TEACHING ALL VOLUMES SUBMIT WORK SEARCH TIEE
VOLUME 1: Table of Contents TEACHING ISSUES AND EXPERIMENTS IN ECOLOGY
Issues : Figure Sets

Figure Set 3: Ecology of Fire

Purpose: To introduce students to the ecology of fire under natural and "controlled" conditions.
Teaching Approach: "Citizen's argument"
Cognitive Skills: (see Bloom's Taxonomy) — comprehension, interpretation, application
Student Assessment: Oral presentation

BACKGROUND


Fire — natural and human caused — is an important and controversial disturbance worldwide and is particularly interesting to many students. U.S. regions characterized by fire vegetation include coniferous forests in the west, the southern pine barrens, grasslands in the midwest, and mediterranean shrubland and chaparral in the southwest. Fires typically occur in locales where plants grow in seasons with rain followed by dry season and dry lightening storms, although these conditions are not always necessary.

Controversies related to fire include: setting fires for agriculture and grazing, control burns in national parks that affect bordering populated areas, the problem of accumulated fuel from decades of fire suppression, encroaching development in fire communities, and control of natural fires in parks and other public lands.

Ecological topics related to fire are types of fire (surface, crown, ground) and adaptations and tolerance of plants to fire, including serotiny (seed release in cones heated by fire). See the resource section on the first page of this issue for links giving more background about these subjects and also controversies and debates.

Numerous more recent studies have shown the response to fire by some species is less direct than the example of serotiny. For example, some plants that are seemingly destroyed during a fire, quickly re-sprout from intact underground stems and roots. These new sprouts benefit from the reduced cover of competing species. Briggs and Knapp (1995) have shown that fire increases production of grasses on the Konza LTER prairie. In addition, annual fires limit spread of woody plants but fire rotations of 4 years or more actually increases their growth (Briggs et al. 2002).

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Bormann and Likens Study

The data in Figure 3A are fire statistics in two parts of the U.S. where fire is (Great Lakes region) and is not (northeast) an important ecological phenomenon.

In the Great Lakes region several pollen studies of lake cores indicate that catastrophic fires have been frequent in the northern hardwood forest. Heinselman's (1973) core data from the Boundary Waters Canoe Area in Minnesota show a natural fire rotation of about 100 years in the presettlement period. Interestingly, this is the region that Clements studied as he developed his climax hypothesis. As a result of widespread, fires old and even-aged stands of white pine are common in the Great Lakes area.

In contrast, in their book Pattern and Process in a Forested Ecosystem Bormann and Likens (1979) argue that fire has not historically been an important disturbance in northern parts of the northeast U.S. — and that this remains the situation today. They present evidence that Native Americans did not set fires to forests in northern New England. This is in contrast to southern/central New England, Pennsylvania, and New York, in addition to the Great Lakes forests. Bormann and Likens argue that Native American populations in the Hubbard Brook area in northern New Hampshire, for example, were small and migratory. While Native Americans in other regions apparently set fire for agriculture, summer travel over land, and driving herds of deer, those of the northern New England region farmed little, traveled by canoe in summer, and killed deer by stalking.

The data in Figure 3A clearly show that human-induced fires are much more common than fires caused by lightening. In addition, fires in Vermont and New Hampshire (Green and White Mountains) are quite rare compared to the those in national forests in Pennsylvania, Wisconsin, Minnesota, and Michigan. Northern New England forests have been called "asbestos forests" because fires are so relatively uncommon. Hurricanes and other wind events are much more important vectors of disturbance here (see Foster 1997). Factors limiting fire in northern New England include: precipitation throughout the year, resistance of dominant trees to fire, limited litter accumulation, and many sites (e.g. valleys) protected from high winds. Students from western states in particular may be surprised that fires are uncommon in some locales; the New England example helps us better understand variables determining fire as an important ecological consideration.

The degree to which Native Americans changed the landscape with fires continues to be a controversial topic in the ecological literature. In her paper on forest fires set by Native Americans Russell (1983) concludes that no strong evidence supports their large scale burning of New England forests in precolonial times. She proposes instead that these peoples did increase the numbers of naturally occurring fires in areas around their habitation sites. Russell analyzed documents written before 1700 and includes several interesting quotes such as this one written by Adriaen Van der Donck who lived on an island in the Hudson River:



In addition to giving these first-hand accounts Russel's paper is illuminating because it nicely outlines the controversy as to whether Native Americans were "aboriginal pyromaniacs" (Raup 1937). Faculty especially interested in this topic could lead a stimulating class discussion about the types of evidence ecologists can use to address this question.


Literature Cited

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Minnich Study

Minnich studied fires in the Mediterranean-type habitat of southern CA and northern Mexico. Although vegetation type is similar in both areas, fire histories have been quite different since the early 20th century. Before this time lightening-set fires were common on both sides of the border. Sheep and cattle farmers also set fire to improve grazing, and both natural and set fires could burn for months. However, in 1892 the Los Angeles coastal plain region became the nation's first federal forest reserve, and fire suppression was initiated north of the U.S/Mexico border.

Minnich tested the hypothesis that fire suppression has resulted in recent severe and large-scale wildfires in southern CA by comparing burns in southern CA with those in northern Mexico. He found that during the study period of 1972-1980 total area burned was similar in the two areas, but size of burns differed. Large burns (< 3000 ha) occurred more often in southern CA while small burns (>100ha) were more frequent in northern Baja California. Median burn size in southern CA was about twice the size measured in Mexico (3500 ha as opposed to about 1500 ha).

Minnich concludes that:

This study therefore gives evidence to support the Forest Service's more recent policies on controlled burns.

According to Minnich, Figure 3B shows "a broad gradient of increasing fire area northward in Baja California [that] shifts to a pattern of infrequent small to very large fires north of the border. Divergences in fire size between the two countries are most evident in chaparral."

In Figure 3B, the slopes of fire size distributions for chaparral are different for the U.S. and Mexican sites. In Baja California burns less than 800 ha were more frequent than in southern CA.


Literature Cited

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