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Landscape Structure

Primary Question (chapter 6): How have land uses changed in the South, and how might changes in the future affect the area of forests? (chapter 1): What are the history, status, and projected future of terrestrial wildlife habitat types and species in the South? (chapter 3): What are the likely effects of expanding human populations, urbanization, and infrastructure development on wildlife and their habitats?

Landscape structure refers to the spatial arrangement of forests and the habitats that they provide. The arrangement of forest cover has an influence over many functions of forest ecosystems, especially as wildlife habitat. For example, a large contiguous forest area has a large share of interior forest and relatively little edge-forest, while a patchy forest with the same total area has proportionately more edge habitat. In addition, configuration of forest cover influences the options available for active stand management. For example, small patches of forests may be less economical to manage for timber production due to economies of scale in timber harvesting, prescribed burning, and other treatments.

Interspersion of land uses, road construction, and the splitting of land parcels can cause fragmentation of forests. The effects are a decrease in the average size of contiguous forest patches, a decrease in the amount of forest, and losses in connectivity and interior habitat. Forest microclimate, species dispersion, and interspecies population dynamics are also influenced by fragmentation (chapter 1). However, the effects of fragmentation on wildlife may be slight in areas with forest cover that exceeds 60 to 70 percent (chapter 4). This is the situation across a large portion of the South.

On the other hand, afforestation of nonforest land can reverse fragmentation. That is, forest patches can become better connected if intervening nonforest areas are converted to forest cover.

Data are not available at the regional level for measuring changes in forest fragmentation over time. However, recently compiled data provide a “snapshot” of forest cover and fragmentation, thereby allowing comparisons of these measures among subregions of the South. Using maps derived from satellite images taken in the early 1990s, the extent of fragmentation in the South is shown by mapping two measures. One is the share of forest that is interior forest, defined as a forest “cell” surrounded on all sides by other forest cells so that it is not strongly influenced by environmental gradients emanating from edges. The other measure is “edge-influenced forest,” defined as areas adjacent to nonforest uses (chapter 6).

The highest concentrations of interior forest in the South are in the Blue Ridge Mountains, the Cumberland Plateau, and the Allegheny Mountains (fig. 20). The Great Smoky Mountain National Park and a part of the Daniel Boone National Forest in Kentucky—just west of where the boundaries of Virginia, West Virginia, and Tennessee meet—form the cores of these two areas. Other large areas where the share of interior forest is high include the Ouachita Highlands and Ozark Mountains of Arkansas, a region just north of the Mobile Bay, and the Apalachicola area in the Florida Panhandle.

Maps of edge-influenced forests (fig. 37) indicate several areas where forest is fragmented. The Southern Appalachian Piedmont (commonly referred to as the Piedmont) has a relatively high share of land in the edge-influenced category, especially in North Carolina. Two other contiguous blocks are in an area spanning northern Mississippi and western Tennessee, and an area west of the Cumberland Plateau between Alabama and the Ohio River. In both of these areas, agricultural cover types are interspersed with forest cover, fragmenting it into smaller patches and reducing the amount of interior forest.

Three parts of the South may be especially susceptible to fragmentation in the future due to current conditions coupled with forecast population growth (chapter 1 and chapter 6):

• The Piedmont is a heavily fragmented patchwork of agriculture, urban, and forest land uses. Forecasts of urbanization in this ecoregion would mean reduced forest cover, increased fragmentation, and cause other modifications to forests (chapter 6).

• In North Carolina, Tennessee, and Kentucky, the Blue Ridge, Ridge and Valley, Cumberland Plateau, and Allegheny Plateau ecoregions are susceptible to increased fragmentation. While much of this area is heavily forested, localized areas of urbanization and increasing road density could alter the structure of affected forested communities. A similar situation may arise in the Ozark and Ouachita mountains.

• In the Mississippi River Valley, West Coastal Plain, and Interior Low Plateau, forest habitat is scarce due to conversion of large portions of these forests to agricultural uses. Additional relatively small changes in forest structure in these ecoregions could reduce their overall habitat suitability for many species.