Ash tree variability may offer restoration path post-beetle decimation

Since 2019, scientists in Penn State's Louis W. Schatz Center for Tree Molecular Genetics have focused research efforts on Oregon ash (Fraxinus latifolia), a species that plays a critical role in the Pacific Northwest by providing wildlife habitats, stabilizing streambanks with an extensive root system, keeping waterways cool through shade and serving as a food source for birds and insects.

In a study recently published online before inclusion in a print edition of Molecular Ecology, the researchers found significant genomic variation across the range of Oregon ash. The genetic makeup of the trees that stretch from California to British Columbia varies, the researchers found, depending on their location in the species' range, influenced by demography, range connectivity and environmental adaptation. This variability could inform the development of hardier ash trees, resistant to both the invasive beetles and rising temperatures -- critical needs, the researchers said, because no other tree species occupies its niche in the region's ecological balance.

Estimates of the genomic change required to adjust to future climate projections, known as genomic offset, were greatest in the northeastern and lowest in the southern portions of the species' range, according to the researchers. These findings, they explained, suggests that the regional distribution of genomic variation may be critical to the trees' long-term survival.

"The emerald ash borer recently has been observed in Oregon and British Columbia, and we know what to expect in terms of mortality in Oregon ash," said study senior author Jill Hamilton, associate professor in ecosystem science and management and director of the Schatz center. "Our research aims to quantify and preserve genetic diversity for future resistance breeding efforts. Penn State is leading international conservation genetics efforts to preserve genetic diversity across ash species that will be critical to developing breeding and reforestation programs."

To reach their conclusion, the researchers analyzed genomic variation from more than 1,000 individual trees from 61 separate populations around the Oregon ash range, extending from California, Oregon and Washington into British Columbia. Sequencing of the samples collected by researchers from Penn State, the Forest Service and other partners revealed that, despite evidence of connectivity between populations of Oregon ash, the patchy distribution of populations may impact the tree species' long-term evolutionary potential.

To develop breeding programs that will produce climate-resilient, emerald ash borer-resistant trees, it will be necessary to prioritize conservation of range-wide genomic diversity, Hamilton pointed out.

"Leveraging the power of landscape genomics enables us to identify regions and Oregon ash populations of greatest conservation concern and can improve selection of populations for expanding collections for the breeding programs," she said. "Given the immediate risk of the emerald ash borer to the Oregon ash range, there is a need for proactive conservation. This study represents the first application of genomic data to conservation and restoration for Oregon ash."

Anthony Melton, who was a postdoctoral scholar at Penn State and is now professor of biology at the University of Montevallo in Alabama, spearheaded the study.

Contributing to the research were Trevor Faske, Southwest Biological Science Center, U.S. Geological Survey, Flagstaff, Arizona; Richard Sniezko, Dorena Genetic Resource Center, U.S. Forest Service, Cottage Grove, Oregon; Tim Thibault, The Huntington, San Marino, California; Wyatt Williams, Forests Resources Division, Oregon Department of Forestry; and Thomas Parchman, Department of Biology, University of Nevada Reno.

The U.S. Department of Agriculture National Institute of Food and Agriculture supported this work.