Interior Forest on the Wane in the United States

Human activities often fragment forests. Photo by Larry Korhnak.

Interior forest, which can be simply defined as forest area surrounded by more forest, supports a wide range of plants and animals that do not thrive in forest edges or the small patches of woods left by human activities. Many of the nations most important rivers originate in interior forest, which also shelters municipal watersheds and reduces air pollution. Interior forest also offers solitude and renewal: It was time in interior forest that inspired the founders of our national parks and lands.

A new study by Forest Service Eastern Forest Environmental Threat Assessment Center (EFETAC) scientist Kurt Riitters and long-time collaborator James Wickham from the U.S. Environmental Protection Agency shows a continuing fragmentation of U.S. forests and diminishing forest interior not captured in other studies based on total forest area.

“Total forest area is often used to monitor forest loss or gain over time,” says Riitters, landscape ecologist and leader of the EFETAC Forest Health Monitoring Research Team based in Research Triangle Park, NC. “We wanted to test whether using forest area is an adequate method to detect changes in interior forest area.”

The researchers analyzed spatial patterns of forest loss and gain across the United States from 2001 to 2006 at five different spatial scales. Rather than defining interior forest in terms of distance to nonforest conditions—as is commonly done—Riitters and Wickham defined forest interior as forest area that exists in forest-dominated neighborhoods of a specific size.

The researchers found that using forest area as a measure underrepresented losses in interior forest. Between 2001 and 2006, the net loss of forest in the United States was 1.1 percent, but Riitters and Wickham found that 3.2 to 10.5 percent of interior forest was lost, depending on which neighborhood size was used in the analysis. Interior forest losses of greater than 5 percent were typical in the Pacific Northwest and the Southeast.

“The extent of differences between total forest loss and interior forest loss suggests a widespread shift in U.S. forests to a more fragmented condition,” says Riitters. “If recent patterns of change continue, interior forest area will become smaller and more concentrated on public lands. Fewer options will be available to natural resource managers working to ensure the habitat, water quality and other benefits that interior forests uniquely provide.”

Read the full text of the study.

For more information, email Kurt Riitters at kriitters@fs.fed.us

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