Prehistory of Southern Native Plant Communities

Through providing an understanding of the history of native plant communities in the South, this Assessment hopes to put into context the background against which change has occurred. It is important to understand the roles that global climate change and indigenous human cultures played in shaping the plant communities that are considered native or natural today. In this Assessment, only those works that address the Quaternary, 2 million years before present (BP), and later floras are discussed. The primary focus is on the vegetation history of the Holocene, 10,000 years BP.

For the majority of the Quaternary, the climate of the Southeast has been colder than at present (Greller 1988). During this period, there were multiple continental glaciation episodes that did not affect our region directly, but nonetheless had significant impacts on the composition of our native plant communities. These glaciations have been attributed by most to Milankovitch (1941) variations in the orbit of the Earth about the sun. The components of the Milankovitch cycle are expressed at periods of approximately 100,000, 41,000, and 21,000 years (Delcourt and Delcourt 1993). The effects of each of these cycles have been correlated with the relative severity of glacial periods and the rapidity with which glacial advances or retreats occurred.

The coastlines of the Southeastern United States achieved their present approximate position and shape during the early Quaternary (Christensen 1988). Changes in sea level associated with Quaternary glaciations have profoundly affected the vegetation of the historical Coastal Plains, though due to normal coastal processes, most of the evidence of paleocoastal plant communities has been obliterated. Likewise, the major Quaternary glaciations also profoundly impacted the depositional landscape, especially in the Mississippi Basin.

The composition of native plant communities of the Southeastern United States has changed less than that of any other region in the country during the last 20,000 years (Delcourt and Delcourt 1993). This is not to suggest that plant communities in the South have been static over that period. About 18,000 years ago, at the peak of the last major glacial period, the influence of Arctic air masses and boreal vegetation extended to about 33º N. latitude, the approximate latitude of Birmingham, AL, and Atlanta, GA (Delcourt and Delcourt 1993).

These forests were dominated by various spruce species (Picea spp.) and jack pine (Pinus banksiana); fir (Abies spp.) was abundant in some locations. The understories of these forests were generally typical of modern spruce-fir forests, with the exception of the absence of certain prairie elements (Wright 1981). Today, jack pine is essentially limited to boreal forest types and higher elevations in New England, Wisconsin, Minnesota, and northward. Modern boreal forests dominated by spruce and fir are similarly restricted to New England and Canada.

Temperate deciduous forests dominated the landscape south of 33º N. latitude, to about 30º N. latitude, including most of the then Gulf Coast from about 84º W. longitude. The climate of this region was similar to or slightly drier than modern conditions, based on the analysis of the species present in pollen profiles collected from lake sediments deposited during this time. Oak (Quercus spp.), hickory (Carya spp.), chestnut (Castanea dentata), and southern pine species were abundant. Walnuts (Juglans spp.), beech (Fagus grandifolia), sweetgum (Liquidambar styraciflua), alder (Alnus spp.), birch (Betula spp.), tulip tree (Liriodendron tulipifera), elms (Ulmus spp.), hornbeams (Carpinus spp. and Ostrya spp.), tilias (Tilia spp.), and others that are generally common in modern southern deciduous forests were also common then. Pollen of members of the grass, sedge, and sunflower plant families (Poaceae, Cyperaceae, and Asteraceae) were also common in samples from this time period (Delcourt and Delcourt 1993, Greller 1988, Watts 1980).

The vegetation south of 30º N. latitude, in peninsular Florida, was dominated by sand-scrub communities with xeric pine-oak forests in the uplands. Swamps and marshes occupied low-lying and coastal areas (Delcourt and Delcourt 1993, Greller 1988, Watts 1980). The areas that were occupied by coastal marshes at that time are now submerged because sea levels during the time of peak glacial extent were significantly lower than modern levels. The sand-scrub communities still occupy significant areas of upland central Florida (Ricketts and others 1999).

During glacial periods, extensive mesophytic forest communities, similar in character and overall composition to modern lowland and bottomland forests, occurred along major river drainages, especially the Mississippi embayment, the Alabama-Coosa-Tallapoosa Basin, the Apalachicola-Chattahoochee-Flint Basin, and the Savannah River Basin (Delcourt and Delcourt 1993, Greller 1988).

From approximately 15,000 years BP to approximately 10,000 years BP there was a gradual warming trend throughout the region, but the period of 14,000 years BP to about 12,000 years BP was marked by a high degree of climatic variability, including increased seasonality and other climatic extremes (Delcourt and Delcourt 1993). By approximately 10,000 years BP, deciduous forests had expanded northward throughout the region, with pockets of boreal elements remaining only at high elevations in the Appalachian Mountains and in a few other refuges. Broadleaf evergreen and pine forests occupied an area similar in extent to what they occupy today, primarily in the Coastal Plains. Mesophytic and bottomland forest communities continued to occupy the major river drainages of the region (Delcourt and Delcourt 1993).

Although the exact date is in question, this was also the period in which humans first colonized the Southeast. Archeologists date the earliest potential human habitation at approximately 12,500 years BP. Between 12,500 and 10,000 years BP, the human population of the region is thought to have been largely nomadic and very sparsely distributed. Human influence on the region’s vegetation was almost certainly trivial and highly localized.

At about this time, many large herbivores that heretofore had been common in the region went extinct (Martin and Klein 1984). Among these animals were the mastodon, ground sloth, and giant bison. In other parts of the World where large grazing animals still exist, they are known to exert a profound influence on the composition and condition of the native plant communities. Likewise, their extinction would lead to a variety of (largely unpredictable) changes. It is not clear why this guild of plant-eating animals disappeared from the region, but overexploitation by aboriginal Americans and an inability to adjust to climatic changes are most often posited. It is certain that their disappearance altered regional patterns of vegetation (Martin and Klein 1984).

At the beginning of the Holocene (10,000 years BP), the climatic conditions in the Southeast were comparable to conditions today (Delcourt and Delcourt 1993). However, the existence of modern climatic conditions does not necessarily imply the existence of modern native plant communities. Although the major modern community types were flourishing in the Southeast by 10,000 years BP, the understory flora had not yet come to resemble modern herbaceous floras. Mixed hardwood forests dominated the majority of the upper Coastal Plains, Piedmont, and lower Mountain regions. Southern pine communities dominated the middle and lower Coastal Plains, whereas evergreens and some remnant boreal elements occupied higher elevation sites. Canopy openings in the mixed hardwood and high-elevation forest regions are thought to have been infrequent and due either to local edaphic conditions or natural disturbance (Delcourt and Delcourt 1993, Watts 1980).

Evidence of human habitation in the region becomes common at about 10,000 years BP (the Paleo-Indian period), but there is little evidence that these cultures had significant or large-scale impacts on the landscape (University of Illinois 1997).

Around 8,700 years BP to approximately 5,000 years BP, a period of significant warming and drying, often called the hypsithermal period, began impacting the vegetation of the Southeast. During the hypsithermal period, extensive expansions of prairies and savannas occurred throughout the region (Delcourt and Delcourt 1993), and xeric oak and oak-hickory forest types proliferated. Many species with more northerly affinities migrated northward and, to the extent possible, upward in elevation. Given the limited heights of the Appalachian Mountains, many of these boreal elements were extirpated during this period. Others were relegated to isolated refuges (Delcourt 1979, Delcourt and Delcourt 1998). Further retraction of boreal forest elements caused a proportional increase in pine-dominated forests in the Appalachians. The hypsithermal was also responsible for the expansion of sand and scrub habitats in central Florida (Delcourt and Delcourt 1993, Watts 1971). The grasslands and savannas of the time expanded and were also linked to the great interior plains grasslands to the west of the region. As a result, elements of the prairie flora became established throughout the region, first by simple migration, but then also by invading disjunct openings (including glades and barrens) that were forming in the canopy of more mesic forests (Delcourt and Delcourt 1993).

During most of the climatic shifts of the last 100,000 years, most plant migration in Eastern North America occurred along a more or less north-south axis. The hypsithermal was significant because it made conditions favorable for the invasion and establishment of species from the center of the continent.

With the warming and drying of the climate throughout the region, species with more mesic proclivities retreated to shrinking riparian and riverine areas.

During this period, the population density of aboriginal peoples increased substantially. The hypsithermal also saw the transition from Paleo-Indian to Archaic Indian cultures. During this period, the Archaic Indians’ settlements and populations tended to increase in size. Archaic Indians remained; like their Paleo-Indian ancestors, they were largely nomadic but were able to remain in some areas for extended seasons by practicing more concentrated resource usage. Increased resource use was made possible by technological advances that improved the efficiency of the harvest, collection, and processing of, for example, native plant materials. More concentrated occupation had significant but still local impacts on the abundance and regeneration of tree species (University of Illinois 1997).

At the end of the hypsithermal interval, about 5,000 years BP, all of the components of the modern southern forests were in place. As the climate cooled and precipitation increased, species migrated so that communities were reassembled in new form. The boreal elements of the early Quaternary enjoyed a modest expansion. Riparian, bottomland, and wetland plant communities expanded. Grasslands and savannas contracted and retracted westward.

Within approximately 1,000 years of the end of the hypsithermal, the distribution of species within plant communities of the Southeast had more or less stabilized and would see only minor changes until the colonization by Europeans (Delcourt and Delcourt 1993).

At about 4,000 years BP, the Archaic Indian cultures began practicing agriculture throughout the region. Technology had advanced to the point that pottery was becoming common, and the small-scale felling of trees became feasible. Some of their crop plants, such as corn and squashes (Zea mays and Cucurbita spp.), were acquired through trading with cultures from the South that had a longer tradition of agriculture (Delcourt 1987). Other crop plants were selected from local natives on the basis of desirable cultivation and harvesting traits. This period also saw increasing emphasis on some forms of passive agriculture, in which existing perennial plants were cared for to increase or improve their output of desired products such as beechnuts or cranberries. Concurrently, the Archaic Indians began using fire in a widespread manner in large portions of the region. Intentional burning of vegetation was taken up to mimic the effects of natural fires that tended to clear forest understories, thereby making travel easier and facilitating the growth of herbs and berry-producing plants that were important for both food and medicines.

Approximately concurrent with the transition from the Archaic Indian culture to the Woodland Indian culture, around 2,800 to 2,500 years BP, aboriginal groups began to establish relatively large settlements. People from these settlements visited sites to exploit specialized resources such as fish, medicinal plants, and cherts. There was a trend, however, toward more permanent occupations to maintain local agricultural plots (University of Illinois 1997). It was during this time that the Mound cultures began to develop and flourish. Woodland Indian Culture evolved into the Mississippian Indian Culture in large portions of the region approximately 1,000 years BP (University of Illinois 1997). Mississippian Culture agriculture became more highly developed, and villages, both large and small, were able to support a more specialized citizenry (Delcourt 1987). Mounds became larger and more numerous, and the amount of land needed to support these populations increased. The majority of Mississippian Culture sites are associated with wetland, riparian, or riverine habitats, and these people became quite expert at altering local hydrological patterns to keep their villages dry and their fields irrigated, and to supply community water needs. In some places, soil erosion became locally significant.

Indian use of fire in land management continued from approximately 4,000 years BP to approximately 500 or 600 years BP (Adams 1992, Cowell 1998, Delcourt and Delcourt 1997). This practice significantly affected the structure of forest stands and the relative abundance of species over large portions of the region. It is not clear to what extent fire influenced the composition or richness of regional floras.

For reasons that are unclear, approximately 500 years ago, aboriginal populations declined significantly throughout Eastern North America and more broadly throughout the Americas. Most anthropologists attribute this depopulation to the transmission and spread of pathogens brought to North America by Europeans. Some communities are known to have lost 98 percent of their population; in general it seems that approximately two-thirds of the Indian population of the Eastern United States was eliminated in a very short time. As a consequence, large areas that had been cleared, burned, and farmed by native peoples were left fallow. Thus, by the time the first European observers were reporting the nature of the vegetation of the region, it is likely to have changed significantly since the regional peak of Indian influence.

A myth has developed that prior to European culture the New World was a pristine wilderness. In fact, the vegetation conditions that the European settlers observed were changing rapidly because of aboriginal depopulation. As a result, canopy closure and forest tree density were increasing throughout the region.

When Europeans started making regular visits to the New World approximately 500 years BP, and during subsequent colonization (specifically in Florida, but also shortly afterwards northward along the Atlantic coast), they also began introducing Eurasian and nonnative tropical plant species. Exotic plants first became prevalent around permanent settlements, especially along the coasts, and then spread inland along travel routes to other suitable locations.

The earliest exotic plants to become established in the region came originally as packing material (often rough hay) in shipping crates or animal bedding material. Later, food, forage, and medicinal plants were introduced in support of the settlements (Carrier 1923). The introduction of exotic animals (especially hogs, cattle, and rats) also began at this time. These animals also have had a significant and permanent impact on the vegetation of the region.

In June of 1527, a group of Spaniards, including Cabeza de Vaca, began a 10-year expedition from Florida along the gulf coast into Texas and on into the American Southwest (Cabeza de Vaca 1542). In his account of the journey, Cabeza de Vaca reported that: (1) the natives of Florida cultivated large quantities of corn; (2) palmetto was abundant and was used commonly for food, fiber, and fuel; and (3) extensive areas of heavy timber (almost certainly longleaf pine) were present with a considerable amount of large woody debris on the ground. The chronicles of other early Spanish explorers, such as Hernando de Soto and Ponce de Leon, contain similarly superficial accounts of the existing native vegetation. The first really useful and widely available information on the natural vegetation of the Southeast was not published until more than 200 years after the Spanish exploration of the region.