Adapted from: Cathy W. Dahms and Brian W. Geils, Technical 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, Southwestern Region, Fort Collins, CO. This report is available in its entirety on the web at www.rmrs.nau.edu/publications/rm_gtr_295/
Wildfires had an essential part of in the ecology of many of the Colorado Plateau's biotic communities prior to Euro-American settlement. Whether lightning-caused or started by native peoples, wildfires were once quite common occurences throughout the grasslands and forests of the region. These frequent fires maintained an open forest structure in the ponderosa pine forests, and prevented tree encroachment into high elevation meadows and grasslands.
A century of fire suppression and grazing in the Southwest has significantly decreased the incidence of low-intensity natural surface fires, while the number of catastrophic wildfires in the region's forests has increased dramatically, especially during recent drought conditions.
The Forest Service has recommended a significant increase in the use of prescribed fire in many of the region's forests. Prescribed fire is the intentional burning of forest fuels under conditions specified in an approved plan to meet management objectives and confined to a predetermined area; ignition may be either the result of a scheduled management activity or from other sources (e.g., lightning).
In most parts of the region and until very recently, prescribed fire has mostly been used to burn slash piles and for small broadcast burns, but the Forest Service believes that an increase in the number of acres of broadcast burning would enhance forest health. The potential benefits from broadcast burning are numerous and includereduction in fuel loads; stimulation of understory vegetation including grasses, forbs, and shrubs; thinning of overcrowded stands; and nutrient cycling. Over time, prudent use of prescribed burning could reduce the damage caused by wildfire, as well as the costs associated with fire suppression (Moody et al. 1992).
Thinning stands with fire can potentially be done at a much lower cost than with mechanical thinning, although past attempts to do this in the Southwest have met with varying degrees of success (Harrington and Sackett 1990). Generally, fire increases structural heterogeneity and diversity, creating mosaics within stands and over larger areas. It recycles nutrients for use by surviving trees and new vegetation. No other silvicultural technique fully mimics the ecological effects of historical fire regimes.
Burning tends to promote natural regeneration of ponderosa pine, providing favorable seedbeds and enhancing the growing environment for survival (Harrington and Sackett 1990). Repeated burning of the same area would be expected to maintain a sparse understory and relatively open forest conditions by killing some of the previously established regeneration.
A few studies indicate a tendency for understory burning to reduce stand dwarf mistletoe infection levels (Harrington and Hawksworth 1990); significant amounts of crown scorch are probably needed to achieve this effect.
One of the obstacles to more widespread use of broadcast burning in the forests of the Colorado Plateau is the risk involved both to resources and property. In the spring of 2000 a prescribed burn at Bandelier National Monument in northern New Mexico escaped control and burned several thousand acres, including more than 200 homes in the town of Los Alamos. This was certainly not the only forest in which fuel loads are so high that prescribed burning could lead to stand-replacement fires. These areas need to be mechanically thinned first, with much of the cut stems removed from the site, before fire can be safely reintroduced.
Broadcast burns are recommended following most mechanical thinning operations on ponderosa pine and lower mixed-conifer sites. Generally, such fires should be designed to reduce fine fuels, leave coarser fuels, and cause minimal damage to the remaining stand. As with any prescribed burn, timing is critical. Occasionally, stands may benefit from understory burns prior to mechanical thinning, to reduce ground fuels before the additional slash is generated. Because of crown scorch and needle fall after understory burning, fuel loads can increase to high levels within a year or two after treatment. Thus, re-burns are often needed to reduce these fuels.
Like other management practices, prescribed burning is both an art and a science. Every prescribed burn can be a learning tool; the acquired experience of practitioners is invaluable. Documentation and monitoring of prescribed burning activities and their effects is also be crucial for developing and improving burning programs.
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 published 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.
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