Inventories often report that forested lands across the U.S. and elsewhere are declining, but those studies simply look at the total deforested area. A new analysis of forests in the lower 48 found that “forest interior” is disappearing at a much greater rate than total forest acreage, raising concerns about biodiversity and core habitat.
According to Kurt Riitters, a research ecologist at the U.S. Department of Agriculture’s Southern Research Station and the lead author of a study published in Scientific Reports, forest interior has particularly high conservation value because it is less likely to be affected by human influences and so provides more natural forest function than edge forests. Riitters and his colleague James Wickham examined national land-cover maps to determine whether forested pixels were surrounded by other forested pixels, and they found that while total forest area declined by 1.1 percent from 2001 to 2006, interior forest declined by 5.8 percent.
“In order to understand the impact of forest loss, we have to look at the pattern of where the forest was lost and where it was gained,” said Riitters. “The pattern of loss is such that it’s punching holes in what used to be interior forest; it’s taking interior pixels away. But the pattern of forest gain isn’t adding interior forest.”
In a supplemental report, the researchers examined 60 “eco-provinces” around the country and found that the three provinces that cover the Northeast lost interior forest at lower rates than the national average. Two of those regions – the northeastern mixed forest in much of New York and eastern Maine, and the Adirondack-New England mixed coniferous forest in the high elevations of northern New England – lost interior forest at about twice the rate of total forest loss, though half of the former region and two-thirds of the latter are considered forest interior. Surprisingly, the Eastern Broadleaf Forest, which extends from coastal and southern New England down the Appalachians, lost interior forest at a lower rate than its loss of total forest, perhaps because only one-third of this forest is considered interior.
Noting that rates of decline do not apply equally, even in individual ecoprovinces, Riitters made a point of highlighting the Adirondacks as an area where virtually no net loss of interior forest was detected. “There are always forest gains and losses, but the Adirondacks always stand out as being a reservoir of interior forests,” he said. “When you analyze forest fragmentation, there are few places that stand out as having a high percentage of forest and a high percentage of interior forest, but the Adirondacks is one.”
The researchers’ analysis could not identify the drivers of forest loss in each region, though they noted that in the western U.S. it is primarily caused by insects, disease, and fire, while forest loss in the East is usually a result of urbanization. The next step in their study is to collect data that will help them identify the specific causes of forest fragmentation in each region.