Where can you go to find an eastern hemlock tree? Although threatened by the hemlock woolly adelgid, eastern hemlock has an extensive range.
“Eastern hemlock grows throughout the southern Appalachians,” says U.S. Forest Service collaborator and ecologist Kevin Potter. Potter is also a forestry faculty member at North Carolina State University.
“Hemlock grows in the northeastern U.S., the Midwest, the Great Lakes, and even in Alabama,” says Potter. “Those are places that have very different climates and other environmental conditions.”
Hemlock’s range encompasses great variability in the environment and genetic variability in the species. Hemlock often grows in cool, humid areas. In the southern Appalachians, the graceful conifer often grows at the edge of mountain streams. Its branches shade streams, helping them stay cool.
Over the millennia, hemlock’s range has fluctuated in response to cycles of global cooling and warming.
“Over time, different groups of eastern hemlock populations have diverged from each other,” says Potter. “They’re evolving on their own trajectories.” The result is several genetically separate lineages that appear to have evolved to adapt to different environmental conditions.
Anantha Prasad, a research ecologist at the Forest Service Northern Research Station, and Potter evaluated ecological differences between the areas inhabited by these lineages. Their results were published in the journal Biodiversity and Conservation.
The scientists expanded on a 2012 study by Potter and others that revealed clusters of eastern hemlock genetic diversity. For the current study, Prasad and Potter divided the species into four genetic zones – North-Central, Northeast, Southeast, and Southwest.
“We combined genetic information with demographic and environmental models,” says Prasad. “One of our goals is to better understand whether the hemlock lineages are equally at risk or if they have different conservation requirements, especially under future climates.”
The scientists used Forest Inventory and Analysis data to add an ecological dimension to genetic findings. FIA collects information on abundance, regeneration, and mortality for hundreds of U.S. tree species, including eastern hemlock.
Hemlock mortality has been high. Non-native sap-sucking insects known as hemlock woolly adelgids have killed millions of hemlock trees. Half of all hemlock ecosystems in the eastern U.S. have been affected.
Prasad and Potter compared hemlock demographics and genetics across the four genetic zones. The scientists also explored hemlock abundance under current and future climate conditions, as well as its colonization potential.
Apart from small areas in the Northeast and Southeast, the results show that hemlock is not very likely to colonize new areas.
Of the four locations, the Southeast may provide the best future habitat. However, hemlock woolly adelgids are likely to continue devastating southeastern hemlock trees.
“Protecting eastern hemlock’s genetic diversity requires a continued emphasis on efforts to control hemlock woolly adelgid, as well as seed collection,” says Potter.
Since 2003, Forest Service has partnered with Camcore, an international tree breeding and conservation program at N.C. State University. The partners have collected seed from hundreds of mother trees. Seeds are either in long-term storage or planted in conservation orchards in Chile, Brazil, and North Carolina.
“Our results suggest habitat in the North-Central and Southwest zones could shrink, even under mild scenarios of future climate change,” says Prasad. “Seed collection should probably focus on these zones.”