So distinctly dark and quiet is an Eastern hemlock forest that walking through a healthy stand has been described as finding oneself in a “cathedral-like atmosphere.” This native North American conifer is shade-tolerant, and as it grows it keeps its needles on lower branches. This unique feature of hemlock trees creates a densely shaded forest floor where few other plant species can survive. In time, this results in a forest stand nearly entirely dominated by hemlocks.
Economically speaking, Eastern hemlock isn’t a highly valued tree. Hemlock wood is coarse-grained, knotted and subject to splitting, making it difficult to work with. The bark was used in tanneries for making leather during the 19th Century, but as other sources of tannins were developed, the economic utility of hemlocks declined significantly.
The Eastern hemlock forests of North America do, however, play an important ecological role. Eastern hemlock is considered a foundational species, meaning it creates and defines an entire ecological community. These forests serve as important habitat for numerous animals. Several bird species, including the black-throated green warbler, have been identified as hemlock obligates, meaning they depend on this forest tree for breeding habitat. Eastern hemlocks also shade and cool headwater streams, creating suitable habitat for many aquatic species, such as trout.
This ecologically significant species is currently under threat from a tiny invasive insect. The hemlock woolly adelgid, native to Japan, was first discovered in the United States in Virginia in the 1950s and has since spread from Georgia to southern Maine. The adelgid is a small wool-covered insect and isn’t capable of moving long distances on its own. Yet, it has caused widespread hemlock mortality throughout North America. In some adelgid-infested stands of the Northeast, hemlock mortality has exceeded 95 percent.
As the climate warms, this invasive insect will continue to move north, potentially spreading throughout the entire range of Eastern hemlock in North America.
As the adelgid has steadily swept across the eastern United States, not all infested trees have responded similarly, however. In several forest stands, amongst an expanse of skeletal trees, occasional individuals appear to be healthy, in spite of heavy adelgid infestation. University of Rhode Island ecology Prof. Evan Preisser and post-doctoral researcher Chad Rigsby want to know why, and how, these trees are surviving.
Preisser said it’s highly unlikely that among the many millions of Eastern hemlock trees in North America there wouldn’t be individual trees that had developed some degree of resistance to the adelgid. It’s plausible that naturally occurring genetic mutations would create this type of resistance, he added. In the fight against this invasive insect, these resistant trees would have thicker armor to defend itself, so to speak.
To test their hypothesis, potentially resistant individual trees must first be located. And that, as Preisser said, is no easy task. In fact, he likens finding resistant individuals to finding a needle in a haystack.
However, the destruction that the adelgid has brought upon hemlock forests may actually make that task a bit easier.
“The best way to find a needle in a haystack is to burn the haystack and sift the ashes,” Preisser said. “Essentially, adelgid is a highly selective forest fire. It is going through forest stands and removing all the hemlocks.”
Any trees left standing after such an intense attack, it can be reasoned, are likely still alive because of some genetic mutation that left them more resistant than their neighbors, and not simply because of luck.
After locating potentially resistant individual hemlock trees, the next step is to determine if there are things about their genetic makeup that might indicate to researchers why those trees appear to be resistant.
Rigsby, the post-doctoral researcher in the Preisser lab, is well prepared to orchestrate such tests. Previous research of his includes working on resistance mechanisms of ash trees to the invasive emerald ash borer. Rigsby is currently applying those skills and techniques in the investigation of hemlock resistance.
He and Preisser are interested in hemlock resistance from a conservation perspective because that this information could help inform conservation, such as breeding efforts, and the planting of hemlocks, Rigsby said.
The hope is that eventually the trees that possess the specific resistant traits will be used in breeding programs to create a stock of healthy adelgid-resistant hemlock trees that can be planted throughout Eastern forests.
“It is becoming increasingly clear to me,” Rigsby said, “and I think the evidence is mounting in the face of the broader scientific community, that this really is the only approach to long-term continental hemlock conservation.”
Mary Mallinger is a graduate student at the University of Rhode Island pursuing a master’s degree in ecology.