Benefits of Fire for Oaks

Frequent fires favor oaks on high-quality sites by giving oaks several regeneration advantages over competing species. These advantages include: (1) opening the understory by reducing fire-intolerant competitors; (2) allowing oak to dominate the advance regeneration pool; (3) preparing seedbeds and acorn caching; (4) discouraging acorn and seedling predators; (5) xerifying sites; and, (6) increasing flammable fuels and the probability of additional fires. Each of these points are discussed in this section.

Open Understory and Reduce Fire-Intolerant Competitors

A regime of frequent burning over long periods of time creates an open structure in pine or hardwood stands. In hardwood stands, long-term burning tends to eliminate small understory stems outright and gradually reduces the midstory and overstory canopy through mortality resulting from fire wounds. Increased light reaching the forest floor in these open stands maintains the vigor of oak advance regeneration. Loftis (1990) demonstrated that elimination of the subcanopy with herbicides encouraged development of advance regeneration of red oak in mature mixed hardwood stands in the southern Appalachians. Frequent burning should create a stand structure similar to that created by injecting understory hardwoods with herbicides (Van Lear and Watt 1993).

In a study in central New York, mortality of saplings caused by accidental spring fires ranged from 35 percent in ironwood (Ostrya virginiana) to 93 percent in hemlock (Tsuga canadensis), but only 12 percent of the oak and hickory roots stems were killed (Swan 1970). Others have reported similar selective discrimination of fire against shade-tolerant competitors (Brown 1960, Carvell and Maxey 1969, Kruger 1992). Fire may also result in a major reduction in shrub cover (Nyland and others 1982, Abrams 1988, Reich and others 1990). As a result of this selective reduction of competitors, oaks may increase in relative density even if the absolute density remains unchanged (Lorimer 1993) (Table: Functional Role of Fire in the Ecology of Oak Regeneration).

Allow Oak to Dominate Advance Regeneration Pool

Frequent fire allows advance oak regeneration to accumulate. Nearly all hardwood species sprout in a regime of annual winter fire (Thor and Nichols 1974, Langdon 1981, Waldrop and others 1987). Hardwood sprouting is more vigorous after periodic winter burns because of greater carbohydrate reserves in that season (Hodgkins 1958). Thor and Nichols (1974) noted that even with periodic and annual winter burning, oak stems tend to increase at the expense of competing hardwoods. After two periodic winter burns and eight annual winter burns, oak stems comprised 61 and 67 percent of the total stems, compared to 51 percent on unburned plots (Van Lear and Watt 1993).

Annual summer fires eventually eliminate all hardwood sprouts (Langdon 1981, Waldrop and others 1987). Biennial summer fires also gradually eliminate hardwood sprouts, but oaks succumb more slowly than many other species. Oaks, in the absence of prolific root sprouters, such as sweetgum, gradually dominate the advance regeneration pool in mature mixed hardwood stands because of the tenacity of their sprouting (Carvell and Tryon 1961, Waldrop and others 1987). Increases in the number of oak sprouts and, more importantly, the number of top-killed oak stems up to 6 inches (15 cm) in ground diameter with basal sprouts appearing after summer broadcast burns suggest that periodic summer burning favors oak even more (Augspurger and others 1987, Van Lear and Watt 1993).

Cumulative Morality of Hardwood Roots

When repeated burning occurs in stands with mixed advanced regeneration, oaks have an advantage over less fire-resistant vegetation, which is killed by fewer fires of lower intensity (Waldrop and others 1987). This loss usually exceeds species gain through invasion. Thus, the frequency of the fires is as important to reduction of fire-susceptible species as is fire intensity (De Selm and others 1990). Carvell and Tryon (1961) reported large increases in advanced oak regeneration in West Virginia where stands had been burned several times over a 20-year period. Keetch (1944) found that oak sprouting was stimulated by a single prescribed fire and was maintained by three successive fires. Sander (1988) stated that effective hardwood competition control may require as many as three or more burns at 2- to 3-year intervals (Van Lear and Watt 1993).

Single prescribed fires have been shown to enhance oak regeneration little. Johnson (1974) reported a spring fire in a 102-year-old northern red oak stand failed to increase oak abundance in the understory, failed to control competing vegetation, and killed 58 percent of existing seedlings. Wendel and Smith (1986) reported similar results after a single spring burn in a central Appalachian oak-hickory stand. Teuke and Van Lear (1982) found only slight benefits to oak regeneration after single winter burns in western South Carolina and northeastern Georgia.

Prepare Seedbed and Encourage Acorn Caching

Fire removes excessive litter from the forest floor, thereby preparing a favorable seedbed. Areas of thin litter are preferred by squirrels and bluejays for acorn burial (Galford and others 1988). An important ecological finding is that jays collect and disperse only sound nuts (Darley-Hill and Johnson 1981, Deen and Hodges 1991). If these acorns escape predation, they will establish first-year seedlings, but seedlings from freshly germinated acorns are unable to emerge through a heavy litter cover. Germination and first-year survival are best when acorns are buried about 1 inch (3 cm) deep in the mineral soil (Sander and others 1983, Van Lear and Watt 1993).

Although removal of thick litter may expedite the germination process by encouraging the caching of acorns by squirrels and jays, it is important that not all the humus layer be consumed. The humus layer keeps the surface of the soil porous, so that radicles from uncached acorns can more easily penetrate the soil. Humus also retains moisture and provides support for the new seedling (Carvell and Tryon 1961). The intensity and severity of a prescribed burn will determine the amount of organic matter that remains on the site (Wells 1979, Van Lear and Watt 1993).

Discourage Acorn and Seedling Predators

Fire helps to control insect predators of acorns and new seedlings. Martin and Mitchell (1981) illustrated how insect populations can be reduced or eliminated directly or indirectly by fire (Table: Effect of Fire on Insects). Insect pests act as primary invaders, secondary invaders, parasites, or scavengers on acorns (Gibson 1972). Many of these insects spend all or part of their lives on the forest floor. Infestations, which can vary from year to year and even from tree to tree in some areas, are a major cause of oak regeneration problems (Marquis and others 1976, Van Lear and Watt 1993).

Annually about 50 percent of the acorn crop in Ohio is destroyed by the larvae of Curculio weevils, acorn moths, and gall wasps. Other insects attack germinating acorns and oak seedlings. However, recent studies indicate that prescribed burning may reduce populations of oak insect pests when conducted under proper conditions and at the appropriate time in the insects life cycle (Galford and others 1988). A reduction in insect predation would allow more acorns to be scattered and buried by jays and squirrels, thus enhancing the probability of successful germination, and also encourage subsequent seedling establishment. Burning may also reduce rodent habitat, eliminating another source of acorn predation (Hannah 1987, Van Lear and Watt 1993).

Xerify Sites

Severe or frequent fires dry out the surface of forest sites by consuming much of the forest floor and perhaps even organic matter in the mineral soil. Since fires also expose the site to greater solar radiation through canopy reduction (Van Lear 1990). Adequate advance oak regeneration in the Southeast is generally found more often on xeric sites than on mesic sites (Sander 1988). Crow (1988) cited the lower frequency of fire in recent years as a major factor in the failure of oak to regenerate on mesic sites. Conversion of mesic sites to more xeric conditions by intense fires or by a long regime of low-intensity fires could explain in large part the ability of oaks to dominate sites where more mesic species normally occur. Mesic sites may only have burned during cyclic periods of dry weather, which have apparently occurred in the Southeast for millennia (Van Lear and Watt 1993).

Increase Flammable Fuels

Frequent fire in oak stands may also increase the production of legumes and grasses, which benefit numerous wildlife species, but also create a more flammable understory. At the turn of the century, summer fires were quite common in the southeastern United States as farmers burned the land to facilitate grazing. They had learned from early settlers, who in turn had learned from their Indian predecessors, that growing-season fires best maintained an open forest with a rich herbaceous layer (Komarek 1965). Thor and Nichols (1974) noted an increase in herbaceous vegetation after frequent burning in mixed hardwood stands in Tennessee. Similar findings have been reported in pine forests of the Southeast by numerous researchers. Therefore, frequent fires create and maintain a ground cover that encourages the return of fire, which favor the establishment of oak advance regeneration (Van Lear and Watt 1993).

Northern Red Oak Reproduction

Although oaks in general are relatively fire resistant because of their thick bark, the resistance of northern red oak ranks behind that of bur oak (Q. macrocarpa Michx.), black oak, and white oak (Lorimer 1985). Nevertheless, prescribed burning has potential to promote the accumulation of red oak reproduction in at least three ways: (1) by eliminating or reducing the number of fire-sensitive understory competitors, including shrubs and shade-tolerant trees; (2) by reducing overstory density by killing trees with thin, fire-sensitive bark; and (3) by killing back the tops of oak reproduction, which increases the root-shoot ratio and ultimately the root mass of those that survive by resprouting. The first two factors increase light on the forest floor, which creates conditions more conducive to the accumulation of oak reproduction. The third factor increases the probability of rapid height growth after overstory removal. Northern red oak is well adapted to surviving fire because of the concentration of dormant buds near the root collar. These buds often remain an inch or more below the soil surface, where they are protected from lethally high temperatures (Korstian 1927, Isebrands and Dickson 1994).

Contradictory Evidence

In addition to this large body of literature that documents the benefits of fire for oak regeneration, there are also studies that reveal contradictory evidence.