Hemlocks are under attack. U.S. Forest Service scientists and their partners are working to save the native conifers from the hemlock woolly adelgid (HWA), an invasive insect from Japan. Carolina hemlock (Tsuga caroliniana) trees can survive HWA infestation for a decade or more but often die within four years.
Carolina hemlocks grow in tiny, isolated populations in the southern Appalachian Mountains. They are more closely related to Asian hemlocks than the more common, familiar eastern hemlock (Tsuga canadensis).
Carolina hemlocks are most often found in drier, rockier places than eastern hemlocks. The two species do not typically grow on the same sites and do not hybridize.
Part of the conservation strategy for Carolina hemlock is collecting seeds from the trees for gene banks, conservation planting, and future restoration efforts.
Kevin Potter, a population geneticist and SRS cooperator at North Carolina State University, along with SRS research geneticist Dana Nelson and plant genetics lab technician Sedley Josserand and Camcore partners Angelia Campbell and Robert Jetton, wondered if Carolina hemlock’s small population size — combined with its isolation — might make the species susceptible to inbreeding and low genetic diversity.
This would make the species an especially urgent target for gene banking efforts.
The team conducted the most extensive seed sampling of Carolina hemlock populations to date. Their findings were published in Tree Genetics & Genomes.
“We learned that there are high levels of inbreeding in nearly all of the Carolina hemlock populations,” says Potter. “About half of the genetic differentiation occurred among populations, rather than within. That’s highly unusual for a conifer.”
Gene flow between populations was very low, estimated at less than one migrant per generation. Close to two-thirds of the populations contained unique genetic markers, and several had three or four. This suggests high levels of genetic differentiation among populations. Most of these unique markers were located close to the center of the species’ geographic range.
One possible explanation for the inbreeding and differentiation is the Carolina hemlock seeds: although they’re winged, they’re the largest of any North American hemlock. “Wind is an effective mechanism for spreading genes through the seeds and pollen. But the large genetic differences between populations suggest that these seeds have not routinely covered the long distances between them,” notes Potter.
Their analyses indicate that there were two or three places near the southern end of the range where Carolina hemlocks persisted through the Pleistocene. During periods of glaciation, the species would have endured at lower elevations and latitudes and then spread to other locations during intervals of warming.
Interestingly, populations at the northern and southern extents of the species’ range showed greater genetic differences.
Environmental stress and climate change are more likely to impact these small, peripheral populations because they tend to have less genetic diversity. And they may have adapted to different conditions and evolved separately from populations closer to the center of the range.
Scientists and land managers can apply the study results to target conservation efforts, to prioritize the populations where seed collection is most needed, and to inform HWA-resistant tree breeding programs.
“Carolina hemlock is a really unique and interesting tree. Its entire range is threatened by the hemlock woolly adelgid,” says Jetton. “Because it’s located on dry sites, it’s more susceptible to fire than eastern hemlocks.”
While scientists continue to refine and deploy HWA control measures, the hemlock seeds collected by Camcore can be used to restore populations that have suffered considerable mortality or loss of genetic variation. Restoration efforts will need to select seed sources close to the target sites because of the genetic differences between populations.
“We have to act immediately and quickly if we hope to preserve the genetic diversity of Carolina hemlock,” concludes Potter.
Potter, other SRS researchers, and NC State partners are continuing this work. The scientists are documenting additional populations and counting trees across all populations, assessing their condition — including HWA infestation and mortality rates, and collecting additional genetic materials.
For more information, email Kevin Potter at email@example.com.