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3.3 Forest Conditions

3.3.1 Forest Population Density

To examine the potential influence of the expanding wildland-urban interface on forests of the region, we construct a simple index. For each county in the South, we divide the number of people by the area of forest in square miles. The resulting forest population density index (FPD) provides a measure of the population pressure on existing and future forests. For example, a high FPD indicates a relative scarcity of forest benefits for people in the county.

Clearly, FPD is a very general measure of human influence, but it helps to define where population effects on forests may be most concentrated and where they may change most in the future. Forecasts of the FPD to 2020 were constructed from population and land-use forecasts.

3.3.2 Landscape Pattern

Measuring the configuration of forests in a county requires spatially explicit data. The basis of the analysis was a fine-scale (0.09 ha) grid-based map of land cover in each county developed from satellite images of the South.

A forest fragmentation indicator was constructed from the land-cover maps as defined by Riitters and others (2000). Land cover was lumped into forest and nonforest classes, and the index was calculated based on the amount and connectivity of forest pixels within a fixed-area around each pixel. The "forest" class included shrubland, woody wetland, and three upland forest types on the land-cover maps. A value representing the forest fragmentation indicator was assigned to the center pixel. The pixel value thus describes the forest fragmentation condition within which that pixel of land cover occurs. Forest fragmentation values were constructed for two different neighborhood sizes: 7-ha (-acres) and 66-ha (-acres). Six forest fragmentation classes were defined:

a. Perforated - Most of the pixels in the surrounding area are forested and this pixel appears to be part of an inside edge of a forest patch. In other words, this pixel is near a nonforest inclusion within a forest.




b. Edge - Most of the cells in the surrounding area are forested and this cell appears to be part of the outside edge of a forest patch.




c. Transitional - About half of the pixels in the surrounding area are forested and this pixel may appear to be part of a patch, edge, or perforation depending on the local forest pattern.




d. Patch - Most of the pixels in the surrounding area are not forested and this pixel is part of a forest inclusion or patch of forest on a nonforest background.




e. Interior - All of the pixels in the surrounding area are labeled as forest in the land-cover map.




f. Nonforest - Essentially none (< 0.5%)of the pixels in the surrounding area are labeled as forest in the land-cover map.

Cells labeled water or with missing values were excluded, and data were summarized for counties and ecological sections. Fragmentation was summarized in two ways: (1) the share of area that is interior forest as defined above and (2) the share of "edge-dominated" forest, defined by summing the shares of area in edge, transitional, and patch categories. This scheme leaves out the "perforated" category, which may indicate an intensively managed forest area, but is neither interior forest nor clearly "edge-dominated."