Historical Vegetation, Land Use, and Fire Regimes of Loblolly Pine Forests

Natural and Cultural History

Forests of the southeastern United States have changed considerably since the last glaciation as climate has shifted and tree species have migrated. Pollen records from sediment cores indicate that the modern pine-dominated flora of the Coastal Plain dates back at least 5000 years before present (Delcourt 1980, Watts and Stuiver 1980). Research based on 19^th-century land survey records from Georgia (Cowell 1995) and Alabama (Black et al. 2002) have highlighted strong physiographic influences on presettlement forest communities. In the coastal plain, pines were the dominant species and hardwoods comprised only a small percentage of witness trees. In the Piedmont, oaks and hickories were the dominant species, whereas pines comprised only 27% of the witness trees in Georgia and 13% in Alabama. Pine dominance in the Piedmont was highest in the uplands (33% of witness trees) and lowest on floodplains (9% of witness trees) (Cowell 1995). Historical records from DeSoto’s expedition in the mid-16^th Century described a similar shift in vegetation, transitioning from open pine savannahs in the Coastal Plain of north Florida and southern Georgia to hardwood-dominated forest in the Piedmont near the border of Georgia and South Carolina (Hudson 1997).

Shifts in the major pine species also occurred along these gradients. On a journey from Augusta to Savannah in 1773, William Bartram documented changes in vegetation from longleaf pine-dominated grasslands in the upper coastal plain to a mixed forest of loblolly pine, oaks, and other hardwoods in the Piedmont (Harper 1998). In the coastal plain, dominant pine species varied with soil moisture status, with longleaf pine dominant on the drier sites and loblolly largely restricted to more mesic areas near wetlands or streams (Schultz 1997). In the Piedmont, shortleaf pine was the dominant species on drier sites, whereas loblolly pine was associated with more mesic areas. Taken as a whole, this evidence suggests that although loblolly pine was probably a very common species in presettlement forests, it had nowhere near the widespread dominance in forest communities that it does today.

Even before the advent of European settlement, native societies exerted considerable influences on the structure and composition of Southern forests. A study of presettlement forests in Alabama based on witness tree records and archaeological data found that tree species composition varied predictably with distance from native population centers (Foster et al. 2004). Although there is considerable disagreement about the size of native populations, there were probably at a minimum 4 million people in North America before the arrival of Europeans, with at least 400,000 living in the Southeast (Krech 2000). Native Americans utilized fire for a variety of purposes, including land clearing, hunting, warfare, and vegetation management. In the Southeast, native people also practiced shifting agriculture in which small patches of forest were cleared, farmed for several years, and then abandoned once soil fertility was exhausted (Williams 1989, Cowdrey 1996). Over longer time scales, forest landscapes changed as populations centers were established, expanded, declined and were ultimately abandoned. For example, Desoto’s expedition in the 16^th century encountered large settlements with extensive agricultural development, but also passed through areas of uninhabited wilderness that had once supported large native populations (Hudson 1997).

However, it is doubtful that the magnitude of native impacts on the forest came close to the drastic and widespread land use changes that resulted from European settlement in the 19^th century. The rate of land clearing peaked in the 1870’s at 308,000 ha/year (760,000 acres/year) in the Southeast and 371,000 ha/year (916,000 acres/year) in the South-central states (Williams 1989). Land clearing declined gradually throughout the remainder of the 19^th century. Rates of clearing had decreased to 150,000 ha/year (370,000 acres/year) in the Southeast and 235,000 ha/year (580,000 acres/year) in the South-central states by the first decade of the 20^th century. In many cases, lands were cleared, farmed, abandoned, and than cleared and farmed again multiple times.

After 1910, the total area of cleared farmland in the eastern United States began to decline steadily (Hart 1968). In the southeastern United States, much of this abandoned farmland was concentrated in the Piedmont regions of South Carolina, Georgia, and eastern Alabama (Hart 1980). Much of the cropland abandonment in this area can be attributed to loss of cotton as a major cash crop. Cotton farming declined in the Piedmont because of soil erosion resulting from poor farming practices, difficulties in farming associated with small ownerships, and steep slopes that limited the potential for mechanization. Legislation and government programs such as the Clarke-McNary act (1924), the Agricultural Conservation Program (1930), the Cooperative Forest Management Act (1950), and the Conservation Reserve Program (1960) all encouraged landowners to withhold land from agricultural production and plant trees (Williams 1989). The present-day predominance of loblolly pine forests across the southeastern United States is a direct result of this legacy of human disturbance and land use.

See also: A History of People and Fire in the South.

Fire Regimes

Although it is widely agreed that fire was an important element of pre-European forest ecosystems, there is very little data from which to infer historical fire-return intervals in the Southeast. Frost (1995) used information from a variety of sources to develop a map of presettlement fire frequencies across the Southeast. Within the current range of loblolly pine, estimates of fire frequency ranged from 1 to 6 years, with highest frequencies in the Coastal Plain and lower frequencies in the Piedmont. These differences resulted from fewer lightning strikes and from topographic complexity that limited the potential for fire spread. In contrast, (Cowell 1995) estimated that historical fire frequency varied between 10 and 50 years in the Georgia Piedmont, based on the argument that the historical hardwood-dominated forests of this region would have been unlikely to develop under shorter fire frequencies.

These discrepancies emphasize the large degree of uncertainty in our understanding of historical fire regimes in the Southeast. Results of dendroecological studies from other high-frequency, low-severity fire regimes emphasize that presettlement fire regimes were quite variable over both time and space (Heyerdahl et al. 2001, Hessl et al. 2004). Spatial variability in terrain, soil moisture, and dominant plant species would have fostered spatial variability in fire occurrence. Drier sites dominated by pines, grasses, and other pyric vegetation probably burned more frequently than more mesic, hardwood-dominated sites. Long-term fluctuations in climate and native populations, as well as chance events, likewise would have resulted in variable fire return intervals on a given site.

Compared to the presettlement era, wildfires have a relatively minor impact on forests in the modern landscape. Prescribed fire was commonly used throughout the settlement period for land clearing and to maintain pasture for hogs and other livestock (Williams 1989). Uncontrolled fires resulting from lightning or accidental ignitions were also common during this period. Beginning in the 1940’s, however, effective fire prevention and suppression activities greatly reduced the impact of wildfires (Brender 1974). The mean percentage of total forested area burned annually in Georgia between 1997 and 2001 was only 0.1 % for Piedmont counties and 0.2 % for Coastal Plain counties (Georgia Forestry Commission, unpublished data). Instead, prescribed fires now account for the majority of acreage burned in Georgia and across most of the Southeast. During the same 5-year period, the mean percentage of forestland burned each year by prescribed fire was 1.7 % in Piedmont counties and 5.5% in Coastal Plain counties (Georgia Forestry Commission, unpublished data). Rates of prescribed burning are generally higher on public lands than on private lands, and are lower in the densely populated wildland-urban interface than in rural areas (Zhai et al. 2003).

For background information, see: Fire Regimes.