Forest Fragmentation in the United States

National Land Cover Data (NLCD) was reclassified into three categories: forest,
other natural (e.g., grassland and wetland), and anthropogenic use (e.g.,
agricultural and urban). Three new grids were created, one for each edge type
(forest, forest, forest natural, and forest anthropogenic). The values in these
grids were calculated as the number of edges with the appropriate type in the
window divided by the total number of forest edges, regardless of neighbor.
These grids represented forest connectivity (forest forest edges), naturally
caused forest fragmentation (forest natural edges), and human-caused forest
fragmentation (forest anthropogenic edges).

In the map, forest connectivity is displayed in green, natural fragmentation in
blue, and human fragmentation in red. Pure green identifies areas where most or
all forest edges are shared by another forest pixel. Pure red areas are where
forest edges are largely shared with human land use. Pure blue areas show where
most or all forest edges are shared with another natural land cover type.
Different mixes of the three edge types can produce other colors. Two common
examples in the map are yellow and cyan. Yellow identifies areas with roughly
equal amounts of forest connectivity and anthropogenic fragmentation. Cyan is
where forest connectivity and natural fragmentation are approximately equal.
Black represents areas with no forest in the window, and white represents
ignored areas, mostly water, as well as state boundaries.

With few exceptions, forest fragmentation by other natural land cover types is
confined to the western United States, while most human-caused forest
fragmentation is in the East and Midwest. The yellow and red areas around
Yellowstone in northwest Wyoming are a result of the wildfires in 1988. The
burned areas are classified as "transitional" in the NLCD, which are treated as
anthropogenic use. The Mississippi River valley was largely forested at one
time but has been almost entirely converted to agricultural use, resulting in a
display of black and red.

Las Vegas, Nevada, is visible as a patch of red in the Mojave Desert due to an
"urban forest" effect from trees planted by residents. Riparian corridors are
highly visible in arid and developed areas, especially the West and Midwest. In
arid areas, climate often confines trees to riparian zones that are displayed
in shades of blue. In the intensely farmed Midwest, intact and restored
riparian vegetation is depicted in yellow or red. Southern Atlantic coastal
plain riparian zones are wider; forest is better connected and is shown in
green.