Hemlock History Repeating Itself?

Scientists trying to save eastern hemlock trees from widespread insect attacks may have uncovered a case of déjà vu, dating back millennia.

“Our hypothesis is that 7,000 to 8,000 years ago, insects hammered the eastern hemlock in a similar way to how it’s being hammered now,” says Kevin Potter, lead researcher on a paper published in Conservation Genetics. “We think we may see a genetic signature of an ancient and widespread insect epidemic.”

Scientists from NC State and the U.S. Department of Agriculture analyzed the DNA of 1,180 hemlock trees from 60 different sites, stretching from Nova Scotia west to Wisconsin all the way south along the Appalachian Mountains into northern Georgia and Alabama. The results offer a peek into hemlock history, dating back to when glaciers retreated some 12,000 years ago.

As expected, the hemlock appears to have survived in refuges near the ice-free Southern Appalachians, home to four different genetic groups, the most in any region, as well as high measures of within-population genetic diversity. But scientists were surprised to also find plenty of genetic diversity in the New England states glaciers once covered.

The most logical explanation, backed up by previous research and fossil evidence, is that an insect infestation more than 5,000 years ago killed off massive numbers of hemlocks, causing a genetic bottleneck in which some genetic variation was lost as the species struggled to survive. The epidemic appears not to have been as bad in New England as elsewhere.

The modern threat to the hemlock is real and close to home.

“Drive through Newfound Gap in Great Smoky Mountains National Park, and you’ll see a mountain covered by dead hemlocks,” Potter says.

The culprit is a tiny aphid-like insect, the hemlock woolly adelgid, which kills the trees in as little as four years. More than half of the hemlock’s eastern range has been infested over the last decade, and drought has aggravated the threat to the slow-growing tree.

That’s why researchers are banking seeds to ensure the survival of the hemlock, which plays a key role in forest ecology. Co-author Robert Jetton, one of Potter’s colleagues at Camcore, an international tree breeding and conservation program at NC State, will use this research to help choose the locations where certified tree climbers will collect cones high in the trees.

Those seeds will be used in conservation plantings in adelgid-free locations, or go into cold storage to possibly be used in breeding efforts to create insect-resistant hemlocks. It’s a slow process, given that hemlocks take 30 years or longer to produce seeds.

“With forestry research, you have to take a very long view, but we hope to provide some assistance for the hemlock,” Potter says. “The results of our study suggest that the hemlock managed to survive a very serious threat a long time ago. We’re hoping we can help it survive again.”