Unlike many native tree species that face direct threats from land development, the endangered butternut tree in Ontario is battling an invisible and unrelenting enemy: butternut canker. First recorded in 1967, the invasive fungal disease has sparked a rapid decline in butternut populations across the province, stretching from the Ottawa region to the Maritimes
Sean Fox, a senior research associate at the University of Guelph’s Arboretum, noted that the invasive fungus causes a slow death for the tree, where cankers form on the trunk, branches and exposed roots.
“Butternut is unique in that it has a broader range in Ontario and stretches into Quebec, the Maritimes, and into New Brunswick, I should say, at least, so more abundant historically, but it’s been in great decline in the past 50 years or so because of a disease called butternut canker.
“It causes little kind of black oozing lesions on the branches. You could go up to the forest and find a butternut [where] you might find a few of these lesions up in the canopy, if you look hard enough. But after many years, and sometimes even decades, more and more of those lesions form to the point where the tree can’t transport water through the system anymore, and then eventually the tree will decline and die,” he explained.
While the canker is a primary concern, native butternuts face another complex challenge: hybridization. Because butternuts rely on wind-pollination, native trees are easily pollinated by non-native species like Japanese walnuts, which are often planted in nearby yards or parks.
Since butternuts rely on wind pollination, pollen can travel significant distances; native butternuts are easily pollinated by non-native species. Butternuts share much of the same genetic material as other walnut tree species, meaning they can reproduce without major genetic barriers.
Hybrid Japanese walnut and butternut trees are immune to the cankers. These hybrids often outcompete native butternut trees, making it harder for native butternuts to survive.
In the face of the challenges the butternut tree faces, the Forest Gene Conservation Association (FGCA) has been leading a conservation project in Ontario to help save the species.
“We’ve been working on recovery for over 30 years of this species,” said Kerry McLaven, chief executive director of the FGCA.
Their recovery program focuses on butternut trees that show some resistance to the canker disease.
“What we have found over the years, and thanks to research from the U.S. and elsewhere, is that there are sometimes trees that are left, even though others around are dying, and so what we’re trying to do is identify where those trees exist,” said McLaven.
If the FGCA is able to identify a pure butternut tree showing resistance to the canker, they document the special trait it is showing and archive the tree in an orchard.
“What we do is we take little branches off the tops of mature trees that are showing these characteristics. We graft them, which means we kind of put them on top of black walnut rootstock or little seedlings, and then we plant them in an orchard. So essentially, the top part of those trees are a clone of that original parent tree,” said McLaven.
According to McLaven, survival hinges on how well the tree heals from the canker and whether it retains a healthy crown and the ability to produce seed. The goal is to ensure the species can continue to reproduce and share its genetics with future generations.
So far, there are just over 200 trees in the butternut recovery program’s archive. The program is hoping to help the butternut trees survive and expand their population in Ontario.
“All we’re doing as humans is trying to reconnect trees that wouldn’t otherwise be able to be doing this together. So, we’re just sort of facilitating that size of population that we would rather see in a natural setting.”
The FGCA is currently asking the public to help locate pure butternut trees. Anyone who spots a healthy, native butternut is encouraged to reach out to the organization to see if the tree is a candidate for the recovery program.
For more information, visit www.fgca.net.
