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Genetic Studies Reveal a Tree’s History to Ensure its Future

U.S. Forest Service Eastern Forest Environmental Threat Assessment Center
July 6th, 2015 at 1:00PM

Ponderosa pines stand tall in front of Yosemite Falls in California. (USFS Photo by Kevin Potter) It can reach heights of 200 feet and live 500 years, and occupies landscapes across the western United States. Some say its bark has an unforgettable smell resembling vanilla or even cinnamon, and this tree is one tough cookie. It grows in a variety of soils and climates and survives fires that consume other species. It is also an ecologically and economically valuable tree that provides food, habitat and ponderous (heavy) lumber.

It is the iconic ponderosa pine. But the world is changing, and ponderosa pine is vulnerable to climate shifts, high-intensity wildfires and bark beetles — as well as development that replaces trees. To keep the ponderosa pine standing tall, researchers are looking for answers in its genes.

“To conserve the genetic foundation tree species need to survive and adapt in the face of threats, forest management decisions must consider how genetic diversity is distributed across species’ ranges,” says Kevin Potter, a North Carolina State University scientist cooperating with the Eastern Forest Environmental Threat Assessment Center. For five years, he has led a study of the genetic diversity between and within what researchers have believed to be two varieties of ponderosa pine. Results were recently published in the journal Tree Genetics & Genomes.

Potter and partners from the Forest Service and Bureau of Land Management collected and examined genetic samples from 3,113 trees from 104 ponderosa pine populations across the West. Patterns of genetic diversity support dividing ponderosa pine into two varieties, and suggest that genetically similar clusters within these varieties appear to be associated with differences in latitude. An earlier study published in the American Journal of Botany revealed similar patterns using a different technique for analyzing ponderosa pine’s genetic variation.

In the recent paper, the researchers also found genetic evidence of ponderosa pine refuge locations during the last ice age and potential paths of colonization and expansion after that time period. “Our results show patterns of genetic variation that suggest ponderosa pine populations may have survived the last ice age in areas north of southern Arizona and New Mexico,” says Potter.

The small isolated populations are also more genetically unique and show less genetic diversity than other populations whose genes have mixed across the landscape, so they may be most at risk. “We also discovered evidence of recent common ancestry within each of the two varieties of ponderosa pine, but not between the varieties. Such findings have important management and conservation implications,” says Potter.

To plan for conservation of existing trees and restoration of lost populations, managers should treat the two ponderosa pine populations differently and strive to maintain genetically diverse populations of trees and seedlings on the landscape and off-site (such as in stored seeds and pollen).

“Genetic diversity is an essential component of long-term forest health because it provides a basis for adaptation and resilience to environmental stress and change,” explains Potter, who has performed similar studies on eastern and Carolina hemlock, Fraser fir and pine species from Mexico and Central America. “Understanding genetic diversity can ensure the effectiveness of management and conservation efforts for ponderosa pine and other important, widespread tree species.”