SHARE A pocket of dead hemlock trees affected by the hemlock woolly adelgid stand in Great Smoky Mountains National Park along Anthony Creek Trail, on Wednesday, July 6, 2016. The invasive species causes needle loss, which leads to tree death unless treated. (CAITIE MCMEKIN / NEWS SENTINEL) Jesse Webster points out woolly adelgid ovisacks on a hemlock tree off of the Anthony Creek Trail in Great Smoky Mountains National Park at on Wednesday, July 6, 2016. The invasive species causes needle loss, which leads to tree death unless treated. (CAITIE MCMEKIN / NEWS SENTINEL) Forester Jesse Webster stands in a patch of forest where an invasive species, the hemlock woolly adelgid, killed multiple hemlock trees near the Anthony Creek Trail in Great Smoky Mountains National Park on Wednesday, July 6, 2016. The loss of hemlocks at such a rapid rate has drastic implications on the forest ecosystem. (CAITIE MCMEKIN / NEWS SENTINEL) Maple and oak trees sprout where hemlock trees once stood, off of the Anthony Creek Trail in Great Smoky Mountains National Park at on Wednesday, July 6, 2016. The loss of shade from the hemlocks, due to the woolly adelgid infestation, has changed the vegetation sprouting on the forest floor. (CAITIE MCMEKIN / NEWS SENTINEL)

By Morgan Simmons of the Knoxville News Sentinel

TOWNSEND — On a recent summer afternoon, the Anthony Creek Trail at the west end of the Great Smoky Mountains National Park presented stark contrasts between sunlight and shadow.

Much of the trail was shaded beneath the branches, but in patches of forest where the hemlocks had died, the ground was bathed in bright light. Beneath these gaps in the canopy grew various sun-loving species like blackberry and wild grape. High winds had pruned the dead hemlocks of their limbs and their tops, and the trees still standing looked ready to fall, too.

Forests throughout the Smokies are undergoing a major ecological shift due to the hemlock die-off caused by the hemlock woolly adelgid, a nonnative forest insect pest that was discovered in the park in 2002. Trees that for years have suffered in a state of decline are now dead and toppling to the ground, creating a void that forestry experts say will affect everything from stream temperatures to forest composition.

"We saw dramatic disturbance with the chestnut blight, but at least chestnuts could be replaced by oaks and other deciduous trees," said Jesse Webster, forester for the park. "There is no shade-tolerant species to replace hemlocks. They moderate stream temperatures and stream flows. When the hemlocks are gone, that lets in a dramatic amount of light the forest floor hasn't received in hundreds of years."

The infestation

The park contains more than 87,000 acres of hemlock forests. By focusing on high-priority areas like campgrounds, busy roadsides and select backcountry sites, park biologists have treated some 15,000 acres of hemlocks using a combination of chemical and biological agents. To date, those efforts have protected approximately 15 percent of the park's hemlock forests, leaving 85 percent in a state of chronic decline because of the sap-feeding insect.

Because hemlocks were not commercially valued, the trees generally were overlooked by logging companies before the park's creation in 1934. As a result, some of the park's oldest and largest trees are eastern hemlocks that escaped the loggers' ax. Their evergreen canopies attract Neotropical warblers and other songbirds, and the shade they provide along stream corridors helps cool water temperatures in the summer for trout and other sensitive species.

The widespread hemlock decline has increased the threat of hazard trees throughout the park. Last spring, park officials announced the temporary closure of Parson Branch Road near Cades Cove because of the concentration of hazard trees. Crews identified 1,700 such trees within falling distance of the gravel road that stretches 8 miles through a forest of dying hemlocks.

Unlike high priority areas such as the Elkmont entrance road and Cosby Campground, the Parson Branch Road was low on this list for treatment because of its low traffic volume and lack of old-growth hemlock stands.

The park's hemlocks were hit hardest in 2007 and 2008 when the adelgid infestation coincided with severe drought. Bitter cold winters in 2013 and 2014 reduced the adelgid population, but only temporarily — biologists say the population is likely to rebound this year because of mild temperatures last winter.

"We'll never see the complete eradication of the hemlock woolly adelgids in the park," Webster said. "Our goal is to reduce the population to a tolerable level where there is a natural balance between the adelgids and the predator beetles we're releasing as biological controls."

Dramatic changes

About a mile from Cades Cove along the Andrews Ridge Trail, Webster stopped at a "light gap" next to the trail where hemlock branches littered the ground. The tops of two large hemlocks had broken off, and that's all it took to trigger changes in the forest understory. Blackberries were starting to grow, and rhododendron already was beginning to crowd out the native ferns that thrive in the cool, moist shade of the hemlock forests.

Webster said one of the park's major concerns is that these new canopy openings will promote the spread of invasive plant species such as princess trees and oriental bittersweet.

"Forests are about constant change, but that's not what is happening here," Webster said. "These changes are too dramatic, too large-scale. Normal change would be lightning strikes and wind. What we're seeing with the disappearance of the hemlocks is more like a canopy fire."