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Potential Forest Vegetation?

E.L. Braun (1950) developed a generalized map of potential major modern forest types for the Eastern United States (fig. 24.2). The map was developed from surveys to project potential forest species compositions that best characterize the eastern forest region. Braun's classic study is still an important standard for vegetation maps today. Paleoecologists have produced generalized vegetation maps for periods in the past. Delcourt and Delcourt (1984) produced a vegetation map for dominant plant species occurring 18,000 years ago (fig. 24.1). When Braun's modern vegetation map is compared to the Delcourts' map of vegetation, it is evident that dramatic changes have occurred. The presence of glaciers 18,000 years ago, and their absence now, account for much of the difference.

Eighteen thousand years ago the southeastern landscape was powerfully influenced by a massive glacial ice sheet, which created an unusually dry climate. This dry climate limited tree distribution and growth and was conducive to frequent and widespread fire. The glacier's subsequent retreat resulted in extreme continental temperatures, changing sea levels, and disrupted vegetation and wildlife communities. The arrival of humans about 12,000 years BP and their use of fire in the Americas complemented the natural fire regimes. The dominance of oak, hickory, and southern pines throughout much of the Southeast is due to extensive disturbance by humans and nature for millennia. In fact, the vegetation composition and distribution, which complemented the diversity and abundance of wildlife at the time of European contact, was primarily the result of American Indian management of southern landscapes with frequent burning for over 12,000 years.

Because of fire exclusion during much of the recent past and the public's desire for undisturbed forests, unprecedented changes in vegetation composition and distribution are occurring across the Southeast. Oaks, hickories, and pines can survive for hundreds of years in the overstory, but will they remain dominant in our forests without fire? Oaks do not regenerate in extant oak stands on high-quality sites; rather, they convert to more shade-tolerant species (Loftis and McGee 1993). In the absence of disturbance, oaks are not able to regenerate and will not maintain their historical dominance (Abrams 1992, Brose and Van Lear 1998, Loftis and McGee 1993). In the Southern Appalachians, cove forests and upper ridges are increasingly dominated by dense understories of shade-tolerant shrub species, such as rhododendron and mountain laurel, which out compete shade-intolerant oaks, hickories, and southern pines (Baker and Van Lear 1998, Elliot and Hewitt 1997, Hedman and Van Lear 1995). Without disturbance, beech, maples, and other shade tolerant species will gradually dominate southern forests. In the southern Coastal Plain, the once dominant fire-dependent longleaf pine type now occupies less than 3 percent of its original range (Landers and others 1995).

The "potential" or "natural" vegetation map developed by Lucy Braun is a reflection of the Clementsian model of forest succession that dominated ecological thought until the mid-1950s. This model considered disturbance as a relatively unimportant event in the long-term order. However, ecologists now recognize the importance of disturbance. It has been disturbance, repeated over and over for thousands of years, on different temporal and spatial scales that led to the dominance of oaks, hickories, and southern pines in southeastern landscapes and provided the habitats that supported diverse and abundant wildlife populations.

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content: Wayne D. Carroll
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created: 4-OCT-2002
modified: 08-Dec-2013