Slow, heroic comeback for American elms
JOEL BANNER BAIRD, Free Press Staff Writer5:09 p.m. EDT May 1, 2015
Can a tree outrun a fungus?
Possibly, if the American elm is given a head start, experts say.
The iconic, vase-shaped giant has a lot of catching up to do.
Handicapped for at least six decades by a vulnerability to the fungal Dutch elm disease, the once-plentiful tree survives today mostly in the wild as a stunted, short-lived specimen.
Several groomed, disease tolerant varieties show promise. But even their most ardent champions want us to move slowly in the rescue of Ulmus americana.
Although spring planting season is short in Vermont, cautious optimism is in the air. Local campaigns to re-introduce elms to urban and rural landscapes have roared to life.
Municipal efforts are accompanied by heavy equipment, mulch trucks and caution cones.
Less visibly, the banks of Vermont’s river floodplains are bristling with slender, barely budding elm saplings.
Those scrappy elms are in it for the long haul, said Rose Paul, director of critical lands and conservation science for The Nature Conservancy in Vermont.
Fortified by ever more diverse reproductive encounters, subsequent generations of those elms just might have a fighting chance against Dutch elm disease, or DED.
The ongoing experiment has a practical side, Paul continued: American elms are uniquely suited to anchor and restore about 50 acres of fragile, erosion-prone floodplain managed by the conservancy.
This past week, a corps of student volunteers from the Stafford Technical Center’s Forestry Program dug in along the Hubbardton River.
Through mid-May, a total of about 7,000 disease-tolerant elms will be planted along the banks of the White, Barton, and Black rivers, Paul said.
Paul outlined the American elms’ credentials:
- Untroubled by prolonged soaking, Ulmus americanaquickly develop deep roots that anchor a river bank. The plant’s shallower roots are sturdy, too — and literally give pause to a swift current.
- Eddies created by the roots allow more water-borne sediment to settle.
- Detained by elm roots, nutrient-rich sediment boosts the growth of elms and other shoreline plants — and contributes to clearer water downstream.
- The fast-growing tree’s uptake of nutrients from soggy soil protects downstream aquatic habitat from over-fertilization from nitrogen and phosphorus.
- As elms grow, they armor themselves with thick, strong bark that deflects the periodic battering by flotsam and ice floes.
- Finally, as trees mature, they develop cavities in their loose, upper bark — a variety of sheltered niches for other creatures to set up house.
“In their heyday, elms were our tallest, oldest trees on the floodplain,” Paul said. “We’re missing that habitat component now. Structurally, elms are like apartment complexes for wildlife.”
The Dutch elm disease fungus, likely an Asian exotic that spread courtesy a bark-eating beetle, encountered little native resistance as it swept across the U.S. in the middle of the 20th century.
Some isolated elms were spared. Others — a very few — tolerated the fungus, matured, and reproduced.
Their progeny, however, were not universally blessed with genetic safeguards against infection. Most of them died.
The mechanisms of tolerance to Dutch elm disease remain a tantalizing mystery to biologists, and are almost certainly linked to a combination of factors, rather than a single silver bullet, said Paul.
A rule of thumb applies to a species’ successful adaptation to changing conditions: the greater diversity, the merrier.
“It’s like in the casinos: We’re talking about working with the odds,” Paul said.
Weeding out weaklings
The Nature Conservancy is improving those odds through work with U.S. Forest Service biologist James Slavicek to unlock some of the secrets of the tree’s genetic roulette wheel.
At his Delaware, Ohio, research station, Slavicek curates pollen from good-sized, disease-tolerant elms, sampled from around the species’ Eastern range.
The result: a bank of seeds endowed with far-flung pedigrees.
Saplings from those seeds are dosed with the fungal infection. Weaker and disease-susceptible varieties literally weed themselves out of the running.
Slavicek propagates cuttings from the survivors and sends the shoots back out into the world, where they face further trials.
Many of them the hybrids find a home in Vermont.
Nature Conservancy lead biologist Christian Marks transplants them at a nursery at Green Mountain College in Poultney, and oversees further rounds of research .
Among the saplings he culls are those that fare poorly in cold weather.
Those that make the final cut will be planted in test plots he terms “sentinel sites” that do double duty with habitat restoration.
Marks, together with Rose Paul, will spend the next two weeks populating the sentinel sites with elms primed with hybrid vigor. Over the course of the next three years, they plan to plant about 7,000 of them.
“Once those elms are in the ground, they’ll do the work on their own,” Paul said. “They’ll be setting seed for years, and those seeds will be washing through the watersheds. That’s the vision.”
Another branch of America’s love affair with its elms is taking place in towns and cities.
For the past week or so, crews in Burlington have been transplanting disease-tolerant elms in the central downtown.
Previous spring’s plantings are flourishing.
Burlington City Arborist Warren Spinner took an hour out of his Tuesday morning to place that work in perspective.
He began by dipping into an archive of old, black-and-white photographs.
Gracious, tree-lined streets dominated — a virtual monoculture of elms that persisted through the first half of the 20th century.
An undated postcard shows a canopy of American elm trees on South Winooski Avenue in Burlington. (Photo: Courtesy Special Collections at University of Vermont)
Dutch elm disease struck in the late 1940s, and the photographs document more and more aggressive tree pruning. Finally, they capture the elms’ wholesale removal.
The species’ disappearance was profoundly disconcerting to city residents, Spinner said.
He summarized the elm’s appeal: The high-reaching trucks preserved pedestrians’ views of city vistas; the spreading canopies created acres of shade.
The concentration of elms in Burlington — and in many other American towns — dramatized a fatal over-indulgence of a single species, Spinner said.
The aftermath of the die-off begat a policy of biodiversity, deeply engrained in the Queen City’s tree plan.
All of the eggs, so to speak, are no longer in a single basket.
Trees in the trenches
Among the city’s approximately 11,500 street, park and cemetery trees, about 400 — 3.5 percent — are elms, Spinner said. They don’t make the top 20; they don’t show up on his office’s pie-chart of species.
If an elms-specific disease or insect threat were to arrive, the city’s canopy wouldn’t dissolve, as it did in the 1950s – 1970s, Spinner said.
Row after row of elm stumps greeted the young arborist when he signed on with Parks and Recreation in 1979.
Several years later, municipal and volunteer efforts to reforest the city gained momentum under then-Mayor Bernie Sanders.
Elms remain a favorite among urban foresters. They fare remarkably well amid high levels of air pollution, beneath compacted soil and awash with road salt.
They’re resistant to drought as well as flooding.
“They’ll grow anywhere; they’re like weeds,” Spinner said. “They’ll grow out of a crack in the concrete or in a swamp.”
The Burlington roster is relatively diverse: Princeton American elms share the Queen City with seven other varieties, including the Valley Forge, New Harmony, Triumph and the Delaware.
In 2013, the city put a Princeton elm to work on stormwater duty, its roots extending into a giant filter-like box beneath a sidewalk on Cherry Street.
Looks and shade aside, it earns its keep through flood control and the filtration of street runoff.
Another Princeton is planned for a stretch of sidewalk at the flood-prone junction of South Winooski Avenue and Main Street.
So far this spring, a dozen or so elms — in more conventional streetside plantings — have been added to the heart of downtown.
Vandals, voles and mutants
The Princeton elm and other commonly marketed Dutch elm disease-tolerant elms are propagated through cuttings from a parent tree — a much faster track to maturity than sorting through and nurturing seed-derived individuals.
Another advantage of cuttings: If harvested from a disease-tolerant tree, they retain the precise DNA code of the host plant: the very definition of a clone.
Slavicek, in a phone call, deemed the predictably serviceable elms like the Princeton to be boons, but mostly in the short run.
Ultimately, that commitment to uniformity won’t hold up, he added: “The DED fungus is mutating all the time — just as all of life mutates.
“The good news is that people are planting elms,” he continued. “The Princeton, for instance, is a good tree. But sooner or later, the fungus will mutate to a form that can get around its DED-tolerant characteristics.”
Prudent, scattered urban plantings should minimize the damage, Spinner said.
Meanwhile, the Burlington arborist contends with other hazards to downtown elms. High on the list is vandalism.
But even the relative wilds of Vermont’s flood plains offer no lifetime guarantee to young elms.
A conservation planting project three years ago proved no match for a hungry population of meadow voles, conceded Rose Paul.
Re-planting efforts will include vole-proof plastic stem-jackets, she added.
Is it all worthwhile?
Most certainly, Paul said: “There’s something almost mystical about a really large elm.”
Contact Joel Banner Baird at 802-660-1843 or [email protected] Follow him on Twitter at www.twitter.com/vtgoingup.