Using Prescribed Fire to Manage Competing Vegetation
Low-value, poor-quality, shade-tolerant hardwoods often occupy or encroach upon land best suited to growing pine. Unwanted species may crowd out or suppress pine seedlings. In soils with a high clay content and in areas with low rainfall during parts of the growing season, competition for water, nutrients and growing space may significantly lower growth rates of the overstory. Furthermore, understory trees and shrubs draped with dead needles and leaves act as ladder fuels allowing a fire to climb into the overstory crowns. In most situations, total eradication of the understory is neither practical nor desirable. However, with the judicious use of prescribed fire, the understory can be managed to limit competition with desired species while at the same time providing browse for wildlife.
Burning is most effective in controlling hardwoods less than 3 inches in diameter at the ground line. Periodic fires throughout the rotation can keep competing vegetation below this 3-inch threshold. The most desirable season for burning and the frequency of burns will vary somewhat by species and physiographic region. Generally, a winter (dormant season) fire results in less root kill than a late spring or summer burn. One system recommended in both the Piedmont and Coastal Plain is a dormant season burn to reduce initial fuel mass, followed by two or more annual (if enough fuel is present) or biennial summer burns.
If not controlled, the hardwoods will form a midstory and capture the site once the pine is harvested. If a large pine component is wanted in the next rotation, these unmerchantable hardwoods must be removed during site preparation - an expensive proposition. Generally, fire is required in combination with other treatments involving heavy equipment, chemicals, or both. In many locations the preferred system is a combination summer burn and herbicide treatment.
Managing competing vegetation in the Southern Appalachians
In the southern Appalachians, as throughout the South, prescribed fire is used to control competing vegetation. The goal of these burns is not to eliminate competition in stands, but rather to control the size of hardwoods, reduce wildfire hazard, and facilitate stand regeneration. Low-intensity fires are generally effective in top-killing most hardwood stems up to 3 inches in diameter (Van Lear and Waldrop 1988). Summer fires are most effective in killing hardwood rootstocks; however, numerous summer fires in successive years are necessary to eliminate hardwoods from the understory. Winter fires have been shown to be less effective in hardwood control. For example, after 40 annual winter burns on the Santee Experimental Forest in South Carolina, hardwood sprouts were more numerous, although smaller (<3 ft), than on unburned plots (Waldrop and others 1987). Although not documented, frequent understory burning in pine stands in the Appalachians would likely have similar effects. Periodic burning at about 5-year intervals will effectively control the size of sprouts developing from top-killed rootstocks (Van Lear and Waldrop 1988).
Although controlling understory hardwoods with prescribed fire is normally thought of as a silvicultural tool for pine or mixed stands, prescribed fire can also be used in mature hardwood stands to control the composition of advance regeneration. Numerous studies have documented the benefits of using prescribed fire for oak regeneration. These studies have important implications to a major silvicultural problem in Appalachian hardwoods; i.e., the regeneration of oaks on good-quality sites (Van Lear and Waldrop 1988).
Prescribed fire shows promise as a tool for the control of mountain laurel and rhododendron in the southern Appalachian Mountains. Fire suppression in this region has resulted in dense stands of these evergreen shrubs. These thickets compete with and substantially limit reproduction and growth of both woody and herbaceous vegetation (Van Lear and Johnson 1983, Swift et al. 1993) and therefore are thought to have a major negative impact on white pine and hardwood species. Hence, the objective of many prescribed burns in the southern Appalachians is the control of these evergreen species. Studies have shown that fires initially decrease density of moutain laurel; however because it sprouts quickly following fire treatment, in time it regains dominance in the understory. For example, Hooper (1969) reported that a prescribed fire killed the tops of 70 percent of all laurel under 0.5 inch d.b.h. and 70 percent of the rhododendron under 1 inch d.b.h. However, seventeen months after the burn, almost all of the top-killed laurel and rhododendron resprouted. A fell-and-burn treatment used to restore a degraded pine/hardwood community in the Nantahala National Forest also reduced basal area of evergreen shrubs, but their density increased due to prolific sprouting (Elliott et al. 1999). Nevertheless, burning allows planted pine and other species to become well established in the mid- and overstory before mountain laurel sprouts begin to dominate the understory (Clinton et al. 1993).
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