Beech Leaf Disease Threatens NYC Trees: Here’s the Good News

Photo: Jeff Chien-Hsing Liao for New York Magazine

There is a worm growing in Brooklyn, millions of them, actually, and, for a microscopically thin nematode not quite as tall as this letter I, it moves surprisingly fast. The nematode, known to ecologists as Litylenchus crenatae mccannii and first discovered in 2012, causes beech leaf disease. It nests in the beech’s buds, replicates wildly, and causes havoc for its leaves, eventually killing the tree. If you’ve hiked in northern New Jersey forests recently, you might have seen beech groves lose their entire canopy in a year. If you live in Connecticut, you can look up in a shady beech canopy and see the telltale dark bands on the leaves. If you live in New York, you might not have noticed it at all yet, but ecologists and foresters have. “It’s present,” said Malcolm Gore, the arborist for the Prospect Park Alliance, though the infected trees are still alive, for now.

A healthy American beech, with its wide-spreading branches and rounded crown, is a movie-star tree, stately and leafy, six or so stories tall, the leaves edged like a saw, the bark smooth like the legs of elephants — a perennial target for lovers’ initials. Beech trees often come in clusters, as the matriarch sprouts dense groves, producing nuts that are a nutritious staple of birds and critters galore. Once, they covered North America, but after the glaciers, they were confined to the East. Beech trees are everywhere in New York; half the state’s woods are made up of a combination of maple, birch, and beech, and thick stands of beech are to be thanked for making shade and preventing stream-bank erosion — beech are the region’s Brita filters.

Almost 200 beech trees live within Prospect Park, one of more than 175 species within its 585 acres. “The hardest thing to do is to choose a tree,” Gore said. “How do we decide that this tree is more important than that one?” Over the past few weeks, he and his team have been spraying the base of the park’s beeches with PolyPhosphite 30, a fertilizer, that, if things go as planned, will activate the trees’ natural defenses, make their cell walls stronger, and prepare them for attack, however the attack will come: Theories for how the nematodes travel include in the mouths of birds, via birds’ guts, or spread directly by the wind.

The forests in Prospect Park have taken their share of seemingly random natural hits: emerald ash borers and ash yellows, a bacterium that lacks cell walls and attacks a tree’s vascular system; oak wilt, a fungus that causes the tree’s bark to swell and rupture; and, of course, chestnut blight, the first plague the city really paid attention to, which arrived more than 100 years ago. There have also been climate-driven traumas. Last year, a fire in Prospect Park burned as the Fire Department worked to control it. The hoses from the trucks barely stretched into the area known as the Ravine. The next day, handwritten notes from locals began appearing along a fence. “May your ash bring light to new trees soon … Thank you for all the life you bring Brooklyn … Thanks for reconnecting me to the earth … You mean so much to us — I hope you heal and feel better!”

When green returned to the park this spring, the burned woods looked pretty good. “We probably lost one large oak tree,” Gore said. “We’re having some good regeneration in there. We’re also having some invasives, though it’s looking pretty positive. We won’t have a canopy gap there, as was feared.”

Beech leaf disease will be devastating — of that there’s no doubt. But Gore is already looking beyond it. New York employs some of the finest urban foresters in the country, masters in a field that barely existed 20 years ago. Far more than their counterparts in the rural areas of the country, they are trained to move fast and tackle multiple crises at once, all while remaining invisible to the city that surrounds them. Ask an urban forester about the nematode and the answer may be oddly upbeat: If beech trees die, the forest will change. But that doesn’t mean it can’t remain healthy.

A 150-year-old American beech in the New York Botanical Garden’s Thain Family Forest.
Photo: Jeff Chien-Hsing Liao for New York Magazine

How does New York triage the care of 10,000 acres of woodlands? With something called the forest matrix: In 2014, the Parks Department divided its natural areas into 1,156 plots and graphed the conditions in each one, with forest health on the vertical axis, ecological threat on the horizontal. Alley Pond Park, in northern Queens, has areas in all four quadrants within its 600 acres: high health, low threat; high health, high threat; low health, low threat; low health, high threat. The city’s foresters prioritize high-health, high-threat forests. Forests that are healthy are cheaper to care for.

On a rainy morning last spring Kristy King, the city’s chief of natural resources, took me on a tour of Alley Pond Park with Clara Pregitzer, who works for Natural Areas Conservancy, a nonprofit that works to protect urban forests and wetlands. King, leading the way, was reading Parks Department survey maps on her phone. “Let’s head in here,” she said, crossing from open field into the woods on a dirt trail. King and Pregitzer met about 15 years ago, when they both worked at the Parks Department. Pregitzer, the daughter of a forester, had just arrived from Tennessee, where she earned her graduate degree in ecology and evolutionary biology. “My job was to go and map vegetation in Van Cortlandt Park,” she said. “I saw an owl that summer and was completely amazed by how nice the forest was.”

After growing up in South Carolina, King worked as a marine biologist. “White lab coat and poop from cetaceans,” she recalled. “But I felt like my work wasn’t helping people.” On a visit to New York, she saw a horticulturist working in Central Park. “And I said, ‘You can do that?’” A short time later, she graduated from Columbia with a degree in conservation biology. “It felt like everybody else was doing stuff with the Wildlife Conservation Society, studying, say, wild dogs in Africa, and I was studying insects in Manhattan community gardens.”

The Parks Department began to take care of the woods in Alley Pond Park about a decade after the city’s landscape was transformed by bankruptcy in the 1970s, when just the barest minimums of parklands were maintained — ball fields and playgrounds mostly. As the 1980s ended, the Parks Department was assembling a team of ecologists who would reinvent how the city treated its trees: the Natural Resources Group, headed by Marc Matsil, a Brooklyn-born scientist who had returned from working as a park ranger in Alaska. The city, as a bureaucratic operation, had lost track of its forests, and NRG staffers spread out to map them, discovering places that, owing to white flight and deindustrialization, had become nearly wild.

“NRG started with discovery and mapping to protect. Then came cleanup,” said King as we walked through the rain. “Like literal cleanup — getting the old cars out of the woods.” Today, there are 12 Central Parks’ worth of forest in the city, and we’re not talking about street trees here; we’re talking about acorns falling, saplings sprouting, maple trees sending out whirlybirds, birds shitting seeds.

One reason urban forests don’t get the respect they deserve is that people make assumptions: that they are less than, not so natural, degraded owing to their proximity to highways, factories, and those creatures that make things less natural just by being around — i.e., people, apart from, say, me and maybe you. And yet the forests in New York City are among the most beautiful in the state, with trees as old as any in the Adirondacks and just as tall. The majority of the trees here are native — the canopy of Staten Island’s forests is 97 percent native. If you were to blindfold naturalists from New England and take them into some of the forests in Queens, they might think they were back home, as opposed to mere yards from Grand Central Parkway, though the planes landing at La Guardia might tip them off.

In 2007, the city launched PlanNYC, the sustainability program that included the Million Trees Project, which was less a matter of ecological science than a spur-of-the-moment brainchild of a Republican mayor (Mike Bloomberg) and a philanthropic nature-oriented celebrity (Bette Midler). King eventually managed half of the plantings, working with area nurseries on sapling and seed provenance, as NRG ecologists tried to keep pace.

Planting a million trees meant finding places to put them. “The question was, ‘Do we have enough room for a million trees?’” recalled Morgan Grove, a former U.S. Forest Service researcher based in Baltimore who worked on the project, “and the answer was, ‘Yeah, you’ve got a lot more room than you think for a million trees, but it’s not going to happen within city streets.’” Alley Pond Park and a dozen other natural areas eventually became the sites where most of the new trees were planted.

Eventually, King and Pregitzer brought us to our first stop, a forest of mixed quality: oak, hickory and a smattering of beech, which so far haven’t shown any signs of infection, though the nematode is likely already here. “So right now,” said Pregitzer, “we’re in an okay place in the forest matrix.” We looked up to see a healthy canopy of native trees overhead, but the understory was greening early with swaths of bright-green vine. “Multiflora rose,” King announced, an invasive plant that found its way from Asia into the northeastern U.S. thanks to American horticulturalists. In the very early spring, it photosynthesizes before most of the neighboring plants, an adaptive advantage. As new multiflora and other invasives such as bush honeysuckle spread, a native oak sapling might struggle to rise above them.

“The trees are native, but what you have in the understory is fragmented and degraded,” Pregitzer said. In a different ecological era, fragmentation could make space for new saplings, but with pests like the nematode around, fragmented canopy can act like a wound that gets infected, shutting down a forest’s ability to heal itself.

“See, in big rural forests, canopy gaps are good,” said Pregitzer. “That’s how you get your next generation of trees, but here we have so many other plants waiting in the wings. They love the sun, and they’re like, ‘Now is my chance!’”

City foresters will pull back some multiflora rose here to make room for saplings — a planned intervention to encourage natural processes, though Pregitzer has seen her share of unplanned interventions. In Marine Park recently, some teenagers burned a quarter-acre of phragmites and mugwort. “I came back the next year, and all the native wildflowers were there.”

After another few minutes, King pointed out century-old tree trunks covered with lichen, a native witch-hazel bush blooming, its yellow flower a lone mark of color in a still-brown grove.

“Okay, so this is healthy,” Pregitzer announced.

“I was just going to say,” said King.

“Look at the difference. You still have stuff in the understory, but it is not as dense,” Pregitzer said.

“This is a monster tree,” said King.

“A red oak,” said Pregitzer.

“And I love the ilex over there,” King said. “And that’s more witch hazel.”

“Tulip trees, red oak, cherry, witch hazel underneath nice leaf litter,” Pregitzer said, nodding. “This is a very healthy forest, in other words. It has survived a lot of things, and if it is helped to stay healthy, it may continue to do so.”

Beech leaf disease creates telltale dark bands on the tree’s leaves.
Photo: Craig Brodersen/Yale University School of the Environment

In places where the nematode has made greater inroads, more aggressive action is underway. The 147-acre Marshlands Conservancy is a lovely if degraded forest near I-95, just over the Bronx border into Rye. It’s also an experimental forest, a sort of jury-rigged one, thanks to Justin Bowers, a forest ecologist who is finishing up his doctoral work at City University of New York. It is one of ten similar sites stretching across the metropolitan area where forest managers can learn how to deal with beech leaf disease — or, more generally, how to help a forest sustain itself when it loses a suite of trees. “The beech were already showing signs of decline,” said Bowers, “but they were live beech.” Now almost all of them are missing their crowns, a beech apocalypse.

“It’s devastating,” said Rich Hallett, as he walked into the preserve with Bowers, “and it happens so fast.”

Hallett is a research ecologist with the U.S. Forest Service. If Bowers, who grew up in the Boston area, could pass as a grad student on a hike, then Hallett’s leather drover’s hat hints at his experience as a lumberjack. He started in the service in Alaska, studied forest regeneration in Michigan, and eventually settled in New Hampshire after grad school. “I got into forestry to get away from people,” he said. Now he is working in the biggest city in the country.

Around 2000, when the emerald ash borer first began killing trees at an alarming rate, the Forest Service realized it needed eyes in urban areas. Hallett’s boss in New Hampshire knew he’d been dating a woman at Columbia. “‘I know you’re going down to see Mary all the time,’” Hallett recalls his boss saying. “‘You’ll be going there anyway …’”

In the years since, the rural forester became fascinated by city trees and the New Yorkers who love them. (He married Mary too.) There have been surprises. He suspects that rats ate the bark of oak-hickory saplings he planted in Kissena Park in Queens during a particularly cold winter in 2014, girdling the trunks like hungry rural deer. But he speaks of urban forests as astoundingly tenacious. As we walked into Marshland’s woods, we saw black cherry, tulip trees, a young sweet-gum grove, older groves of oak and hickory, then mostly second-growth forest of red, white, and chestnut oak as well as the dead and dying beech. They are to be replaced with white oak.

We entered a fenced-off area of the preserve to inspect dozens of oak saplings that Bowers planted with a group of volunteers. Each has a tag that lists its origin: acorns collected in Bergen County, New Jersey, or in the Bronx or Westchester, then sprouted at a Forest Service facility and transported here. As we stooped to inspect them, Bowers’s eyes moved between studying the saplings and anxiously minding the perimeter. “I’m watching for deer,” he said. Deer consider young trees delicious.

In one set of plots, Bowers has planted saplings in existing forest gaps; in the others, saplings have been planted in gaps that are imminent as beech trees die. “Oak really need light to take off,” Hallett explained, “and you wouldn’t normally plant oak in a spot like this.” This area is shaded for the moment. “But,” he continues, “there’s no competition, and we know those beech are going to go away. And then light is going to come flooding in here, and now these baby trees are going to be ready to go.”

They’re anticipating decline. “It won’t be that long for these trees to go, the next several years,” Bowers said. “And you’ve got plots you didn’t do anything to and there’s already a monoculture of stilt grass.” He points to a line of the grass, which races across the woods like a beautiful bright-green fire.

In the East, oaks contribute more to the web of food in which insects and birds live than any other tree. They are superstars, and they are everywhere. The saplings Bowers has planted will help answer an important question: Will oaks from warmer latitudes fare better than local ones? The oaks themselves might have migrated to these cities if the planet weren’t warming faster than they can move. Bowers is simply matching their spread to the speed of the changing climate.

We see saplings from the Bronx and New Jersey, each marked with a small red flag. They are doing well so far, a 98 percent survival rate. “We’re very happy,” Hallett said. The goal is a native tree canopy that is resilient to future disturbance. “And if in the future,” Hallett continued, “before they start hatching acorns, we find out that, Oh, this is a bad idea to bring southern progeny north, we can go and cut every one of the ones that are problematic.”

In the past, foresters worked only on long timescales. A rural forester marked a stand of woods for trimming; the results arrived decades later. Cities are quicker. “You know when I got to New York City,” Hallett said, “I heard that term ‘a New York minute.’ Well, the Parks Department is totally on that scale, and it was antithetical to the whole idea of what forestry is from my standpoint. And yet they’ve got to get stuff done, right now, and they’re not messing around. If you can harness that energy, you can learn a lot really quickly about managing forests. We can teach our rural partners what we’re learning here.”

Despite the heartening work I saw at Alley Pond Park and in Rye, neglect is still the default, and a host of plant and animal species are poised to take advantage of our carelessness in ways we are just beginning to understand. Take the work of Gisselle Mejía, who, as a graduate student at CUNY, had studied the Million Trees initiative from the vantage point of soils — where tree plantings worked, where they failed, and the ways soil composition contributed to that outcome. Recently, though, she’s been studying the life cycle of Rhamnus cathartica, better known as common buckthorn.

Buckthorn, a large shrub or small tree with dull-green oval leaves, is said to have emerged in its current form in Europe. It was brought to the U.S. in the early 1800s as an ornamental, and on maps you can see it spread as American settlers do, from the Northeast, along the Erie Canal and into what was called the western frontier and then Ohio. Its seeds are so nonnutritious that birds excrete them almost immediately.

Buckthorn has only recently been spotted in New York City forests in just four of the 1,156 forest plots charted by King, Pregitzer, and their colleagues, but it moves fast. It comes in quietly and minds its own business in the understory until something opens a hole in the canopy. “With buckthorn, there are little saplings all through the woods, and they’re basically just waiting for something like that to happen,” Mejía said. The arrival of beech leaf disease is exactly the opportunity buckthorn is looking for: Where mature trees fall, the shrub can take over in a matter of months.

The spread of buckthorn is the latest proof, if we needed any, that there is no such thing as a healthy unmanaged forest — not anymore. Either we intervene or risk ecosystem collapse. “What we’re thinking is the forest of today is not going to be the forest of tomorrow,” Mejía said.

That message has been clear at least since the 1980s, when hemlocks throughout the East Coast were growing sick and dying. Eventually scientists determined that a sap-sucking insect called the woolly adelgid was causing the trees to lose their needles. Forty years ago, the hemlock grove at the New York Botanical Gardens in the Bronx had several thousand healthy trees. Now it has fewer than 100.

Not long ago, Todd Forrest, who began working at the garden just as the infestation started, showed me the trees that had grown in to take the place of the lost trees. A similar sort of adaptation would inevitably follow the death of the beeches, whether steered by humans or not.

After Forrest became manager of the hemlock grove in 1999, he began by methodically surveying what was there and recording how things were changing in the face of ash borers and adelgid and chestnut blight. He knew that attempting to restore the area to a past life was hopeless. “Why the hell would I want to manage this to hemlock?” he said. “I could. But then I’m committing to a spraying and planting program from now till the end of time. So I had a thought. We should manage for” — he paused for emphasis — “the natural regeneration of native species, whatever they are.”

Forrest noted that seven oak species are native to the garden’s grounds. He thought about just letting the woods be, seeing what happened next. “Maybe that is the best thing to do in the long run,” he said. “Maybe all these artificial distinctions of native and nonnative are just sentimental and nostalgic, not science. But what I know is that the vast majority of the world is in the middle of that experiment of leaving things alone. So let me try my own experiment.”

We arrived at a trail that had been closed off since the mid-1980s, where Forrest proceeded to tell me what amounted to a parable, though the moral was, like our future in an ecologically distraught world, murky. The area, he said, had long been dominated by Amur cork trees, a tree native to East Asia that is deemed invasive on our acutely western vegetative timeline. “So in 2008, we removed mature cork trees,” Forrest said. “Then, we planted native trees, a selection. In 2009, the next year, a carpet of cork-tree saplings popped up. It was incredible, just a carpet of cork trees. We came back, removed seedlings of cork. A few years later, a handful of species we planted survived, but most of the trees growing there were not the trees we planted. They were mostly tulip trees. They came up — and they are here now — because we freed that growing space the next year.”

What’s his point? “We didn’t choose our suite of native plants,” he said. “The forest — or you could say the system — came in and did its thing. So now you have this mix! Yes, you have a handful of cork trees. But they’re not dominant. If you can break that cycle of invasion by making the best choice that the system allows you to make, then you are at least giving the system a chance to take off.”

Forrest walked me back toward the garden’s entrance. One of the things that is radical about his plan is that it is not that complicated, though an important difference between his work and the work of the ecologists at the Parks Department is funding and size. The NYBG’s budget for the former hemlock grove was underwritten by John Thain, onetime president of Goldman Sachs and CEO of Merrill Lynch, together with his wife, Carmen: Since 2011, it’s been known as the Thain Family Forest. On a per-acre basis, the NYBG had more than 15 times the money the Parks Department does — and that was before Eric Adams cut the department’s budget by almost $55 million. The city was meant to have a new 26-person trail-care team, but those jobs were eliminated. They were DOGE’d before DOGE — and then DOGE itself canceled Biden-era programs for urban forests.

Politicians tend to think of caring for trees as something volunteers do once a year on Earth Day, and money tends to flow to new construction rather than long-term care. “Deferred maintenance really plagues all aspects of our cities,” said Sarah Charlop-Powers, the executive director of the Natural Areas Conservancy. “The problem is that unlike a water main that can be replaced if it breaks, if we were to let the forests go away, we would not be able to replace them.”

If there’s a benefit to the beech leaf nematode’s arrival in the city, it may be that a tree-killing parasite feels like an emergency, something more like a water-main break than a slow, ignorable drip. Maybe the microscopic worm will move the tree-loving public to, at last, pay attention to the wild urban spaces it takes for granted, if people even remember they’re there. High health, low threat: All of the city’s forests could be brought there, if only we knew to hurry up and try.