Fire and Hardwood Hammocks

Upland hardwood hammocks extend along the coastal plain, from eastern Texas to North Carolina. These mixed evergreen-deciduous hardwood ecosystems have some of the highest number of tree and shrub species per unit area in the United States (Platt and Schwartz 1990). Fire in hardwood hammocks has not been studied as extensively as it has in many other southeastern ecosystems (Christensen 1981). There are two reasons for this absence of information. First, compared to pinelands in the South, hardwood hammocks cover a much smaller area and are not as economically valuable. Secondly, hardwood hammocks are generally considered fire-sensitive, climax vegetation types that are developed in the absence of fire (Monk 1965, 1968). The large intense fires typical of pine flatwood and scrub vegetation are very rare in hardwood hammocks (Platt and Schwartz 1990). Fully developed canopies in hardwood hammocks suppress the development of understory vegetation, and therefore available fuel. Live oak, often a dominant species in hardwood hammocks, generates a litter bed that is dense and holds moisture, and thus is not prone to burn (Myers 2000). Fires that do occur in hardwood hammocks are often patchy, of low intensity, with flame lengths usually less than two feet tall (Myers 2000).

Fires, when they do occur in hardwood hammocks, usually start in adjacent areas and spread into the hammock. Hardwood hammocks, though, are most likely to be found in areas protected from fire. For example, a hardwood hammock stand studied by Laessle and Monk (1961) was sheltered from fire by a swamp and stream to the west, a state highway and sand road to the north and east, and a rural settlement to the south. Lightning strikes do occur in hardwood hammocks, but are not likely to start a fire once the trees have leafed out, which is before the lightning season (Platt and Schwartz 1990).

Fire does, however, play an important role in the formation and extent of hardwood hammocks. In the absence of frequent fire, hardwoods have encroached upon or replaced longleaf pine forests, pine flatwoods, dry prairies (Huffman and Blanchard 1991, Sechrest and Cooper 1970) and other fire-maintained communities. The practice of fire suppression, therefore, increased the amount of hardwood hammock acreage in the southeast. Initially, in the transition from a pine forest to a hardwood forest, fire suppression will lead to an overall increase in species diversity (Platt and Schwartz 1990). Over time, continued fire suppression will lead to the development of fire sensitive species such as Magnolia grandiflora and Fagus grandifolia. Many hardwood hammocks throughout the south are in a continued state of succession due to fire exclusion; it is likely that current hardwood hammocks are not composed of the same species that were before European settlement (Platt and Schwartz 1990).

After formation of a hardwood canopy, a severe fire would revert the area to a hardwood thicket, while a low intense fire would maintain the canopy. Many fire sensitive species typical of hardwood hammocks, such as small live oak, can easily be damaged by fire but are capable of resprouting. Large live oak trees, however, are protected from fire by thick bark. With further absence of fire, mesic species will establish in the hardwood hammock, and once they are established the susceptibility of these species to damage from fire decreases with age of the stand (Laessle and Monk 1961).

Due to the fire-sensitive characteristics of hardwood hammocks, those with natural fire barriers should not be burned. However, fire can be allowed to naturally spread into these communities while burning adjacent forest types. Thus, prescribed burning is used in the management of hardwood hammocks to reduce hardwood encroachment into adjacent fire dependent ecosystems (Huffman and Blanchard 1991), such as dry prairies, pine flatwoods, and longleaf pine forests.