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Fragmentation of Forest Communities in the Eastern United States
Riitters, K., J. W. Coulston, AND J. D. WICKHAM. Fragmentation of Forest Communities in the Eastern United States. FOREST ECOLOGY AND MANAGEMENT. Elsevier Science Ltd, New York, NY, 263:85-93, (2012).
Driven principally by land use changes associated with an increasing human population, fragmentation is an ever-present threat to forests in the eastern United States. Griffith et al. (2003) documented the overall trend of landscape fragmentation between 1973 and 2000. By 1992, forest fragmentation was so pervasive that only 10% of the eastern forest qualified as intact forest at a relatively local spatial scale of 66 ha, while 40% of it was within 90 m of forest edge and small perforations in otherwise intact forest were common throughout the region (Riitters et al., 2002; Riitters and Coulston, 2005). Between 1992 and 2001, the cumulative impacts of additional small and dispersed forest losses included a decrease of interior forest area and a reduction in the spatial scales over which forest was the dominant land cover (Wickham et al., 2007, 2008).
Forest fragmentation threatens the sustainability of forest communities and therefore the beta diversity of forestland in the eastern United States. We combined forest inventory data with land cover data to compare 70 forest communities in terms of the amount and ownership of intact (i.e., not fragmented) forest, and the proximate causes (i.e., adjacent land cover) of fragmentation. Only 45% of total forestland area was intact in 4.41-ha neighborhoods, but that varied from 13% to 78% among forest communities. Among 10 community groups, the proximate causes of fragmentation reflected their typical geographic context, and the relative importance of fragmentation by development (roads) was higher in mostly-forested neighborhoods than in less-forested neighborhoods. Fragmentation was also higher on privately owned forestland than on public forestland. Because of the regional dominance of only a few forest communities and private land ownership, the total regional area of intact forest was driven more by the total area of those strata than by their fragmentation characteristics. Forest communities exhibiting either a low total area or low percentage of intact forest are subject to relatively higher risk of shifts in stand composition towards edge-adapted and invasive species. Such changes in stand composition could result in local extirpation of communities, and homogenization of forest communities at broader spatial scales. The results provide insight for targeting land management strategies to maintain the diversity and regional distributions of intact forest communities.