Forest Edge Density

The Indicator

A forest edge is the outer band of a forest patch, an area that may vary in width depending on the parameters considered. For this application, forest edge has been calculated as a band roughly 300 feet wide. The edge of a forest patch is influenced by environmental conditions adjacent to the patch and is thereby different from the forest interior. Forest edges have significant gradients of solar radiation, temperature, wind speed, and moisture between the forest interior and the adjacent land. If the adjacent land is developed in residential, commercial or industrial uses, edge effects can also include noise, artificial light, exotic species, human disturbance and predation by cats and dogs. Generally speaking, edge can be considered a measure of forest fragmentation, the breaking up of large forests into smaller and smaller pieces.

Depending upon perspective, edge may or may not be desirable. It can promote overall biological diversity at the local scale by providing habitat for species dependent upon two or more land cover types, but the creation of edge conditions often occurs at the expense of interior conditions, which are now far more rare. Thus, overall biodiversity may be reduced on large scales. Finally, some edge-dependent species of both plants and animals have come to dominate, and in some cases have parasitized, native species that are dependent upon larger, unbroken forest patches.

Forest edge habitat conditions tend to be more expansive in those watersheds that (1) contain a significant amount of forest and (2) have smaller mean patch sizes (thereby reducing the amount of interior forest habitat present).

Forest edge density was calculated as the total length of forest edge in a watershed, divided by the land area of the watershed. Land cover is based on the National Land Cover Data set (NLCD).

Indicator Use

Since different species ultimately have different habitat requirements, edge density as depicted here should be used with caution. For some species 300 feet may represent a reasonable distance to approximate edge habitat conditions. For other species this distance may be substantially smaller (or larger). Nonetheless, edge density can provide an overall snapshot of forest fragmentation on a regional scale.

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