Fire Ecology

Fire is one of the leading natural forces that has shaped nearly all land-based ecosystems for several thousand years. Fire is especially important in reg­ulating the species composition of vegetation. Fire is particularly important in forests of cold northern regions, such as Canada and Siberia, and in sa­vannas, grasslands, and shrubby vegetation types in temperate and tropical regions, such as Australia and California.

Fire has become more important in tropical forests due to human dis­turbance. Fires are caused naturally by lightning and by people both acci­dentally and intentionally for management purposes. Native peoples around the world have used fire to maintain favored vegetation types and manage wildlife.

Surviving Fire

Some plant species have adaptations that allow them to survive or repro­duce after fire. Survival adaptations include sprouting from underground roots (aspen, grasses), sprouting from stumps (birches, oaks), and growing thick bark that insulates trees from fire (many species of pine and oak). Adap­tations that lead to reproduction after fire include serotinous cones on species such as jack pine and lodgepole pine. Serotinous cones are held high in the forest canopy in closed condition and do not shed their seeds unless scorched by fire. Buried seeds of some species survive in the forest floor un­til fire kills the tree canopy, allowing sufficient light to stimulate seed ger­mination (pin cherry and some geraniums).

Some plant species have none of these adaptations to fire. They survive fire in refuges, such as rocky areas without continuous fuel to carry fires, or wetlands that rarely burn. They may also grow in parts of the world where rainfall is frequent and fires are rare.

Natural vegetation types of the world experience several “fire regimes,” or characteristic occurrence of fire in terms of the frequency and intensity of fire. A fire regime with very frequent, low-intensity fire (two- to ten-year recurrence) occurs in moderately dry climates supporting grasslands and sa­vannas around the world. Frequent, low-intensity fires (ten- to forty-year recurrence) constitute the fire regime in many temperate and boreal forests dominated by tree species with thick bark, especially oaks and pines. The fires kill invading tree species with thin bark while allowing oak and pine to survive.

A plant ecologist measures lodgepole pine seedling growth in Yellowstone National Park in Wyoming. Seven years after the fire of 1988, there were ten times as many seedlings in the area than before the fire.

A fire regime of moderately frequent, high-intensity fire (thirty- to one- hundred-year recurrence) occurs in oak and manzanita-dominated chapar­ral in California and in dry boreal forests dominated by trees with serotinous cones, such as jack pine in North America. These high-intensity fires kill the forest from the ground up and initiate a new, young forest. Infrequent, high-intensity fires (one-hundred- to five-hundred-year recurrence) occur in many conifer forests of the Rocky Mountains and wetter parts of the bo­real forest.

“Helpful” Fire

A number of ecosystems have a regime in which fires are rare, including hemlock and sugar maple forests of eastern North America, arctic tundra, and very dry deserts. Surprisingly, fire may still be important in these sys­tems. For example, lightning strikes in maple forests of Michigan sometimes burn a fraction of an acre of forest, called a spot fire. These spot fires are usually invaded by oak trees, which then live for up to three hundred years. These spot fires have a long-lasting impact where they occur and they en­hance biodiversity by maintaining fire-dependent oak as a component of the forest landscape where big fires never occur.

Many vegetation types around the world require fire for their mainte­nance over time, and they are replaced by different vegetation in the ab­sence of fire. People have suppressed fire during the nineteenth and twentieth centuries in many parts of the world. Fire suppression in savan­nas and prairie remnants has allowed invasion by forest in many cases. Restoration of prairies requires the use of “prescribed fire,” purposely set by people, to reestablish the fire regime required by the prairie plants.

Pine forests throughout the United States (ponderosa pine in the west, white pine in the east) were formerly kept in a parklike condition with open understories by the occurrence of surface fires. After several decades of fire suppression, these forests have accumulated a high density of trees, includ­ing the invasion of other species such as spruce and fir. The buildup of high fuel loads and smaller trees that can function as a ladder to carry fire into the crowns of large pines means that fires become more intense than in the past, possibly too intense to be controlled by fire fighters, and intense enough to kill the old pines.

Fire and Wildlife

Fires kill relatively few numbers of wildlife species directly. The major im­pact of fires on wildlife is that it alters their habitat. Any substantial alter­ation in habitat is sure to affect some species positively and others negatively. For example, if an old-growth boreal forest of pine, spruce, and fir is re­placed by a young aspen forest after a fire, then a whole suite of conifer- dependent birds, such as spruce grouse, gray jay, and boreal chickadee, will fare poorly after the fire. Conversely, birds that prefer young aspen forest such as ruffed grouse will increase in population.

Forest fires generally only consume 10 to 20 percent of the wood in tree trunks, leaving many standing dead trunks referred to as snags. Snags are good habitat for woodpeckers that seek insects living within the dead wood and cavity-nesting birds that use the cavities excavated by the woodpeckers. Deer and elk also prefer young post-fire forests, whereas the pine marten prefers mature forests.

If a major forest fire were to burn an entire forest, for example, an en­tire national park or wildlife refuge, then the entire habitat after the fire would be young forest, and those species that lived in mature forests could be ex­cluded from the park. Conversely, if there were never any fires, those species of wildlife that require young, regenerating forests would be excluded.

An ideal solution to this problem is to have relatively small fires occur on a regular basis so that a mix of young, middle-aged, and mature habitat is always present to accommodate all species of wildlife that could live in the area. This concept is known as landscape diversity. The distribution and size of fires on the landscape over time is, together with human disturbance such as logging, the most important factor in determining landscape diver­sity and the consequent ability of the landscape to provide for a variety of wildlife.

References

Johnson, Edward A. Fire and Vegetation Dynamics: Studies from the North American Bo­real Forest. Cambridge, UK: Cambridge University Press, 1992.

Pyne, Stephen, J. Fire in America: A Cultural History of Wildland Fire and Rural Fire. Princeton, NJ: Princeton University Press, 1982.

Whelan, Robert J. The Ecology of Fire. Cambridge, UK: Cambridge University Press, 1995.