Adaptation and a ‘Noah’s Ark for trees’ offer hope for endangered black ash

UNIVERSITY PARK, Pa. — With the invasive emerald ash borer beetle decimating black ash across North America, according to the U.S. Forest Service, a team led by researchers at Penn State conducted an expansive study of genetic diversity in black ash seeds collected across the tree’s geographic range and stored in seed banks. The research was done with an eye toward someday reintroducing the ecologically and culturally important tree back into U.S. and Canadian forests. As part of the research, the team established three large common gardens — experimental plantings — of black ash that someday may play a role in producing seeds needed to bring the species back.

Black ash is a vital wetland tree renowned for regulating forest hydrology, supporting rich biodiversity, and serving as the foundational material for basketry and cultural heritage for Indigenous communities, including Wabanaki, Iroquois and Great Lakes tribes.

In findings published today (Aug. 25) in Annals of Botany, the team reported that differences in climate have caused black ash trees in different regions to evolve different seed traits that affect how seeds and seedlings develop over time, and these differences affect how well their seedlings survive and grow. That knowledge will be critical in selecting seed for eventual reforestation, the researchers said.

“From a conservation perspective, many of the populations represented in these collections are trees that don’t exist on the landscape anymore, so we like to consider this a little bit like the Noah's Ark for black ash,” said study senior author Jill Hamilton, associate professor in the Department of Ecosystem Science and Management and director of the Schatz Center for Tree Molecular Genetics in the Penn State College of Agricultural Sciences. “We need the collected and preserved germplasm for emerald ash borer-resistance breeding and future restoration.”

To conduct their wide-ranging study, the researchers — led by study first author Kyra LoPiccolo, a doctoral candidate in the intercollege graduate degree program in plant biology, administered by the Huck Institutes of the Life Sciences — analyzed collections of black ash seeds obtained from the U.S. Department of Agriculture’s National Plant Germplasm Network and Canada’s National Tree Seed Centre. The team evaluated variation in seed form and structure and early life-history traits of black ash using X-ray imaging to look inside seeds without damaging them, and measured seed characteristics.

More than 700 seed lots from different regions were X-rayed and those images were used to train a machine learning model — a type of artificial intelligence (AI) — to develop an algorithm that would automate measurements of their characteristics. AI then enabled the team to characterize approximately 35,000 seeds. The researchers also engaged in climate modeling to predict the success of seeds sourced from different climates.

Finally, the team, germinated 22,000 seeds at Penn State’s Mendel’s Way Farm and Greenhouses to gauge their early life growth and production, and then used those seedlings to establish three common garden experiments: at the Ag Progress Days site near Penn State’s University Park campus, as well as in Syracuse, New York, and Quebec City, Quebec, Canada. The team partnered with State University of New York (SUNY) College of Environmental Science and Forestry and the Canadian Forest Service on the common garden plantings.

A common garden experiment is a scientific method used to test if differences observed in traits among populations are due to genetics or the environment, said LoPiccolo. Researchers collect plants from different locations and grow them together in the same shared environment. If the traits stay different, the cause is genetic. If the differences go away, the cause is the environment.

Importantly, much of this trait variation occurred among trees originating from the same environment, rather than simply reflecting differences among environments. The researchers reported that 43% of the seed trait variation observed on average occurred within populations, and 14% was observed between populations. This means that individual black ash trees can differ substantially from one another even within the same population, providing genetic diversity that may help populations adapt to change.

To understand those numbers, explained LoPiccolo, imagine a forest containing many black ash trees.

“Even trees growing side by side often produced noticeably different seeds,” she said. “This is important because it means each population contains a lot of genetic diversity, which gives natural selection ‘raw material’ to work with if environmental conditions change.”

The team also found that climate mattered. Trees from different climates produced different seed traits, they reported. Trees from warmer and drier climates tended to produce heavier seeds, seedlings that developed more quickly and taller seedlings after one year. These traits may be adaptations to those particular environments, LoPiccolo pointed out.

The researchers found that early growth affected later success. They reported that seedlings that started with heavier seeds and reached developmental milestones sooner generally had higher first-year fitness. In biology, fitness means how well an organism survives and eventually reproduces. Since reproduction in trees takes up to 50 years, according to the U.S. Forest Service, the team used first-year survival and growth as early indicators of fitness.

Finally, the team discovered that black ash populations from more continental climates — places with larger seasonal temperature swings, with hotter summers and colder winters — tended to have better survival and greater height accumulated during the first year.

Because the emerald ash borer has killed so many ash trees over such a large area, Hamilton, suggested that a large, effective collaboration similar to this study will be needed to devise a plan to rescue black ash across its range.

“We need to work with different state, federal and university partners across borders — the trees don't know borders, we do,” she said. “So, figuring out ways to work with partners in this kind of broad scale is absolutely what is needed for our forest species because you can't go into the wild and see many of these black ash populations anymore. We have to figure out good ways to work across borders effectively and ensure that we have resources that can help us understand tree genetics, climate adaptation and pest resistance in the short and long term.”

Contributing to the research were: Jacob Mazza, undergraduate research assistant who graduated with a degree in forestry; Lucia Anderson, undergraduate research assistant who graduated with a degree in biology; and Dolzodmaa Davaasuren, who graduated with a doctoral degree in informatics from the College of Information Systems and Technology, now with Uber Technologies Inc., San Francisco.

LoPiccolo recently presented this research at the North American Forest Genetics Society meeting and received the People’s Choice award for best student presentation.

The research was supported by the U.S. Department of Agriculture’s National Institute of Food and Agriculture and McIntire Stennis Capacity Grants under award numbers PEN04815 and PEN05027, as well as the Schatz Center for Tree Molecular Genetics, the Huck Institutes of the Life Sciences and the graduate program in plant biology — all at Penn State. This content is solely the responsibility of the authors and does not necessarily represent the views of the funders.