What can the emerald ash borer reveal about the long-ago loss of American chestnut?

Ash trees are vanishing from forests across the Northeast, Midwest, and parts of the South as they fall victim to the invasive emerald ash borer. (Photo by K. Stoll/National Park Service)

For more than 70 years, the American chestnut tree has, mostly, been absent from the landscape. But historical accounts of its grandeur and the tons of nutritious seeds it produced every fall have made it loom large in New England’s forest memory.

Now, as another tree disappears — the ash — scientists working in the White Mountains of New Hampshire hope they can document its demise to gain a new, deeper understanding of how species losses ripple through ecosystems.

When most American chestnuts were killed by chestnut blight in the first half of the 20th century, the loss of the species was felt both culturally and ecologically. The trees were among the largest in the forest, reaching heights of more than 100 feet. Scientists know the species took with it an autumnal chestnut crop that was nutritious for people and wildlife alike. They also suspect that the die-off disrupted tree-specific communities of insects and soil microbes. But regarding what those communities looked like, and to what extent their loss rippled through the forest ecosystem, they can only make educated guesses.

“More or less, the species has been deleted from the landscape,” said UNH professor Jeff Garnas, who studies forest ecosystems and health in the Northeast. “… That happened 100 years ago, and we really have very little ecological understanding of what the consequences of the loss of (American chestnut) are,” he said.

Now, Garnas and the other researchers working on the Ash Protection Experiment at Hubbard Brook Experimental Forest, in the White Mountains, hope they will be able to shed light on the effects of tree loss events by studying another as it unfolds.

Ash trees, members of the Fraxinus genus, are vanishing from forests across the Northeast, Midwest, and parts of the South as they fall victim to the invasive emerald ash borer. When the beetle infests an ash, the tree typically dies within three to five years. The emerald ash borer has killed hundreds of millions of ash trees across North America since it arrived, likely during the 1990s, according to Southern Forest Health. 

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The emerald ash borer was first detected in New Hampshire in 2013 and has since made it to every county in the state. The future looks grim for ash in North America, Garnas said. And at Hubbard Brook, where the ash borer was first detected in July 2021, scientists are beginning to see trees dying off as it takes hold.

“As unfortunate as it is, the emerald ash borer stands to delete another, in this case, genus of trees … from our forests,” he said. “And so we wanted to take the opportunity to ask questions about the fundamental feedback between the tree species and associated biodiversity and ecosystem function.”

That feedback includes many relationships among forest organisms, from the microbes and fungi in the soil to plants and insects living beneath the tree canopy — all of which are shaped by trees. The ash tree protection experiment will allow scientists to track the impacts of ash borer on those communities and relationships, Garnas said. And while the exact impacts of the American chestnut’s decline all those years ago will remain a mystery, Garnas hopes the findings of this experiment will, among other things, help scientists get a better idea of what they may have looked like.

The experimental setup includes dozens of forested research plots in Hubbard Brook Experimental Forest, each with seven to 15 “large, mature ash” trees, Garnas said. Ash trees in half of the plots were injected with a powerful insecticide at the outset of the study — which began in a flurry right after the invasive beetles were found at Hubbard Brook in 2021, he added.

The insecticide, emamectin benzoate, is derived from a compound produced by soil microbes. An injection can protect a tree for three to four years, Garnas said. However, he said, though the treatment has worked to protect trees from the emerald ash borer in the context of this experiment, it’s not a solution for ash loss more broadly. 

“At a forest scale, you know, it’s really not going to be feasible to fight this invasion with chemical pesticides. … It’s more of a spot treatment,” he said.

By preserving the trees involved in the study, the Ash Protection Experiment may provide a system to study ash for years to come, even when others of their kind have long since died, Garnas said. Researchers elsewhere use the pesticide to preserve some ash trees as seed stock. The conservationists who undertake this work, Garnas said, hope the emerald ash borer will eventually be vanquished, or at least managed.

“With the hope that some of the other management tools will come to fruition and be, you know, as effective as they can be, people are sort of anticipating being able to at least keep some ash in the forest so that it’s there for when it can thrive again,” Garnas said. “You know, whether that’s 10 years or 100 years off – It’s hard to say at the moment.”