Steady loss of hemlock trees could devastate ecosystem

OTTO — Chelcy Ford looked up into the early afternoon sunshine and pointed to the naked, brown branches of the hemlock trees surrounding her. The bare-limbed evergreens are a familiar sight here in the Southern Appalachian Mountains, where nearly all of the hemlocks are dying after being infested by the woolly adelgid.

Unlike many other scientists trying to figure out how to save the trees, Ford and a small group of researchers at the Coweeta Hydrologic Laboratory here are letting them die.

Their hope is that by studying the dying hemlocks they can begin to answer the question that seems to puzzle everyone: What will happen when they’re gone?

The woolly adelgid and its distinctive white, snow-like egg sacs have moved through the region faster than anyone had predicted, and scientists say all of the hemlocks in Western North Carolina’s forests could be gone in the next five years.

As more research continues, scientists are finding that the hemlock plays a critical role in its ecosystem, and without another species to replace it, the hemlock’s loss could mean major changes for the area’s forests.

“We lost the American chestnut,” Rusty Rhea, an entomologist with the U.S. Forest Service, said of the tree species that was essentially wiped out by a fungus in the middle of the 20th century. “It went away and the oak trees slid in there and took its place. It was a pseudo-substitute.

“When the hemlock goes away, there’s not going to be a pseudo-substitute. That habitat is going to be lost.”
The threat to the hemlocks

The woolly adelgids were thought to have come into the United States on ornamental plants imported from Japan in the 1920s, appearing first in Virginia in the 1950s and then making their way up to the Northeast in the 1990s.

The bugs made their way into the Southern Appalachians around 2002, and now many hemlocks in neighborhoods all over Western North Carolina display the distinctive white, cottony balls on their branches. Without any natural predators, the adelgids have flourished, feeding on Eastern and Carolina hemlocks, which are only found in the Southern Appalachians. The adelgids inject toxic saliva into the trees that effectively kills them over a period of years.

“It caught us off-guard,” said Chris Ulrey, a plant ecologist with the Blue Ridge Parkway. “All of a sudden it popped up all over Western North Carolina, and it just spread.”

Ulrey and others who spend a lot of time in the area’s forests said the loss of the hemlock in the Southern Appalachians has progressed much faster than anyone had predicted. Ulrey said nearly all of the trees on the Blue Ridge Parkway are infested. About one-quarter of the hemlocks in the Great Smoky Mountains National Park are already dead, and forester Tom Remaley estimates that all the untreated hemlocks in the park will be dead in two to five years.

“Really cold winters will really slow it down,” Jim Vose, a researcher at Coweeta, said of the adelgids. “We don’t have those conditions. We have ideal conditions for adelgids to reproduce.”

In the Southern Appalachians, hemlocks are typically found in riparian zones, areas of vegetation near streams, and are a keystone species, meaning they play a unique role in the ecosystem.

“There’s only a few that are keystone species that if you lose them, it can disproportionate effects on the ecosystem,” Vose said.

The streams and areas surrounding them are home to a large number of fish, including trout, along with insects, salamanders and hundreds of invertebrates. The hemlocks themselves are home to a few different species of migratory birds.

“We have the highest diversity in riparian zones of anywhere in the country,” Vose said. “Those environmental conditions are going to change.”

Scientists at Coweeta, a research arm of the forest service, started examining the effects of the loss of the hemlocks in 2004 by setting up plots in riparian zones in their laboratory in Otto.
Studying the effects

The first study published by Ford and Vose this July determined that the hemlocks are taking up much less water as they die – about 10 percent less annually and 30 percent less in the winter and spring.

This increase in water in the areas where hemlocks grow means that stream flow could increase and the riparian zones could become wetter, which could have consequences for all of the animals and plants in the area.

The hemlocks, with their dense canopy, also provide shade that helps to regulate stream temperatures and keep the ground moist. The loss of the hemlock’s canopy could change the temperatures in streams and riparian zones that are critical to the survival of everything from the trout to the worms in the soil.

The loss of the tree may mean that many birds will lose their homes. And there are other considerations — the hemlock’s needles play a role in the acidity of the soil and the tree’s tannins are already washing into streams in the Great Smoky Mountains National Park.

“Just from the basic water balance to the sunlight hitting the forest floor, it will really have a cascading effect to species in the area,” said Mark Cantrell, a biologist with the U.S. Fish and Wildlife Service. “I think because of the widespread and catastrophic level of impact, we can expect there to be significant changes.”

The biggest changes might be when the hemlocks, which can weigh up to 2,000 pounds, fall down. This will send huge amounts of wood onto the forest floor, causing danger to campers and increasing the risk of forest fires. The large trees can also alter the region’s streams, changing the water’s acidity and causing blockages, which could lead to flooding, said Will Blozan, president of the Eastern Native Tree Society and an arborist who lives in Asheville.

Without another evergreen species to take the place of the hemlocks, hardwood trees may move into the area. But these trees cannot provide the same functions as an evergreen, and the thick rhododendron that often thrives in hemlock stands will make it difficult for other large tree species to move in.

As more research comes out of the laboratory, scientists are beginning to discover just how devastating the loss the species will be. Vose said understanding these effects will help researchers determine how to minimize the impact of the loss of the hemlock in the Southern Appalachians.

“Everyone’s just saying it’s going to be bad,” Ulrey said, “But we’re not going to know until it starts happening.”
Trying to save some hemlocks

While the Forest Service, the National Forest Service and the Blue Ridge Parkway all use both chemical and biological controls to stop the adelgid, their cost and sheer number of hemlocks make it impossible to save large portions of the trees.

The Forest Service is able to treat less than 5 percent of the hemlock population on its land in WNC. Great Smoky Mountains National Park has treated about 1,000 acres of trees in conservation areas and another 1,000 near the roadside. The Blue Ridge Parkway has treated about 2,000 trees in its forests.

The agencies have also released beetles that are natural predators of the adelgid into the forests, although there are too many adelgids and not enough beetles. They are also working on getting more species of beetles, which Rhea said is necessary to control the adelgids.

The goal is to try to save some portion of the hemlock population with the chemical treatments. Scientists hope that they can use these stands to repopulate areas that are being lost and that the beetles will keep the future adelgid population in check.

In the future, however, the loss of the hemlocks is going to be coupled with the loss of other tree species as more non-native pests enter the forests in WNC, Rhea said.

“It’s just one thing after another,” he said. “The additive effect of all of these things is going to be catastrophic.”

News notes are courtesy of Southern Forests Network News Notes
www.southernsustainableforests.org