Euro-American Settlement. Journal of Forestry 92:39-47.

Covington, W. Wallace, P.Z. Fule, M.M. Moore, [and others]. 1997. Restoring ecosystem health in ponderosa pine forests of the Southwest. Journal of Forestry 95(4): 23-29.

Dahms, Cathy W. and B.W. Geils, tech. eds. 1997. An assessment of forest ecosystem health in the Southwest. USDA Forest Service General Technical Report RM-GTR-295. Available on-line at http://www.srs.fs.usda.gov/pubs/viewpub.jsp?index=470.

Johnson, Marlin A. 1997. Changed Southwestern forests: resource effects and management remedies. Forest Ecology Working Group Session, Society of American Foresters National Convention, Albuquerque, New Mexico, November. 9-13, 1996. Available on-line at http://www.fs.fed.us/r3/resources/forestry/source_info/gen_forestry/1996_paper.pdf
4/14/03.

Swetnam, Thomas W.; C.D. Allen, and J.L. Betancourt. 1999. Applied historical ecology: Using the past to manage for the future. Ecological Applications 9: 1189-1206. Available on-line at http://www.mesc.usgs.gov/products/pubs/258/258.pdf

Swetnam, Thomas W.; J.L. Betancout. 1998. Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest. Journal of Climate 11: 3128-3147.

Temple, Ryan, P. Gagnon, S. Harrington, J. Bailey, Y. S. Kim, D. Larson and W. Zipse. 1999. Assessment of Forest Resources and Communities in the Four Corners Region: Synthesis Report. The Forest Trust. Research Synthesis Report 11, 194 pages. See below.

Temple, Ryan, P. Gagnon, S. Harrington, J. Bailey, Y. S. Kim, D. Larson and W. Zipse. 1999. Assessment of Forest Resources and Communities in the Four Corners Region: Research Summary Report. The Forest Trust. Research Summary Report 11, 6 pages. Available on-line at http://publications.theforesttrust.org/download/pdf/4cornerexec.pdf

U.S. General Accounting Office. 1999. Western national forests: a cohesive strategy is needed to address catastrophic wildfire threats. GAO/RCED-99-65. U.S. Government Printing Office.

Woolsey, T. S., Jr. 1911. Western yellow pine in Arizona and New Mexico. USDA, Forest Service Bulletin 101. 64 p.

Last edited June 6, 2003

Go to top

 
NAU's Program in Community, Culture and Environment Northern Arizona University