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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.



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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

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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

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