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Fire Ecology

Prescribed FireFire is both a scourge and a valuable management tool, depending on the season and the situation.

Fire has played an integral role in the shrub-steppe environment. The bunchgrass component of the native shrub-steppe is a discontinuous fuel bed that prevents many large fires. Prior to manmade disturbances, the historic fire regime was a thirty-two- to seventy-year fire return interval of small, high-intensity fires that removed small patches of the fire-intolerant shrub overstory. Small, infrequent fires maintained bunchgrass openings within the shrub-steppe, providing for both shrub and grassland communities.

However, the historic fire regime has been significantly altered by sociopolitical and economic factors. After the 1900s, human activities interrupted the natural fire interval and patterns of burning. Agricultural development and livestock grazing reduced the light fuels that would normally carry a fire. Livestock grazing also had the effect of suppressing native bunchgrasses and allowing non-native invasive species (e.g., cheatgrass) and native sagebrush densities to increase.

(Note: It is highly possible that Native American’s used fire to shape the landscape, drive wildlife, etc., as they did in other parts of the country. However, until additional factors, such as extensive livestock grazing, introduction of exotic species, and farming, significantly altered the ecology of the area, fire would likely not have had as significant an impact as it does today. Fires set by Native Americans could arguably be considered part of the historic, natural fire regime, if indeed fires were purposely set in the Columbia Basin.) 

Fire suppression organizations developed in the early twentieth century nationwide. Beginning about 1906 through the present, fire suppression efforts have resulted in increased sagebrush stand density. This allows for hotter, more destructive fires, due to the closer proximity of each individual plant, which allows fires to spread within the shrub canopy.

Of even greater impact, though, is the introduction of cheatgrass and other invasive species and noxious weeds. Rangeland “improvements” brought in a variety of non-native grasses, either as purposeful introductions to provide forage enhancement, or as accidental introductions in seed/pasture mixes. Plants such as cheatgrass, tumbleweed and other annual species altered the native plant community structure. The discontinuous fuel that native bunchgrasses provided were invaded by thick, continuous fuels that would carry fires over large areas. Cheatgrass also cures into dry fuel earlier in the fire season than native grasses, providing a longer fire season. A high mortality of perennial grasses may occur if fire burns in a cured litter of annual grasses while perennials are still actively growing. Fires that start in cheatgrass stands often spread to surrounding habitats, resulting in the loss of shrubs from adjacent communities.

Finally, the arrival of settlers brought additional sources of wildfire. Even today, many fires on the Monument are ignited by such sources as cigarettes, sparks from machinery, and motor vehicles. The 24 Command Fire, one of the most destructive in recent history, was caused by a highway accident, and the Wahtoma Fire of 2007 was almost certainly human-related.

Particularly hard-hit by modern, high-intensity fires are sagebrush and other shrubs; sagebrush does not tolerate fire, while native grasses are more fire-tolerant. The natural recovery of sagebrush stands following a fire is further hampered by the presence of invasive species, which often out-compete sagebrush following a disturbance such as fire. Additionally, sagebrush totals only 15-25% of the vegetative cover in sagebrush shrub-steppe communities, and although wind can disperse sagebrush seeds up to ninety feet, most seeds fall within three feet of the canopy, so the natural reintroduction of sagebrush into an area can take decades.

In summary, the contemporary fire regime is a short fire-return interval of large, high-intensity fires that remove large patches of the fire-intolerant shrub overstory. The invasion of cheatgrass has changed the community appearance and altered the fire regime because of an abundance of available and continuous fuel. Natural succession has been altered by cheatgrass such that burned areas do not recover to their former community structure following fire. This has led to a decrease in the fire intolerant sagebrush and a commensurate increase in exotic species, primarily cheatgrass and tumbleweed, thereby creating a cycle that is hard to break.

 

Want to know what fuels fire on the Monument . . .

Fuels

 

The fuel types in shrub-steppe are typically grass and shrub. The fuel is generally herbaceous plants that are dormant, or are nearly dormant. Occasionally, litter and dead/down stemwood from the open shrub overstory contributes to the fire intensity. Fires in this fuel type are surface fires that move rapidly through the cured grass and associated material. In rare instances, brush can become the primary carrier of fire spread; however, brush requires moderate winds (more than eight miles per hour at the mid-flame height) for fire to spread from crown to crown.

Four different fuel types are currently recognized on the Monument.

Native grasslands are characterized by dry, open, grassy areas, with individual grass clumps providing a discontinuous natural fuel. Native perennial grasses and forbs are found throughout this community. Perennial grasses and forbs tend to have long, fibrous root structures that can access moisture throughout the soil profile. Thus, native vegetation in this area remains green during the first half of the fire season, curing out during the late summer, typically July and August. Fires during late summer can burn within these areas. Perennial grasses may suffer high mortality if fires fueled by cured annual grasses burn perennial species during their active growing season, or if they burn at such a high intensity that the crown (the actively growing part of the plant) is damaged or killed. Fires during late summer can burn within perennial grassland areas. Occasionally, depending upon wind conditions, surface fires can move rapidly through the cured grass and associated materials.

Shrub-steppe areas are grasslands that retain a component of shrub as an overstory. Wyoming big sagebrush is the most common, dominant shrub, but there are also communities of three-tip sagebrush, bitterbrush, black greasewood, spiny hopsage, and gray and green rabbitbrush. Generally, the shrubs burn with greater intensity than the grasses and produce longer flame lengths. Sagebrush has volatile, flammable chemicals associated with its foliage. In some areas, the shrubs can burn with such intensity that they permanently destroy the understory plants and create hydrophobic conditions on the soil surface.

Riparian and riverine bottoms are occupied by willow-dominated communities. Because of their proximity to water, riparian and riverine habitats tend to have a high density of shrubs and trees and a greater amount of vertical structure. Native and non-native grasses are found in the understory throughout the community. Vegetation in this area remains green during the majority of the fire season, but as the grasses cure, the understory becomes more flammable. Dried grasses and shrubs can provide ladder fuels that burn into the riparian tree canopy and can kill overstory trees. Occasionally, aquatic vegetation can build up such that open water habitat becomes limited. These situations may require fire to reduce such buildups.

Non-native plant communities are dominated by invasive species such as cheatgrass, tumbleweed and other exotic plants. Cheatgrass germinates in late fall, winter and early spring and cures earlier than native grasses, usually by early June. As the cheatgrass cures, it becomes an available and abundant fuel. Often, fires start within the cheatgrass and spread to other adjacent communities. Subsequently, other plants are exposed to burning earlier in the fire season than they historically would have been. This weakens native plants because they are burned during the peak of their growing cycle, which allows cheatgrass to spread further into native plant communities. This reduces biodiversity and accelerates the fire cycle.

Page Photo Credits — Prescribed Fire - Austin Catlin
Last Updated: May 03, 2013
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