Forest Fragmentation and Roads

Fragmentation refers to the direct loss of forest and the division of the remainder into smaller pieces. Although the actual extent of forest has increased in some areas of the U.S., the spatial patterns indicate extensive forest fragmentation, which affects the habitat quality for mammal, reptile, bird, and amphibian species found in forests. Some species are adapted to edges or other disturbed habitats. However, changes in forest spatial patterns more often result in decreased habitat suitability, reduced ability of wildlife to move through the landscape, and the spread of invasive species from disturbed edges. Even small perforations, areas of nonforest within forested areas, introduce these impacts deeper into the forest.

FHM used landcover maps derived from satellite images to model forest fragmentation across the conterminous U.S. (Riitters and Wickham 2003, Vogelman and others 2001). The findings indicate that forest fragmentation is pervasive and extensive, with three-fourths of all forest found in or near the edges of large, heavily fragmented regional forests. Most of the large interior forests in the U.S. are publicly owned, or unsuitable for agriculture or urban development.

Fragmentation caused by roads is of special interest because the effects of roads extend tens to hundreds of yards from the roads themselves, altering habitats and water drainage patterns, disrupting wildlife movement, introducing exotic plant species, and increasing noise levels. The land development that follows roads out into rural areas usually leads to more roads, an expansion process that only ends at natural or legislated barriers. To analyze nearness of roads at the regional scale, FHM used a national road map to estimate the proportion of land area within certain distances of roads (Geographic Data Technology 2002).

Results showed that 20 percent of all land area was located within 417 feet (127 meters) of the nearest road, and 50 percent was within 1253 feet (382 meters). Only 18 percent of U.S. land area was more than 0.6 miles (1000 meters) from a road, and only 3 percent was more than 3.1 miles (5000 meters) away. Overall, forest land was slightly more remote from roads than other landcover types. While the actual size of a road influence zone depends on local circumstances, the sheer pervasiveness of roads means that few places in the U.S. are immune to their influences.

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