Are our elms on the rebound?
On a recent damp May morning I walked around Dartmouth College in Hanover, N.H., with arborist Brian Beaty. While he is responsible for all of the trees in the center of the campus, our visit focused on a small number of trees that require an inordinate amount of his attention. These were the college’s mature American elms — tall, elegant and, most importantly, healthy. Beaty wants to keep them that way, which is why he checks on the elms almost daily from early spring to late summer, and has his crew look them over every time they drive by one. “We don’t have a lot big elms left,” he explained. Of the hundreds of mature elms that once adorned the college, only 20 remain.
Beaty and his crew hope not only to protect the few remaining mature elms, but to nurture a new generation of disease-resistant varieties to grow alongside them. Similar efforts are underway across the country, including in Vermont, where the state chapter of the Nature Conservancy expects to plant 7,000 American elms over the next three years.
The American elm (Ulmus americana) was once a common tree in American cities, suburbs and forests. That changed starting in 1930, when a shipment of logs from Europe arrived with the fungus Ophiostoma ulmi, which causes Dutch elm disease. The fungus (and a second, more recently discovered species, Ophiostoma novo-ulmi) is spread by a native elm bark beetle as well as by a smaller European species. It lives in the beetles’ guts, and is regurgitated as they feed. Soon after an elm is infected, it develops symptoms — yellowing leaves and dying limbs — and it often dies soon after.
It’s those telltale signs that Beaty looks for on his morning rounds, starting in early spring when the bark beetles begin to emerge. Infections usually appear in June, but can surface throughout the summer. At the first sign of infection, Beaty and his crew cut off limbs that appear to be infected. It’s crucial that the fungus not spread, because infection of the trunk is “usually a death sentence for the tree,” said Beaty.
But Dartmouth does more than monitoring and cutting to protect its last grand elms. Every three years, trees more than 15 inches in diameter are inoculated with the fungicide Arbotect. As we visited 100-year-old specimens along College Street, Beaty pointed out small holes along the root floor of each tree where it had been injected.
The most recent application was in 2013, and the college spent about $8,000 on the fungicide, plus the labor costs for Beaty’s crew to apply it. That’s a price tag most homeowners and municipalities can’t afford. Thanks to the development of disease-resistant cultivars, they may not have to.
As we walked along Tuck Drive, Beaty pointed out a row of young staked elms. They are cultivars, crosses of mature trees that have most likely been exposed to the disease many times, but appear to have some natural resistance — although not complete immunity — to infection. Among the better known varieties are Princeton, Valley Forge and Liberty.
The Vermont Nature Conservancy hopes to add to the list of disease-resistant cultivars. In 2011 ecologists from the organization collected the pollen of four large American elms from Connecticut River floodplain forests and used it to pollinate disease-resistant cultivars. “The offspring from these crosses have been planted at several wildlife management areas in the Northeast Kingdom for field testing,” explained Christian Marks, a Nature Conservancy ecologist.
Over the next three years, the organization plans to plant elms around the state, including in Canaan, Orleans and Pomfret. The newly developed crosses will be carefully monitored and compared to some of the more established cultivars, which will also be planted. If the crosses prove successful, they will be named and could eventually be made available to the public.
Rose Paul, director of Critical Lands and Conservation Science at the Nature Conservancy of Vermont, is leading the elm planting project. Like Beaty, her attachment to this endeavor runs deep. “How many times in one’s lifetime do you have the chance to say you helped to restore a species? I love this project!”
Carolyn Lorié lives with her rescue dog and very large cat in Thetford. The illustration for this column was drawn by Adelaide Tyrol. The Outside Story is assigned and edited by Northern Woodlandsmagazine and sponsored by the Wellborn Ecology Fund of New Hampshire Charitable Foundation: [email protected].
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