Fire Information Continued
Afternoon thunderstorms that release little precipitation occur frequently in the northern Rockies. Yellowstone receives thousands of lightning strikes in a typical summer, but most do not result in fires. A snag may smolder for several days and then burn out because fuels are too moist to sustain combustion or too sparse to permit the fire to spread. The park’s forests have few shrubs; understory fuels are predominantly young trees. The moisture content of both live and dead vegetation tends to drop as summer progresses, temperatures increase, and relative humidity decreases. Fuels have often dried out enough to ignite the first wildfire of the year by early July.
A forested area that has burned recently enough to contain only young stands of trees usually doesn’t have enough combustible fuel to carry a fire. But as the years pass, trees that don’t survive the competition for light and other resources die and eventually fall over. On living trees, older branches die and fall off as they are shaded by new foliage growing above. As a stand grows older and taller, the canopy becomes more broken. This allows enough light to reach the forest floor for a shade-tolerant understory to be established. The accumulation of fuel on the forest floor and the continuity of fuels between the ground, understory, and canopy make older stands more vulnerable to fire. Some forests in Yellowstone may not have burned in at least 300 years and may be particularly prone to lightning ignition.
Nearly all of Yellowstone’s plant communities have burned at one time or another, but their varied characteristics cause fires to behave differently in them. To quickly assess a fire start and its potential to spread, park staff use different vegetation communities as indicators of fuel load, dominant vegetation, and time since the last fire or other disturbance.
The moisture content of dead and down woody debris and the year’s weather trends are the main factors determining the severity of a given fire season. While fires can occur no matter the fuel moisture, many times conditions are too wet for fires to burn. In fact, 85% of all lightning-caused fires burn less than 10 acres. However, when 1,000-hour fuel moistures fall below 13%, fires can grow quickly. If extreme drought continues, all forest types and ages are more likely to burn.
To determine how much water is in the fuel, Yellowstone fire monitoring staff weigh and oven dry fuel samples to determine the moisture content. In a normal fire season, 1,000-hour fuels within the park may average 14–18% fuel moisture. (Dead fuels are classified according to size, and how long they take to dry out when completely soaked; “1,000-hour fuel moisture” refers to the moisture in large fuels such as downed timber that would generally dry out within 42 days. Kiln-dried lumber is 12%.)
Active fire behavior is generally not observed until 1,000-hour fuel moisture contents are less than 18%, and only minimal areas are burned until moisture levels drop to 13%. At that point a fuel moisture threshold is crossed; lightning strikes in forested areas at 13% fuel moisture quickly result in observable smoke columns and, if fuel and vegetation conditions are right, the fire spreads. Below 12%, younger and more varied forest types burn readily, especially when influenced by high winds. During extreme drought years, 1,000-hour fuel moistures may drop as low as 5%.
Depending on the forest type, fuel moisture, weather, and topography, fires can grow in size by isolated or frequent torching and spotting (transport of burning material by wind and convection currents), or by spreading from tree crown to crown. Fires in Yellowstone’s subalpine forests seldom spread significantly through ground fuels only. Like weather, terrain can be either an ally or adversary in suppressing unwanted fire. A few natural barriers such as the ridge from Electric Peak south to Mt. Holmes; Yellowstone Lake; and the Absaroka Mountains along the eastern boundary of the park are likely to prevent the spread of a low-to-moderate intensity fire, but fire may cross these features by spotting, covering a distance of two to three miles.
Fire managers may be able to predict a fire’s behavior when they know where the fire is burning (vegetation, topography, etc.) and the fuel moisture content. However, predicting fire is much more difficult during extreme drought, such as was experienced in 1988 and in the early 2000s.
Ongoing research in Yellowstone is also showing that forests experiencing stand-replacing fires can affect fire behavior for up to 200 years. When a fire encounters a previously burned forest, its intensity and rate of spread decrease. In some cases, the fire moves entirely around the burned area. Thus, fire managers have another tool for predicting fire behavior: They can compare maps of previous fires with a current fire’s location to predict its intensity and spread.
Frequency of Fire
Fire return intervals since European American settlement have ranged from 20–25 years for shrub and grasslands on the northern range to 300 years or more for lodgepole pine forests on the central plateau and subalpine whitebark pine stands. Fire scars on old Douglas-fir trees in the Lamar River valley indicate an average frequency of one fire every 25–60 years.
Until 1900, written records on fires in Yellowstone were sketchy, with generally only large fires reported. From 1900 through 1930, approximately 374 fires burned 11,670 acres. Since 1931, when fire statistics began to be kept more methodically, 1,616 fires have been lightning-caused and 728 were considered human-caused, including those caused by power lines.
The largest fire in the park’s written history prior to 1988 occurred when about 18,000 acres burned at Heart Lake in 1931. In 1989, fire ecologists William Romme and Don Despain suggested that without the fire suppression efforts that began in the 1880s, large fires might have occurred during the dry summers of 1949, 1953, 1960, or 1961. They believe that fire behavior in 1988, in terms of heat release, flame height, and rate of spread, was probably similar to that of the large fires that burned in Yellowstone in the early- to mid-1700s.
In 1988, 50 fires burned a mosaic covering about 800,000 acres in Yellowstone as a result of extremely warm, dry, and windy weather combined with an extensive forest cover of highly flammable fuels. Some of the largest fires originated outside the park, and a total of about 1.4 million acres burned in the Greater Yellowstone Ecosystem.
Some of the areas that burned in 1988 have burned again during the drought conditions of subsequent years, although unique conditions are required for such areas to reburn. Rare, extremely high wind events (greater than 20 mph), or more than 80% ground cover of cured elk sedge (Carex spp.) during very dry conditions, seem required for fires to again carry through areas burned in 1988. Fire behavior of previously burned areas is generally of a very high intensity—probably because of the high fuel load due to dead and fallen trees. Understanding the conditions necessary for recently burned areas (less than 50 years old) to reburn and modeling for the type of fire behavior seen in these areas is a challenge for fire managers in Yellowstone. Continue: Ecological Consequences of Fire