Historic Fire Regimes of Longleaf Pine

The most conclusive sources of fire history information are tree-ring (especially fire scar) and lake-sediment data, techniques that allow reasonably unambiguous determinations of fire dating (Frost 1998). Unfortunately, published data on longleaf pine fire history are not available for either method. The paucity of fire scar studies is a function of the relative lack of old growth trees, the reluctance to destructively sample those few old trees that remain, and the rarity of fire scars (Frost 1998) since typical low intensity surface fires do not damage the vascular cambium of mature longleaf pine trees. Consequently scarring generally does not occur and the fire goes unrecorded. Despite this problem, fire-dating studies could perhaps be carried out in longleaf pine stands with remnant old growth trees by focusing on the occasional trees that had sustained damage sufficient to cause scarring. Such trees are potentially valuable sources of fire history data since once scarred, e.g. in a locally intense fire, a tree becomes much more susceptible to subsequent scarring.

Forest fires produce airborne charcoal particles that can become incorporated as layers in lake-sediments. Interpretation of such data is somewhat problematic since fires occurring anywhere within the vicinity of a given lake may contribute to charcoal accumulation in the lake. Thus, lake-sediment data provide an integrated picture of fire history around the lake rather than a fire frequency specific to a particular site. Preliminary studies utilizing this approach indicate essentially continuous fire beginning around 5,000 y BP. Prior to that date fires were still very frequent, but slightly less so (J. Porter, personal communication). Increasing fire around 5,000 y BP is hypothesized to result from the onset of anthropogenic burning by Native Americans. Thus the available data suggest an ecosystem adapted to frequent fire managed by humans for millennia with even more frequent fire. However, these conclusions are based on analyses from just a handful of sites and much more work is needed.

Given the absence of truly conclusive information, fire history of the longleaf pine region has been inferred from a variety of sources including historical observations, land form, lightning ignition rates, and remnant vegetation patches (Wright and Bailey 1982, Christensen 1988, Harcombe et al. 1993, Frost 1998). Utilizing similar historical sources, different authors have nevertheless inferred significantly different historic fire frequencies. For example, Christensen (1988) suggested fire return intervals in the 3 to 10 year range, whereas Frost (1998) postulated 1-3 year between fire intervals for the outer Coastal Plain and 4-6 years for the inner Coastal Plain. Harcombe et al. (1993) estimated a median fire return time of 3 years for east Texas longleaf forests, but with longer fire free intervals for smaller longleaf patches. One explanation for the differing estimates produced by Harcombe et al. (1993) and Frost (1998) for eastern Texas concerned the definition of patch size. Both studies assumed that larger patches would burn more frequently, given an equivalent rate of lightning ignitions. Frost (1998), however, based his patches, or “polygons”, on Hammond’s (1964) land form map, whereas the polygon definition in Harcombe et al. (1993) was based on an early US Forest Service map of extant longleaf vegetation. One other point to consider about Frost’s (1998) estimates is that they incorporate a certain degree of circularity, at least where management is concerned. That is, the existence of remnant longleaf groundcover vegetation is used to infer a historic fire frequency that is then used to suggest an appropriate prescribed burn regime for maintaining the observed vegetation. Whereas this may be a valid approach, all things considered, practitioners should at least be aware of the circularity.

Historic fire season has also been inferred from lightning ignition data and climate (Komarek 1968, Frost 1998). Lightning strikes in the southeast USA are primarily associated with summer thunderstorms. Consequently, it has been suggested that most “natural” fires occurred during the growing season. Another related suggestion is that most of the landscape actually burned in late spring, at the beginning of the thunderstorm season, when fuels are driest and fires could burn uninterrupted over large distances.

Given the uncertainties involved with fire history reconstruction in longleaf pine stands readers should keep in mind that history is only one of several factors to consider when planning a prescribed burn program. For example, experimental data on fire responses at the population or community level may allow us to tailor burn regimes to meet particular objectives, e.g. maximizing persistence or population growth of rare and endangered species (Gray et al. 2003). A related point is that historical burn regimes can be inferred somewhat from community responses. If it is correct that the disturbance regime that maximizes species richness is the one to which most plants and animals are adapted (Denslow 1985), it should be possible to infer the historic disturbance regime from management experiments. In the case of longleaf pine plant and animal communities the results of management experiments, including prescribed burn studies, reinforce historical interpretations in suggesting the need for frequent, low intensity fire (Glitzenstein et al. 2003, Gray et al. 2003).

Anthropogenic Fire

There is substantial information indicating that humans have used fire in landscape management of southeastern Coastal Plain ecosystems for at least several thousands of years (Pyne 1982). In addition to the lake sediment data discussed above there are numerous historical observations of Native American fires (Pyne 1982). Lastly, phytolith data suggest that fires set by Native Americans may explain the existence of longleaf pine islands in otherwise scrub dominated areas in central Florida (Kalisz et al. 1986). Native Americans apparently burned to improve habitat for game species, to reduce annoying insects, and perhaps for aesthetics. According to Lawson (1709) Native American burning occurred primarily in autumn when fuels began to dry after the summer thunderstorm season and temperatures became more comfortable. European and African Americans rapidly adopted the Native American practice of woods burning (Pyne 1982). For example, Elliott (1824) and Ruffin (1843) allude to the common practice of burning the woods every spring to improve forage for cattle. These references document a shift in season of anthropogenic fire from earlier fires set by Native Americans. Official industrial and government fire suppression policies began towards the end of the 19th century. However, “woods burning” by rural people continued throughout and probably saved what remains presently of the longleaf pine ecosystem (Pyne 1982). Even today, suppression of arson fires is a major occupation of the US Forest Service in southeastern National Forests. Most current fires, however, are legal prescribed burns set by private landowners or state and federal agencies.

See: Effects of Fire Suppression on Longleaf Pine Woodlands and Savannas