Using Prescribed Fire For Oak Management

In the southern Appalachians, regenerating oak stands on productive upland sites is a major problem because of intense competition from yellow-poplar and other less-desirable species. Yellow poplar colonization following burns is common on sites that receive direct solar radiation (Van Lear and Waldrop 1988; VanLear and Watt 1993). For this reason, perhaps, stands with partial canopy cover have shown better results in regenerating oaks suggesting that combining shelterwood harvesting with prescribed fire may be a viable method for regenerating oak stands on productive sites (Abrams 1992, Lorimer 1985, McGee 1979).  Prescribed fire has also been used before clearcutting, as a means of promoting advanced oak regeneration before regeneration harvests.  These methods of using prescribed fire in the management of oak are explained below.

Using fire in combination with shelterwoods

If oak and hickory regeneration is present in the understory, a two-step shelterwood harvest combined with a prescribed fire can be used for release (Brose and others 1999a,b). This shelterwood-burn technique appears to be a reasonable mimic to the disturbance regime of oak-hickory forests before Euro-American influence. It has considerable value as a silvicultural method, a wildlife management tool, and a means for restoring habitats such as oak savannas and open woodlands.  

In this method, the initial shelterwood cut reduces the basal area to about 50 sq. feet/acre (11.5 sq. m/ha) removing low-value stems. The regeneration is allowed to develop for 3 to 5 years. During this time, oak and hickory regeneration develop large root systems but exhibit little height growth while their competitors do the opposite. When the root collar diameter of the oak regeneration is about 0.75 inch (2 cm), a growing-season prescribed fire with flame lengths of 3 to 4 feet (about 1 m) is used to kill the regeneration layer. This treatment will completely kill the less fire-tolerant competing hardwoods (Christianson 1969), invading eastern white pine (Blankenship and Arthur 1999), and rhododendron and mountain laurel. Few oaks and hickories will be killed by the fire, and most will sprout and grow vigorously. Regeneration should be inventoried 2 to 3 years later to determine whether additional fires are needed.

When using the shelterwood-burn technique, care must be taken to protect dominant oaks from basal fire damage. Directional felling during the logging operation is recommended so that the resultant slash is not abutting the trees. Otherwise, slash must be removed from the bases of dominant oaks to prevent fire damage. Generally, damage to dominant oaks is not a problem when burning in uncut stands because fuel loadings are considerably lighter. Graphs or equations can be used to predict mortality of several oak species after fires of varying intensity (Loomis 1973).

Using fire in combination with clearcutting

Prescribed fire can be used to regenerate oak after clearcutting. Vigorous, abundant sprouting of oaks occurs following broadcast burning of clearcut areas (Van Lear and Watt 1993). Burning of clearcut hardwood or mixed pine-hardwood stands promotes better quality oak sprouts by forcing them to develop from the groundline. Over 97 percent of all oak sprouts developing after broadcast burning of logging slash in the southern Appalachians were basal sprouts, versus 71 percent for unburned areas (Augspurger and others 1987). Suppressed buds higher on the stump are apparently destroyed by the intense heat of the fire. Sprouting from buds at or below groundline is encouraged by fire, reducing the likelihood of rot being transferred from stumps to new sprouts (Roth and Sleeth 1939). Poorly formed tops of small (<15 cm) oak stems killed by fire are replaced by more desirable sprouts, which are more likely to develop into sound timber trees than other types of oak regeneration (Roth and Hepting 1943, Teuke and Van Lear 1982).

Many questions remain unanswered regarding the effects of broadcast burning on stem quality of regenerating oak stands. Some research suggests that fire may cause multiple sprouting from top-killed advance regeneration (McGee 1979 in Van Lear and Watt 1993). In addition, some rootstock may be damaged or even consumed by fire, thereby reducing the regeneration potential of the stand. However, observations of dozens of hardwood-pine stands regenerated using the fell-and-burn site preparation technique (Abercrombie and Sims 1986) suggests that broadcast burning under carefully prescribed conditions is favorable to quality oak regeneration in the southern Appalachians.

Intense fires can sometimes result in the introduction of oak in the succeeding stand (Van Lear and Watt 1993). Nowacki (1988) documented cases in northern Wisconsin where clearcutting of old-growth maple-hemlock stands and slash burning resulted in even-aged stands dominated by northern red oak. Lorimer (1989) suggested that these oak stands probably developed from acorns brought into the burned area by birds and animals. Similar observations have been made following an intense wildfire in the mountains of South Carolina. However, in some cases, intense fires may have unintended effects. For example, Loftis (1990) found that a severe backing fire killed oak seedlings in northern Georgia, and opened microsites for yellow-poplar seedlings.

Using prescribed fire to promote advanced oak regeneration

Obtaining adequate stocking of advance oak regeneration on good-quality sites is a major deterrent to effective oak management. Excluding fire from mature oak stands may have altered the ecology of these stands to the detriment of advance oak regeneration (Little 1974; Van Lear and Johnson 1983). Periodic burning may therefore play a major role in promoting advance oak regeneration (Van Lear and Waldrop 1988).

It has been suggested that an extended period of repeated burns prior to harvest may improve the status of oak in the advance regeneration pool (Little 1974, Sander 1988, Van Lear and Waldrop 1989), especially on better sites (Van Lear and Watt 1993). For example, Thor and Nichols (1974) found that advance regeneration of oaks in central Tennessee was doubled by both annual (for 6 years) and periodic (2 burns, 5 years apart) prescribed fires. Carvell and Tryon (1961) reported large increases in advance 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. A burning regime might include a mix of winter and summer fires adjusted to enhance the relative position of oak in the advance regeneration pool.

One way fire favors oak regeneration is by killing a smaller proportion of oak stems relative to competing species (Van Lear and Watt 1993). The famous Santee Fire Plot study showed that annual summer burns for 5 years in a pine stand in the Coastal Plain killed about 40 percent of oak root stocks compared to 55 to 90 percent of other woody competitors (Waldrop and others 1987, Waldrop and Lloyd 1991). Biennial summer burning killed hardwood root stocks more slowly, but the rate of mortality for other woody species was still significantly greater than that of oak species. Annual winter burning, while not as effective as summer burning in altering species composition, still tends to xerify the site and promote oaks by consuming litter and reducing shading of top-killed understory species.

Another way fire may favor oak regeneration is by increasing height growth rates (Van Lear and Watt 1993). Initial height growth of oak advance regeneration is slow, since most of the early growth goes into the root system (Kelty 1988). Burning can increase the average annual shoot growth of oak seedlings, providing a potential advantage over competing stems (Johnson 1974). Oak advance regeneration occurs as true seedlings or sprouts; the latter have root systems older than the stems and are often referred to as seedling-sprouts (Sander and others 1976). A large root system is necessary for initiation of shoot growth when environmental conditions become favorable (Crow 1988). Thus, a regime of frequent understory burns, perhaps including both growing-season and winter burns during a period of 5 to 20 years prior to harvest, should promote a favorable root/shoot ratio during oak seedling establishment. The timing of the burns would be dependent on the observed vigor of the oak advance regeneration and its competitors.

Once an adequate number of oak seedling-sprouts are present and numbers of competing species have been sufficiently reduced, fire should be withheld to allow the oak advance regeneration to attain sufficient size to outgrow other species which germinate or sprout after the mature stand is cut (Van Lear and Watt 1993). A relatively open stand with few midstory and understory trees would provide adequate light for the oak advance regeneration to succeed. Sander and others (1983) recommend that at least 1,075 advance regeneration oak stems/ha over 1.5 m tall should be present before the overstory is removed.

Herbicides may be required to remove midstory trees that have grown too large to be killed by low-intensity fires (Van Lear and Watt 1993). Loftis (1988, 1990) has convincingly shown that growth of advance regeneration of northern red oak can be enhanced by herbicidal removal of midstory and understory competitors. A combination of herbicide treatment and frequent fire may be required to secure oak regeneration and allow it to maintain its vigor in mixed hardwood forests which have not been burned for decades.

It must be emphasized that a series of burns over an indefinite preharvest period will likely be required to favor oak regeneration (Van Lear and Watt 1993). Single prescribed fires have been shown to have little effect on enhancing oak regeneration. Johnson (1974) reported that not only did a spring fire in a 102-year-old northern red oak stand fail to increase oak abundance in the understory, but also 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. Although one burn may be detrimental to oak advance regeneration in that small rootstocks may be killed, over a series of burns, oaks will be less adversely affected than their competitors. This response will afford oak a competitive advantage that will enable them to favorably respond to subsequent release.

The task of regenerating oaks, particularly northern red oak, is more difficult on cove sites than on upland sites. On moist and fertile cove sites, understory vegetation competes vigorously with oak seedlings and usually overtops them. Therefore, advance regeneration of the desired species is required. In addition, some control of the subcanopy and midstory is necessary to favor advance regeneration. Recent research suggests that fire exclusion from cove sites has created environmental conditions unsuitable to oak regeneration (McGee 1979). However, research on the use of periodic fire for regenerating cove hardwoods is lacking (Van Lear and Waldrop 1988).