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 2.5b shows watershed rankings of average forest patch area or size, expressed as a percentage of total watershed area. Figure 2.5c shows watershed rankings of forest connectivity, defined as the probability that a randomly-selected forested spot on the map is adjacent to another forested spot. All three maps have a similar pattern.
Forest cover is usually continuous where most of the watershed is forested.
In other cases, such as some watersheds in the southwest, forest cover is a minor component overall, and yet is still continuous where it does occur. Compared to potential natural cover conditions, forest loss and fragmentation of the remainder is significant in the northeast United States, along the east coast, and in the Mississippi River valley. The patterns in the mid-Atlantic region are typical of those found in other places in the eastern half of the country. Although the three maps have a similar pattern, the charts illustrate different views obtained by using different indicators. The distribution of watersheds is more or less uniform for the indicator based on percentage of forested area.