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Forest Management

The Increase in Fuel Levels

The Increase in Fuel Levels

Why have fuel levels increased?

  • Fire fuels have accumulated to historically high levels due to a combination of fire suppression, timber harvesting, grazing and other land uses
  • Climate variability and climate change may further increase fire risk due to multi-year dry periods and/or extended drought, overall warmer, drier climate patterns, reduced winter snowpack and longer fire seasons

Fire suppression and exclusion

By 1900, fuel levels were increasing in dry interior western forests. Some interests wanted to emulate Native American use of fire with "light burning" to reduce wildfire severity- an idea similar to scientifically-guided prescriptions for managed fire use being advocated today. C ritics derided such notions as "Paiute Forestry".

This debate effectively ended after the fires of 1910 burned 3.1 million acres of interior western forests and killed 85 people, galvanizing public attitudes that every wildfire was bad and needed to be actively put out. Suppression capacity and effectiveness grew rapidly, especially with the advent of modern vehicles and road systems in the 1940s.

Today, after decades of fire suppression, fuel loadings have increased to levels that many experts agree are abnormally high compared to historic conditions.

Small diameter tree density and brushy undergrowth have increased

Many dry conifer forests now contain high levels of flammable, brushy undergrowth, litter (needles, twigs and leaves) and densely stocked, small diameter trees.

Besides fire suppression, harvesting of larger, more fire resistant trees and livestock grazing, which reduces perrenial grass cover and disturbs soils, providing a foothold for tree seedlings, have also contributed to development of uncharacteristically dense stands.

Trees in these stands are more likely to be killed by insects and disease than more widely spaced trees, which can further increase fuel loadings.

Species composition changes also increase fuels and fire intensity

Fire-adapted ponderosa pine and Douglas-fir develop thick bark and high crowns which protect them from surface fires.

Fire-sensitive species like grand fir are a component of mixed conifer forests, but most seedlings are eliminated under frequent, low-intensity ground fire regimes. Because of f ire exclusion, significantly more of the landscape is now occupied by forests with a dense understory of shade tolerant, fire-sensitive fir.

These trees have thin bark and retain their low branches making them susceptible to virtually all fires in which they often serve as ladder fuels to the overstory and facilitate intense crown fires in areas that rarely experienced them historically.

Climate and fuel interactions may increase wildfire risks

Climate affects wildfire fuels and wildfire occurrence in several significant ways.

The most important environmental variable affecting Pacific Northwest forest composition is the moisture regime (temperature and precipitation) during the relatively dry summers.

The Pacific Decadal Oscillation (PDO), a 20-40 year oscillation in the temperature of north Pacific Ocean currents, is correlated with multi-year periods of wetter or drier summers, and significantly higher fire risks in drier periods. Warmer winter temperatures resulting from global warming may be reducing winter snowpack and lengthening summer fire seasons.

Scientists remain uncertain about relationships between drought in the western U.S., and decadal climate variation and climate change. However, drier, longer summers clearly increase fire risks by reducing fuel moisture content and keeping fuels dry longer.

Also, long-term moisture stress makes trees more susceptible to pathogens such as bark beetles. This exacerbates stress and can push trees past survival thresholds, causing forest die-offs. Extensive stands of small, dead trees are at high risk for wildfire.