When climates change, species move. It’s a fact of life on Earth and probably has been for the past 542 million years, even when species don’t have legs or wings or fins to get them from place to place.
Quaking aspen is one example of a seemingly stationary species that has managed in just the past 20,000 years to expand into the largest range of any native North American tree. These distinctive trees grow across Alaska and Canada all the way south along the Rocky Mountains to Mexico. Since most of their current northern distribution was buried under up to 2 miles of ice just 22,000 years ago, aspens provide a great opportunity to study how species move during climate change.
Researchers at Utah State University were curious whether the aspens that currently live in the mountains of the western United States are stragglers that have occupied this region since the species moved north after the last ice age and have subsequently climbed into the mountains to avoid the warming lowlands, or whether the southern aspens are wayward offspring of the main northern population that found the mountains to be a distant, but suitable, habitat.
In order to find the answer the researchers analyzed short sections of DNA from almost 800 individual trees in 30 different forests spanning the species’ entire range. The genetic variation among these trees showed that forests in the northern part of the aspen’s range are all reproducing with each other across thousands of kilometers, or are at least very recent relatives of one another. In contrast, forests in the south are more isolated and probably represent the remnants of two populations that took refuge on opposite sides of the Great Basin during the last ice age.
This geographic history has interesting consequences for aspen genetic diversity (how much variation there is among members of the same species in their DNA). Southern aspens have lower genetic diversity within a forest compared to aspens in northern forests, mainly because of their isolation and propensity to reproduce by sprouting clones rather than by seeds. However, the amount of variation between forests in the south is much higher than in the north, so that, taken all together, northern and southern aspens embody about the same level of genetic diversity. This means that losing a single forest in the south will eliminate much more of the genetic diversity within the entire species than losing a single northern forest.
Unfortunately, the aspens with the most unique genes are also the most likely to disappear; climate change in the next century is expected to decrease southern aspen’s habitat by as much as 94%. Genetic diversity holds a species’ potential ability to adapt to new conditions. What incipient adaptability to current climate change is stored in the DNA of the endangered southern aspens? It is a bit ironic that the most vulnerable forests house trees whose genetic material could hold the adaptive key to outrunning the cause of this vulnerability. The question is, will this potential manifest before its basis is lost?
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