SOIL AND FIRE

Ecological Change

The unintended consequences of logging, livestock grazing, and fire control resulted in significant changes to species composition and structure – especially in short interval fire-adapted ecosystems. These changes, in turn, predisposed extensive areas to many of today’s wildland fire and forest ecosystem health problems in the interior West.

The following photos (figures 2-3), from the Bitterroot National Forest in western Montana, illustrate the changes that have occurred in species composition and forest structure over a 111-year period in a short interval fire-adapted ponderosa pine forest ecosystem. Each photo was taken from the same place, looking at the same forest, at different periods in time. The photos capture the differences that have developed in species composition and forest structure in the prolonged absence of periodic surface burning. Within these ecosystems, these changes become indicators of potential risk.

Changes in Species Composition and Forest Structure

Figure 2 Bitterroot National Forest 1871 Photo -- This serves as the baseline reference of forest stand conditions that evolved from regularly occurring, low-intensity surface burning. The forest was open and dominated by fire-tolerant, fire-adapted ponderosa pine.

Figure 3 Bitterroot National Forest 1982 Photo -- By 1982, the forest has changed dramatically from the one that existed here in 1871. Over this 111-year period, small trees have established in dense thickets and fire-intolerant tree species now crowd the forest. During drought periods the overabundance of vegetation stresses the site, pre- disposing the forest to insect infestations, disease outbreaks, and severe wildland fire.

In the prolonged absence of periodic surface burning, vegetative growth compounds and dead fuels accumulate. Within the forest, this biomass – in the form of multi-layered tree canopies – can carry flames from the surface where dead branchwood burns up into the tree crowns. In drought years, when vegetation dries, these "ladder fuels" contribute to severe, high-intensity wildland fires.

Figure 4 National forest wildland fire acres burned trend in the 11 Western states. (1945 - 1997

Under these conditions, wildland fires exceed nearly all control efforts and often result in long-lasting damage to the soil and to the watershed.

In 1871, practically all of the short interval fire-adapted ecosystems in the interior West were considered to be relatively low risk. They were typically open and because of frequent fire had little fuel accumulation. By 1982, the situation had reversed. This elevated risk is apparent when evaluated in the context of Western wildland fire trends (Figure 4). Since approximately 1987 – despite better firefighting capabilities – the change in fuel conditions has resulted in an increase in wildland fire acres burned.

For the purpose of this strategy, risk conditions are assigned "condition class" descriptors. In short interval fire-adapted ecosystems, Condition Class 1 (which corresponds to the 1871 Bitterroot N.F. photo) represents low relative risk. As Figure 2 indicates, the Condition Class 1 trend has few small trees and little ground fuel. The scarcity of fuel tends to limit the intensity of wildland fires. At low intensities, wildland fires typically do not kill the larger fire-tolerant trees but often consume small encroaching trees, other vegetation, and dead fuels.

At low intensities, fire is ecologically beneficial because nutrients are cycled. In addition, the soil’s organic layer is not consumed at these low fire intensities. The remaining organic material stabilizes the soil surface and helps prevent erosion.

Because fires in Condition Class 1 areas are low-intensity within these ecosystems, they leave the soil intact and functioning normally. These fires generally pose little risk and have positive effects to biodiversity, soil productivity, and water quality.

Figure 5 – Increased density of smaller trees provides fuel for vertical fire spread.

Condition Class 2 situations develop as one or more fire return intervals are missed, primarily due to well-intentioned suppression efforts, while understory vegetation continues to grow, becoming denser. If this accumulating vegetation is not treated, fires begin to burn more intense -- making them more difficult to suppress. The impact of fires to biodiversity, soil productivity and water quality become more pronounced.

In Condition Class 3 areas within these same ecosystems, fires are relatively high risk. As Figure 5 indicates, the forest is littered with considerable amounts of dead material and is choked with hundreds of small trees that reach into the crowns of the larger, older-age forest above. During drought years, small trees and other vegetation dry out and burn along with the dead material – fueling severe, high intensity wildland fires. At these intensities, wildland fires kill all of the trees – even the large ones that, at lower fire intensities, would normally survive.

Within Condition Class 3 in these short interval fire-adapted ecosystems, wildland fires usually damage key ecosystem components, including the soil. High-intensity fires consume the soil’s organic layer and burn off or volatilize nutrients. When small twigs, pine needles, and other litter are consumed, water runs unimpeded over the surface. Under these circumstances, the soil becomes more susceptible to erosion (Figure 6).

Figure 6 - Buffalo Creek Fire, Colorado These photos, of Colorado's Buffalo Creek Fire aftermath, illustrate soil severely burned and left exposed to rain and runoff. This produced the subsequent 1996 flash flood event that claimed two lives. The ensuing erosion also washed topsoil off the hillsides, clogging downstream watercourses. This erosion reduced future storage capacity of reservoirs and silted over the river's gravel beds - significantly reducing spawning habitat.


At extreme fire intensities, the soil’s capacity to absorb water is often lost. The fine, powder-like ash that follows a severe wildland fire on these sites makes water bead on the surface. These so-called "hydrophobic conditions" result in highly erodable soils.

Condition Class 3 is classified as high risk because of the danger it poses to people and the severe, long-lasting damage likely to result to species and watersheds when a fire burns – particularly in drought years. Firefighters are especially cognizant of hazards in Condition Class 3 situations. In a national survey (Tri-data, 1995), nearly 80% of all firefighters identified fuel reduction as the single-most important factor for improving their margin of safety on wildland fires.

In Condition Class 3, fires become more costly when homes are involved. Throughout much of the interior West, short interval fire-adapted ecosystems are typically located in valley-bottoms where homes and human development are most concentrated. Just as building homes in floodplains exposes homeowners to risk of floods, if hazardous fuels accumulations persist, development in fire-adapted ecosystems may pose a tangible risk to communities.

An example from the 2000 fire season demonstrates the increased costs of fighting fire near people and homes. The Skalkaho Fire on the Bitterroot National Forest covered 64,000 acres of forest interspersed with homes. It employed 755 firefighting personnel at a cost of $7.2 million dollars. Meanwhile, on the same forest within the Selway-Bitterroot Wilderness Area, a fire that burned about the same acreage (63,000 acres) only required 25 firefighters at a cost of approximately $709,999.

Efforts to reduce hazardous fuels on federal lands must be coupled with efforts to assist private landowners to take preventative action in their own communities. Creating defensible perimeters around homes, improving building codes, and employing fire resistant landscaping will help reduce fire risk to communities. These and other such actions can help prevent wildland fires from burning homes, reduce insurance premiums, and reduce suppression cost.

Figure 7 – Homes burning in the Dude Fire, Arizona, 1990.

Excerpted and edited from: http://www.fireplan.gov/cohesive.cfm
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