When the last ice sheet began melting some 20,000 years ago, the climate changed dramatically, and trees responded. As temperature and humidity shifted, tree populations expanded and contracted as natural selection worked its magic. Resulting changes in tree DNA passed on to the next generations.
This past history of trees adapting to climate change is of interest to scientists trying to understand the fate of some heat-sensitive northeastern species, such as red spruce, that seem at risk of losing habitat.
Researchers from University of New Brunswick and Michigan State University investigated the genetic diversity and structure of red spruce, sampling eight populations across the species’ range, from Clingman’s Dome in Great Smoky Mountains National Park in Tennessee to Abraham Lake, Nova Scotia. They focused on microsatellite markers (unique segments of repeated DNA) in the nuclear genome, which indicate patterns of inheritance.
The northern red spruce populations in this study shared more of their DNA with black spruce, suggesting some degree of mixing, which was not unexpected because red and black spruce are known to hybridize. In contrast, the southern populations in the high elevations of the Appalachians were more genetically differentiated from black spruce, leading researchers to agree with earlier conclusions based on fossil data that the southern Tennessee and West Virginia populations may be remnants of a pure red spruce glacial refugium.
The results, published in October 2023 in Frontiers in Plant Science, allowed the researchers to propose an updated scenario of spruce evolution and migration. Over millennia, as red spruce spread north and east, it mixed and hybridized with black spruce, which is more cold-tolerant and persisted all along the edge of the ice, and was therefore among the first trees to re-inhabit glaciated areas. It is this mixed-heritage population of spruce that found a secondary refugium along the Maine coast when the rest of the region became warm and dry, and then expanded its range in the cooler, wetter climate of the past few thousand years.
In contrast to earlier studies, these researchers did not find evidence for low genetic diversity in red spruce, or that red spruce descended from black spruce. Instead, they suggest, the two share a common ancestor. Northeastern red spruce may continue to mix with black, but perhaps this is an age-old strategy for survival. As for red spruce trees in the “sky islands” of the Smokies, they represent important reserves of genetic memory that could be important for sustaining spruce in the warming mountains of the north.