An Ecological Assessment of the Louisiana Tensas River Basin Chapter 2 
Forest Patterns 
Forest patterns are particularly relevant in the southern 
United States because forests are the dominant natural 
vegetation cover. In contrast, natural land cover in the 
western United States also includes grasslands and 
shrublands, so forest patterns alone do not describe 
departures from potential natural vegetation types. We 
used three different indices of forest pattern in the water¬ 
sheds: amount of forest, average forest patch size, and 
forest connectivity. The resulting national rankings of 
watersheds for these forest indices are shown in Fiaure 
2 . 6 . 
The first map (Figure 2.6a) shows the watershed rankings 
of forest area, expressed as the percentage of total water¬ 
shed area. The chart indicates that about 20% (400) of the 
nation’s watersheds are almost completely forested, and 
that about 30% have little forest cover. About 100 water¬ 
sheds have no forests at all when measured at this scale. 
Forest cover is the most common vegetation type in nearly 
all of the watersheds east of the Ohio River. Many western 
watersheds are only forested at higher elevations. 
The two other maps are different ways of looking at 
whether the forests that do occur in a watershed are 
continuous or fragmented into smaller patches. Figure 
2.6b shows watershed rankings of average forest patch 
area or size, expressed as a percentage of total water¬ 
shed area. Figure 2.6c shows watershed rankings of 
forest connectivity, defined as the probability that a ran¬ 
domly selected forested spot on the map is adjacent to 
another forested spot. 
o T3 
t_ 0) 
0 -C 
-O £ 
E a) 
H 
National Rank 
Quintile Data Range 
< 0.19 
0.19-0.28 
0.29-0.42 
0.43-0.76 
> 0.77 
Indicator Value 
Natural Land 
Figure 2.5 
Number of natural land-cover types per 100 square kilometers of 
watershed area. 
