Ponderosa Pine Fire Ecology

Author: Will Moir, USFS Rocky Mountain Research Station, Flagstaff, Arizona.

Unburned grasses in ponderosa pine forest. Photo by Will Moir.

The ponderosa pine forests of the Colorado Plateau have evolved over thousands of years. Over this time the tree has developed several adaptations which help it survive in its dry, often warm habitat. A once common occurrence in these forests which has shaped the pine's particular ecological adaptations is wildfire. Recent studies indicate that the ponderosa pine forests on the southern plateau near Flagstaff, Arizona and along the Mogollon Rim were subjected to low-intensity ground fires perhaps every 2-12 years over historical time. However, beginning in the early 1900s this pattern of fire drastically changed. A fire suppression policy implemented by the United States Forest Service and other land management agencies at this time greatly decreased the occurrence of fire in these forests. The absence of reoccurring fire, coupled with widespread logging and grazing of forest lands, has led to unforeseen changes in forest composition, structure and ecology.

Today's forest is often characterized by dense "dog-hair" thickets of young pines with a thick accumulation of litter on the forest floor. Previously, many pine forests of the region were open stands of large, old ponderosa pine underlain by an understory of native grasses. Small fires maintained this open structure by killing seedlings and encouraging growth of grasses. Some ecologists recognized this change in the nature of these pine forests as a possible problem as early as the 1930s, but changes in forest management did not occur until the 1970s. Fires in many of today's ponderosa pine forests are no longer low-intensity ground fires but rather catastrophic, stand-replacing crown fires.

From about 1910 to approximately 1990, the amount of acres burned by wildfire in Arizona and New Mexico oscillated between a few thousand acres to 60,000 acres annually. This yearly amount is dependent on local factors such as weather and fuel loads on the forest floor. However, beginning in 1992 the amount of acres burned between the two states has skyrocketed, with over 180,000 acres burning in 1997. Prior to fire suppression, the fires in the pine forests of the region behaved in a somewhat predictable manner determined by years of evolution and natural processes. The forest ecosystem of today, in contrast, has possibly reached a point of unstable criticality. A lightning strike may lead to a few trees burning, a few acres burning, or a catastrophic stand-replacing fire sweeping over thousands of acres of forest. Land managers and scientists are no longer able to predict with much confidence what direction fires in the ponderosa pine forests of the Colorado Plateau and the whole Southwest might take. 

Aftermath of the high severity Hochderffer wildfire near the San Francisco Peaks, Arizona. Photo by Julie Crawford.

Fire control personnel with the United States Forest Service and other land management agencies are concerned that more fires might be dangerous, catastrophic fires until fuel loads are reduced below the critical threshold. Extensive tree-thinning projects and prescribed burning are two steps forest managers are taking to try to decrease the danger of high-intensity fires as well as restore the ponderosa pine forests of the region to a more "natural" state.

Despite the bleak appearance of charred black sticks following a major crown fire, native organisms and plants often quickly invade the site and recovery is underway. However, in many areas following these burns invasive species are able to establish themselves, crowding out native species.

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Reintroduction of Fire to Forest Ecosystems

Research:

Fire-Southern Oscillation Relations in the Southwestern United States. A close linkage between fire and climate could diminish the importance of local processes in the long-term dynamics of fire-prone ecosystems. The structure and diversity of communities regulated by fire may have nonequilibrial properties associated with variations in global climate. Successful prediction of vegetation change hinges on a better understanding of climatically driven disturbance regimes and the relative contributions of regional versus local processes to community dynamics. Adapted from a journal article by Thomas W. Swetnam and Julio L. Betancourt.

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.

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.

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