Fire Effects and the Effects of Fire Suppression in Oak-Hickory Forests

Frequent fires ignited by Native Americans maintained open oak-hickory forests with a groundcover of grasses and forbs. Oaks and hickories were favored by these frequent fire regimes because they both have several adaptations that make them resistant to fire, such as their thick bark (See: Oaks and fire). These species dominated the canopy as old, large, fire-resistant trees. Densities of dominant trees probably varied from 20 to 40 per acre. Shrubs, understory trees, and woody debris were likely rare (Barden 1997, Buckner 1983, Denevan 1992, Pyne 1997).  Hardwood regeneration comprised of seedling sprouts were dominated by oak and hickory because these species initially emphasize root development over stem growth and have the ability to sprout repeatedly (Barnes and Van Lear 1998, Brown 1960, Van Lear 1991). With fire excluded for a few years, the well-developed rootstocks sent up vigorous stems that often developed sufficient size and bark thickness to withstand future fires. Where windstorms blew down trees over large areas, the replacement stand was even aged. Consequently, the forest was uneven aged, consisting of even aged patches.

Effects of Fire Suppression in Oak-hickory Forests

Reduction of fire has profoundly changed the oak-hickory forest by allowing the forest to succeed to mixed mesophytic and northern hardwood species such as red maple, eastern white pine, sugar maple, and beech. In the absence of fire, these species become established in the understory, grow into the midstory, and eventually change the composition of the canopy. Stem densities are often hundreds per acre. During the growing season, the dense shade from these fire-sensitive species reduces the abundance and richness of forbs and grasses and inhibits development of oak and hickory regeneration. Consequently, when a dominant oak or hickory dies, its reproduction is not capable of sufficient growth to capture the canopy opening.  Instead, the growing space is filled by mesophytic and northern hardwood species (Abrams and Downs 1990, Crow 1988, Lorimer 1985, McGee 1984).

According to Olson (1996) the brushy character of many oak-hickory sites is the result of an interruption in the chronic fire regime that allows shrubs and hardwoods to capture the site. When the area again burns several years later, these stems are top killed producing a dense growth of sprouts that can dominate the site for decades, especially with occasional fire. On drier mountainous sites, fire exclusion allows ericaceous shrubs such as mountain laurel and rhododendron to move from riparian areas into upland forests (Elliott and others 1999). These shrubs are shade tolerant and evergreen, shading the forest floor throughout the year. Hardwoods cannot regenerate beneath them (Baker and Van Lear 1998), and without disturbance, these heath thickets are the climax plant community on some sites.  Although the forest floor rarely dries enough to support surface fire, this ericaceous shrub layer is flammable; and when it burns, it typically supports intense, stand-replacement fires that alter successional pathways, reduce site productivity, negatively impact streams, and threaten human life and property (such slopes are favored building sites).

Altered fire cycles have also impacted the “low elevation rocky summit” vegetation type where fire historically maintained the hardwood scrub savanna (Hallisey and Wood 1976). Fire exclusion over the past 50 years resulted in an increased hardwood overstory and a dramatic decline in herbs such as blazing star and some woody scrub species such as bear oak (Barden 2000).

Until recently, land managers failed to appreciate the role of prescribed fire in restoring and maintaining open oak-hickory forests (Lorimer 1993). However, research indicates that fire can be used in hardwood stands to establish and release oak-hickory regeneration (Barnes and Van Lear 1998, Brose and Van Lear 1998, 1999, Christianson 1969).