Old Growth Forests and Forest Interior Habitat

Of all the natural biotic communities or habitats in eastern North America, old growth forests are one of the most scarce. By even the most generous estimates, they comprise barely one percent of all forest land in the Southern Appalachians. On national forest land in the Chattooga River watershed, a recent U.S. Forest Service survey establishes that old growth forest communities occupy only about four percent of the watershed area (Carlson, 1995). Plant and animal species associated with old growth are not abundant in the watershed itself, and are barely surviving in the region as a whole.

"Old growth forests are ecosystems distinguished by old trees and related structural attributes. Old growth encompasses the later stages of stand development that typically differ from earlier stages in a variety of characteristics which may include tree size, accumulations of large, dead woody material, number of canopy layers, species composition, and ecosystem function" (USFS, 1989). The last characteristic, ecosystem function, is particularly important from the standpoint of biodiversity at the landscape level.

Large blocks of old growth forest habitat offer a set of conditions which are not present in younger stands. The high degree of variability in chemistry, temperature, humidity, and other physical attributes across the terrain means a more diverse habitat, and a rich collection of organisms in the forest interior. Its diverse micro-environments have allowed for the evolution of some extremely habitat-sensitive species, including a wide range of plants and animals (e.g., Horn 1995, Andrew 1995). With the fragmentation of the forest landscape, many old-growth plants and animals are restricted to islands of remaining forest interior. As a result, many old growth plant and animals species are already listed as threatened and endangered, and others are destined to join them unless further loss of their habitat can be prevented.

As is typical for the Southern Appalachian region, most of the old growth in the Chattooga River watershed occurs in small, isolated fragments. Small patches of any habitat are generally less valuable than larger patches, because they result in small populations--a problem for species that require large home ranges, or have limited ability for dispersal. Small populations can quickly lose genetic diversity (the natural variation within a population that makes it resistant to environmental change); in addition, they are more susceptible to obliteration by chance events such as wildfire and disease. Thus, a highly patchy distribution of small old growth fragments does not provide for the long-term viability of species associated with this forest habitat. Connectivity among patches is also an important issue, since the food chains, reproductive processes and all the other strands in an ecosystem's web of life are coupled with those of adjacent communities, from habitat to habitat across the landscape. Existing old growth fragments should provide centers of distribution for old growth species throughout the forest, but today there are few mature forest corridors between the existing old growth fragments. Essential biological processes can be restored by linking isolated habitat islands with corridors of mature forest across the watershed. Allowing relatively mature forest between old-growth fragments to continue growing will help restore these critical corridors.

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