Next to the paved commercial parking lot in West Knoxville grew a tall black walnut tree. Except for some bare limbs at the top of the crown, the tree looked perfectly healthy.

"It's dying," said Scott Schlarbaum, professor and director of the University of Tennessee's Tree Improvement Program.

Like scores of black walnuts across East Tennessee, the tree was infected with thousand cankers disease. Discovered in Knox County in 2010, thousand cankers disease is caused by a lethal fungus that's transmitted by a small twig beetle that historically has been limited to the American Southwest. Forestry experts say the beetle most likely hitchhiked east of the Mississippi River on black walnut material transported by highway or interstate.

Across the parking lot stood a tall green ash tree. On the bark were small, D-shaped holes barely larger than the head of a pin. Schlarbaum said these were the exit holes made by the larvae of the emerald ash borer, another exotic insect pest that is wreaking havoc on native forests.

"The larvae are eating away at the living layer beneath the bark," Schlarbaum explained. "Eventually, they'll girdle the tree."

Last week marked the release of Fading Forest III, the last of three reports that examine how nonnative insects and diseases are destroying American's native forests. Schlarbaum co-authored the trilogy with Faith Campbell, a natural resources policy expert with The Nature Conservancy.

Together, they've been calling attention to these foreign invaders for 23 years.

In Fading Forests I, their first report published in 1994, they summarized the effect of exotic forest pests on American ecosystems. In Fading Forests III, they examine how pests spread within the country, and how policies governing imports have failed.

Lots of firewood is likely to be burned this holiday weekend, as folks head out to go camping. Burning local firewood when camping is one way to stop the spread of the pests, because exotic insects frequently are spread on firewood transported from infested areas to noninfested areas.

"Things have gotten worse since we started in 1991," Schlarbaum said. "The hemlock woolly adelgid, emerald ash borer, thousand cankers disease -- all of these have come along since we released our first report. Most of these introductions are directly related to our trade with China, but in this shrinking world, we're trading with all kinds of countries."

Schlarbaum said that from his perspective as a forest geneticist, one of the biggest challenges is restoring tree species to the natural landscape after they've been decimated by pests or disease.

"Despite all the efforts to breed for chestnut blight, I don't think we'll see anything more than the occasional American chestnut tree returning to the forest," Schlarbaum said. "The days of huge chestnut groves are over.

"Restoring trees on a landscape scale takes an enormous amount of infrastructure and money.

"In some cases biological and chemical treatments have been good strategies, but remember, those are just controls. When you get into restoration, that's a whole different ballgame."

Schlarbaum said future forests across the South likely will be dominated by yellow poplar and hickory -- species that so far have resisted exotic insect pests and diseases.

"It's going to be a different forest from what we're used to," he said.

The Fading Forests trilogy paints a bleak picture but also offers glimmers of hope.

One thing individuals can do is burn local firewood when they go camping. Great Smoky Mountains National Park, for example, prohibits bringing firewood into the park from adjacent counties that are under state or federal quarantine. The park's basic recommendation -- delivered in time for the Memorial Day weekend -- is for campers to burn wood where they buy it.

Fading Forest III reveals the economic damage caused by forest pests, and how those costs are sure to rise unless agencies like the U.S. Forest Service and the Department of Agriculture's Animal and Plant Health Inspection Service receive more funding.

The report says exotic forest pests often arrive in the U.S. on wooden crates, pallets and other kinds of wood packaging, and that this movement could be curtailed if warehouses were more aware and took proper precautions.

Faith Campbell, the report's co-author, said there's a good reason why Fading Forests III is subtitled "What Choice Will We Make?"

"There are things we can do as individuals, and things corporations and governments can do, to slow down these infestations," Campbell said. "We're not going back to Eden, but we can do a better job of keeping back the next wave."

Contact Morgan Simmons at simmonsm@knoxnews.com or 865-342-6321.