Forest Composition and Structure

Forest rangers marking ponderosa pine for timber sale near Flagstaff, Arizona, 1909. Image 57a by A.G. Varela courtesy of Cline Library Special Collections, Northern Arizona University.

Lt. Edwin Beale, traversing northern Arizona on an exploratory expedition in 1857, had this to say about the forests of this region of the southern Colorado Plateau.

"We came to a glorious forest of lofty pines...The country was beautifully undulating, and although we usually associate the idea of barrenness with the pine regions, it was not so in this instance; every foot being covered with the finest grass, and beautiful broad grassy vales extending in every direction. The forest was perfectly open and unencumbered with brush wood, so that the travelling was excellent... "

There is ample evidence that prior to extensive Anglo settlement much of the ponderosa pine forests of the region were as characterized by Lt. Beale, open "park-like" stands of large, old ponderosa pine underlain by a rich understory of native herbs and grasses (see 1909 photo at right). Periodic small wildfires maintained this open structure by killing ponderosa seedlings and encouraging growth of grasses.

Today's ponderosa pine forests of the southern Colorado Plateau little resemble those which Lt. Beale described. The introduction of livestock grazing, logging, and fire suppression, combined with a series of wet climate pulses early in the century, have resulted in forests which are characterized by "dog-hair thickets," dense clumps of small trees competing for space and nutrients. Grasses and forbes on the forest floor have been dramatically reduced compared to pre-settlement conditions, replaced by thick accumulations of dead pine litter.

Until 20 years ago, snags (dead trees) were systematically removed as fire and forest health hazards. Those that remained were often the focus of timber poachers, aided by the extensive road networks created for legal logging operations. Most managed forests now lack desired numbers of large-diameter snags, which serve important ecological roles such as cavity-nesting sites for many breeding birds and probably for many bats as well.

The loss of native grasses palatable to livestock may be contributing to higher elk browsing of young quaking aspen. Continuous browsing of aspen sprouts eventually results in the death of the sucker root system and the ability of aspen clonal systems to regenerate. There has been large decline in the quaking aspen cover type in just the last 30 years, representing a significant loss of biodiversity at the landscape level. This broadleaf component of mixed-conifer forests is important to many species of wildlife, particularly birds and invertebrates such as the western tiger swallowtail and red-spotted purple butterflies.

The figures below show some significant changes in the abundance of different forest types in Arizona and New Mexico over a 25-year period from 1962 to 1985. The U.S. Forest Service estimates that the extent of aspen stands in Arizona and New Mexico declined by an 46% during this time, while areas dominated by mixed-conifer forests increased by 81%.

Figures courtesy of Southwestern Region USFS Rocky Mountain Research Station.  From General Technical Report RM-GTR-295, http://www.rms.nau.edu/publications/rm_gtr_295/

Research: Changed Southwestern Forests: Resource effects and management remedies. Over 150 years of occupancy by northern Europeans has markedly changed vegetative conditions in the Southwest. Less fire due to grazing and fire suppression triggered a shift to forests with very high tree densities, which in turn contributed to destructive forest fires. Options to deal with these changes include prescribed fire, thinning and timber harvest to mimic natural disturbances and conditions. However, there are barriers to implementing these activities on a scale large enough to have a significant benefit. Adapted from a published journal article by Marlin Johnson.

Restoring Ecosystem Health in Ponderosa Pine Forests of the Southwest. Restoration of ecosystem structure and reintroduction of fire are necessary for restoring rates of decomposition, nutrient cycling, and net primary production to natural, presettlement levels. The rates of these processes will be higher in an ecosystem that approximates the natural structure and disturbance regime. Adapted from a 1997 journal article by W. Wallace Covington et al.

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.

References and Resources:

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Bassett, R. L., Boyce, D. A., Jr., Reynolds, R. T. and Graham, R. T. 1994. Influence of site quality and stand density on goshawk habitat in southwestern forests. Studies in Avian Biology 16: 41-45.

Choate, G. A. 1966. New Mexico's forest resource. Report INT-5. USDA Forest Service, Intermountain Research Station, Ogden, UT.

Connor, R. C., Born, J. D., Green, A. W. and O'Brien, R. A. 1990. Forest resources of Arizona. Forest Service Resource Bulletin INT-69. USDA Forest Service, Intermountain Research Station, Ogden, Utah.

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

Covington, W. W. 1994. Post-settlement changes in natural fire regimes and forest structure: ecological restoration of old-growth ponderosa pine forests. Journal of Sustainable Forestry 2: 153-181.

Covington, W. W. and Moore, M. M. 1994. Changes in multiresource conditions in ponderosa pine forests since Euro-American settlement. Journal of Forestry 92: 39-47.

Covington, W. W. and Moore, M. M. 1994. Southwestern ponderosa forest structure and resource conditions: Changes since Euro-American settlement. Journal of Forestry 92: 39-47.

Dahms, C. W. and Geils, B. W., editors. 1997. An assessment of forest ecosystem health in the Southwest. General Technical Report RM-GTR-295. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO, 97 pp. Also available online at <http://www.rms.nau.edu/publications/rm_gtr_295/index.html>

Ffolliott, P. F. and Gottfried, G. J. 1991. Natural tree regeneration after clearcutting in Arizona's ponderosa pine forests: two long-term case studies. Research Note RM-507. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, CO.

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Fule, P. Z., Moore, M. M. and Covington, W. W. 1995. Changes in ponderosa pine-gambel oak forest structure following fire regime disruption in northern Arizona: Camp Navajo old-growth forest study.Final report for the Arizona Army National Guard, Camp Navajo, Flagstaff, AZ.

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, Ogden, UT, 16 pp.

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Menzel, J. P. 1996. Historical changes in forest structure in the ponderosa pine type, Walnut Canyon area, northern Arizona. M.S. Thesis. Northern Arizona University, Flagstaff, AZ, 154 pp.

Moore, M. and Deiter, D. 1992. Stand density index as a predictor of forage production in northern Arizona pine forests. Journal of Range Management 45: 267–271.

Muldavin, E. H., DeVelice, R. L. and Ronco, F. 1996. A classification of forest habitat types of the southern Arizona and portions of the Colorado Plateau. General Technical Report RM-287. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Ft. Collins, CO.

Pearson, G. A. 1931. Forest types in the southwest as determined by climate and soil. Technical Bulletin 247. USDA Forest Service, Washington, D.C., 27 pp.

Pearson, G. A. 1933. A twenty year record of changes in an Arizona pine forest. Ecology: 272-285.

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Veblen, T. T. and Hadley, K. S. R., M.S. 1991. Disturbance and stand development of a Colorado subalpine forest. Journal of Biogeography 18: 707-716.

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Youngblood, A. P. and Mauk, R. L. 1985. Coniferous forest habitat types of central and southern Utah. General Technical Report INT-187. USDA Forest Service, Intermountain Research Station, Ogden, UT, 89 pp.