Is Air Pollution Adding to Regional Hemlock Death?

Over the past decade, public awareness of the perilous situation facing our native hemlocks in the southern Appalachians has grown. An exotic bug from Asia, the hemlock wooly adelgid (HWA), has ravaged hemlock stands from New England to Virginia and recently the epidemic has moved southward into the mountains of North Carolina and Tennessee. This region had been completely unaffected since the HWA’s first sighting in the early 1950s in New England.

Why the HWA has suddenly migrated past its southern border, and continues to expand at an increasingly fast rate, remains a mystery. Some claim contaminated nursery stock may be behind the quick migration, while others say that the adelgid had plenty of chances over the past 50 years through natural dispersal methods. Others say that a changing and stressed environment sparked the move.

Whatever the reason, scientists are now faced with an alarming situation, as nearly the full range of the eastern hemlock will soon be infested. Not well known is that hemlock stress has been observed in the western North Carolina region for a number of years, in fact, long before the HWA arrived.

In a recent study conducted by Appalachian Voices, scores of stressed and dying hemlocks were found throughout sites in western North Carolina. From Joyce Kilmer Memorial Forest to the Grandfather Mountain area, many stressed hemlock stands were found with no signs of the HWA. Some of these sites had comparable or even greater numbers of dead and dying hemlocks than sites that were infested with the HWA.

This raises the question: what is the current health of our hemlocks, even before HWA arrival? And further, what are the possible connections between air pollution and the spread of the HWA infestation? It is clearly understood that an already stressed system defends itself poorly against new threats, especially non-native pests as destructive as the HWA.

“Most research has only focused on the HWA as an isolated problem not connected with the other stresses faced by the forest,” said Dr. Matt Wasson, director of Appalachian Voices and a Cornell-trained ecologist. “Without this holistic understanding we can expect little real progress in combating the oncoming HWA infestation.”

Hemlocks have natural stresses just like any living system, and some scientists have ascribed their recent decline in non-aldegid areas to drought. But Dr. Orie Loucks, who has studied the species for several decades, points out that hemlocks can thrive on dry mountain slopes and are tolerant of drought. “It’s ridiculous to say that a pack-a-day ozone habit will not make these trees more sensitive to natural stressors like drought,” he said.

In a survey conducted by the Great Smoky Mountains National Park, Forest Researcher Christine Johnson found what she identified as drought-related stress in a number of sites along the eastern side of the park. Branch die-back and slowed growth were observed in Johnson’s study.

Loucks says drought alone is unlikely to be the culprit. “Ozone pollution is well-known to reduce the small roots of trees like the hemlock, which reduces their ability to deal with dry periods,” he said.

Another alarming case is found in the Joyce Kilmer Memorial Forest, where a small native beetle known as the hemlock borer has left a number of dead and dying hemlocks in its path. Although dangerous, the borer is a common pest throughout the Appalachians and requires a stressed forest to reach infestation levels.

U.S. Forest Service Ranger Jim Buckle of the Cheoah Ranger district claims that old age is the culprit in the death of hemlocks at Joyce Kilmer. He noticed little stress beyond the oncoming adelgids. Close observation reveals another story, however, as a number of recently dead and dying hemlocks stick out alongside the Poplar Cove Loop trail.

Loucks’ research discounts old age as the reason for the decline of Joyce Kilmer’s hemlocks. “Repeated studies show that hemlock has a constant annual mortality from 10 to 350 years of age,” he said. “Unlike humans, who are more likely to die after 70, this long-lived tree has no age threshold where it dies at a higher rate.”

Researchers have seen almost direct correlations in the effects of air pollution and the death of our high elevation spruce/fir stands, but the role it plays in the health of hemlocks is poorly understood.

But a study done by Mark McClure of the Connecticut Agricultural Experiment Station indicates that nitrogen from air pollution may well worsen the HWA epidemic. In a controlled experiment working with differing levels of nitrogen and HWA populations, he found that a higher level of nitrogen in the soils led to adelgid populations five times that of the stands with normal nitrogen levels.

Furthermore McClure’s work as stated by Charles Little found that “hemlock growth was not enhanced at all by nitrogen…(and) instead the unfertilized trees grew faster than the fertilized ones, and the foliage on the fertilized trees became discolored.” As we now know too, acid rain contains very high levels of nitrogen.

Very little research has been done to follow up this study in a natural setting. Are we setting the stage for a fast and furious infestation of the adelgid due to the possible effects of drought and air pollution? Edward Wherat, forest ecologist at the University of Maryland says, “When plants are subjected to stresses such as drought or air pollutants and growth decreases, there is often a rise in the [nitrogen] in leaves… (and that) there is good reason to assume that increased nitrogen deposition could stimulate adelgid populations via increased [nitrogen] in leaves.”

Currently scientists are working hard to combat the adelgid with a small beetle that is a natural predator from Asia. “Unfortunately, little focus is put on the preventative measures that we might take to loosen the adelgid’s hold,” said Dr. Harvard Ayers, who’s been studying tree death across the region for two decades. “Neither the National Park Service or the Forest Service have any data to directly link the death of all these hemlocks to natural stressors, which means something else is going on. We think air pollution is playing a significant role, and we should work to reduce emissions, now.”