Excerpted from Status of the Interior Columbia Basin: Summary of Scientific Findings

PNW-GTR-385

Fire Frequency and Severity

Fires can be described by their effects on vegetation and how often these effects occur. The severity classes

are non-lethal (does not kill the dominant layer of plants), mixed (mixed effects), lethal (kills the dominant

layer of plants), and rarely burns. An interval of 0 to 25 years is considered very frequent, 26 to 75

years frequent, 76 to 150 years infrequent, 151 to 300 years very infrequent, and greater than 300 years

extremely infrequent.

Historically, late-seral multi-layer forested types were in moist areas such as bottoms and benches

and were maintained by succession and disturbance regimes. Timber harvesting and the exclu-sion

of fire have resulted in late-seral multi-layer forest developing on the dryer slopes. The amount

of small dead-standing and down material has increased. Considerable small-diameter timber volume

remains, providing opportunities for harvest that could be accomplished in an ecologically beneficial

manner with appropriate technology. Whether revenues from these harvests will exceed treatment

costs is dependent on site-specific conditions, timber markets, and costs of treatment.

Reserve and wilderness management also have caused substantial changes in succession/disturbance

regimes and the associated vegetation structures, although not to the extent of commodity management. In general, the disturbances are of lower frequency, are more acute, and have higher severity in cumulative effects than under historical regimes. Typically, both community structure and patch diversity decline under reserve and wilderness management.

As a result of blister rust mortality of white-bark pine, the early-seral subalpine forest increased substantially. The late-seral montane forest converted to mid-seral forest as a conse-quence of timber harvest patterns, fire exclusion, and loss of western white pine to blister rust. A similar trend of increased mid-seral forest occurred in the lower montane forests. Large trees that are shade intolerant and fire-,

insect-, and disease-resistant species have generally been harvested from both the mid-seral and late-seral communities either selectively or as part of even-age treatments (clearcuts, seed tree cuts). Fire detection and control efforts have largely excluded fire from these areas. Late-seral communities developed over long periods of time, and experienced periodic fires that typically burned through the understory and maintained open park-like communities of large shade-intolerant trees with low susceptibility to mortality from stress, fire, insect, or disease. Harvest patterns, along with exclusion

of fire, have converted much of the late-seral communities to mid-seral communities. Historically,

the mid-seral communities either burned with crown fires and cycled back to early-seral communities or developed into late-seral communities. Not only is there a large increase in the mid-seral communities, but the current communities are more dense and have higher mortality, higher fuel loadings, and

higher susceptibility to crown fire than historical communities.

Fire--At present, fire frequency and intensity are approaching or exceeding those experienced

in the early 1900s, when many wildfires occurred. The advent of improved technology for fire detec-tion, prevention, and suppression led to a decline in fires in the 1960s. However, with steadily

increasing fuel conditions, the amount of wildfire has increased since then. The average cost of

wildfire suppression, fatalities of firefighters, and amount of high-intensity fire during the period

of 1970 to 1995 are double the corresponding amounts occurring from 1910 to 1970.

One of the key results of many of these alterations across the Basin has been the correspond-ing

change in fire potential and behavior. In general, lethal fires have increased. In particular,

the lethal, very-frequent fire regime class has increased substantially, while nonlethal, very frequent

fires have declined from historic conditions.

The occurrence and intensity of wildfires are correlated with lightning storm routes, fuels, local wind patterns, terrain complexity, and roads. Wildland areas with complex terrain or a moderate or high road density have moderate or higher risk of wildfires. Foothill and mountain terrain facing west or east typically have high potential for wind that can cause rapid spread of wildfires. Areas with fuels, roads, and complex terrain that are on lightning storm routes have the highest risk of wildfire. The areas in the moderate category often have a lower probability of wildfire in any one year, but have high fuel accumulations, such that the fires can be very large and intense.

U.S. Department of Agriculture, Forest Service. 1996. Status of the interior Columbia basin: summary

of scientific findings. Gen. Tech. Rep. PNW-GTR-385. Portland, OR: U.S. Department of

Agriculture, Forest Service, Pacific Northwest Research Station; U.S. Department of the Interior,

Bureau of Land Management. 144 p.