Ash, beech and hemlock trees are being threatened by causes known and unknown, changing the forests of the Highlands in drastic, and dangerous, ways
Thirteen years ago, when I moved to Beacon, I asked a lifelong resident, a neighbor who knew more about the city than pretty much anyone, what his favorite local hike was.
“That’d be the one up Hemlock Gorge,” he said, pointing to the cleave in the distance separating Mount Beacon from Fishkill Ridge.
Maps refer to it as the Dry Brook Trail, even though the brook running down the gorge is rarely dry. Colloquially it’s referred to as the Pocket Road Trail, after the road that ends at the trailhead. I have never again heard that geologic feature referred to as Hemlock Gorge, and I probably never will, and not only because my neighbor died this year.
Hike to the top of the trail and you’ll find an area known as The Pines because of the once towering hemlock trees that provided a tranquil oasis.
What happened to them was devastating — and practically invisible.
“This insect the size of the head of a pin comes in and starts feeding,” explains William Schuster, the executive director of the Black Rock Forest Consortium in Cornwall. “They’re tiny, but when you have 10,000 of them on one tree…”
The damage is apparent at the top of the gorge, where the hemlocks look as if they were struck by lightning or ravaged by fire, with barely a pine needle in sight. The hemlock woolly adelgid, which originated in southern Japan, showed up in the Hudson Valley in the 1980s and has been surging of late in part because of increasing temperatures caused by climate change.
The aphid-like insects feed on hemlock twigs from autumn through spring, growing fuzzy white coats to protect themselves from the cold and to hold their eggs. As the swarms feed, the flow of nutrients within the twigs is compromised, leading to needle die-off, and then, the deaths of the trees.
Not all the ash trees will die — only 99.5 percent of them.
Hemlocks are not the only trees in the Highlands undergoing rapid change. Earlier this year, a friend who had an ash tree cut down in his yard was told by the arborist that, in the next few years, every ash tree in the Hudson Valley would be dead because of the emerald ash borer, another invasive insect from Asia. As with the hemlock woolly adelgid, no native predators feed on them, and the trees have not had nearly enough time and generations to develop resistance.
Alarmed, I asked two friends who both have backgrounds in forestry if this could be true. Are all the ash trees about to die?
They said it wasn’t true: Not all the ash trees will die — only 99.5 percent of them. About 0.5 percent will survive because they have a natural resistance to the ash borer — as long as someone doesn’t cut them down because they heard all the ash trees were dying.
Sometime later I asked one of those friends if she was familiar with the disease ravaging beech trees.
Which one, she asked?
There’s the old one (Birch Bark Disease, which has been around for decades), the new one (Beech Leaf Disease, which appeared in Ohio eight years ago and is encroaching on the Highlands,) and the new new one (an unnamed leaf necrosis that showed up in the Highlands within the last few months).
A forest is always in flux, even if the changes often creep forward too slowly to be noticed by even frequent visitors. Sometimes, however, those glacial changes can speed up to seem like landslides. Time in the forest is speeding up.
Ashes, ashes, all fall down
I have been walking with Jonathan Rosenthal and Radka Wildova for maybe 15 seconds when they spot an ash tree. I ask them how they do that so quickly.
“We dream about them,” Rosenthal says.
Rosenthal and Wildova are scientists at the Ecological Research Institute in New Paltz who run a program called Monitoring and Managing Ash that enlists the public to combat emerald ash borer and protect the few trees that can resist them.
It’s mid-June and we’ve come to High Falls to hike the D&H Canal and check in on what might be one of those trees. We’ve barely left the parking lot of the D&H museum when Rosenthal and Wildova spot a mature, healthy ash that both say they were not aware of. They are delighted, entranced and a bit confused as to how it’s in such good shape.
Since the tree is in someone’s yard, Rosenthal suggests it might have been injected with a systemic insecticide that has been shown to protect individual trees from emerald ash borer. Because the treatment is expensive and must be done every few years, it’s not suitable for widespread application. But for ash trees with particular historical or sentimental value, or that might (like this one) crush a house if toppled, it remains an option.
“I’d love to go take a closer look,” says Rosenthal. “But, you know…”
Meaning we would need to trespass, and whoever owns the tree may not take kindly to three strangers poking at it. We take to the woods instead, bypassing several ash trees that have recently been cut down.
In order to locate trees with ash borer resistance, Monitoring and Managing Ash asks people to keep an eye out for “lingering ash.” The theory is that if you notice a clump of dead or dying ash trees in which one tree appears not to be dying, it’s reasonable to assume the tree might have at least some natural resistance.
No leaves, no photosynthesis, no life.
To determine if an ash is fully resistant, the tree needs to be monitored for a few years. Last year, someone told Rosenthal and Wildova about such an ash tree at the D&H Canal; when they visited last summer it appeared to be healthy despite several dead ash trees within sight.
But it was not resistant. As we approach, Rosenthal and Wildova point out the several signs of its impending death by borer. There’s the die-back in the leaf canopy: many of the branches are barren. There’s the epicormic shoots emerging from the lower parts of the tree, often a desperate attempt to increase leaf production to compensate for the lost canopy and chemical energy. (No leaves, no photosynthesis, no life.)
However, both conditions could be caused by other stresses besides the borer, so once we reach the tree, they look for signs of infestation.
It doesn’t take long. Underneath a piece of bark, the insect’s looping, serpentine “galleries” can be seen: Trails caused by larvae as they tunnel. Several D-shaped holes are found around the trunk, which is the particular shape that the borer makes when it emerges from its larval stage and exits the tree as a beetle.
Emerald ash borers start at the top of the tree, and subsequent generations work their way down. If you’re seeing the D holes at eye level, Rosenthal says, “you can kiss your ash goodbye.”
The forests of New York are under attack from more than 150 invasive pests, more than any other state, according to Rosenthal. The primary reason for this is New York City’s role as a global shipping hub, with invasive bugs traveling inside shipping crates, pallets and packing material.
Once they escape, there’s just enough forests within New York City itself —Central Park, the Bronx — for them to establish themselves before spreading into the thickly wooded suburbs.
The gilded wealth of those suburbs also plays a role, in the form of exotic and expensive imported trees. “You want to be the first on the block with that new tree,” says Rosenthal.
That was the case with the first American tree epidemic that can be traced back to human interference: The chestnut blight that destroyed almost every mature chestnut tree in the early 20th century. Arriving on imported Japanese trees, the fungus was discovered on chestnut trees at the Bronx Zoo in 1904. By the 1940s, it was everywhere.
Chestnut trees live on mostly as scrubby underbrush.
The massive, stately chestnut trees of young America are now few and far between. Today the species lives on mostly as scrubby underbrush, still shooting up from chestnut roots, but felled by the blight before they ever mature.
This may be the future of the ash. Since the emerald ash borer needs a thick trunk to tunnel through, younger and reedy ash trees are safe until they mature. That may be largely how ash trees in the Hudson Valley survive: As thickets and understory, ecological ghosts haunting the forests where they once were found in soaring abundance.
That is, unless enough naturally resistant trees are found and protected. “Every living, standing, healthy tree provides some hope,” says Rosenthal.
Researchers at the U.S. Forest Service have been collecting scions from resistant ash trees to grow grafted clones. Field tests are underway in Ohio and Delaware to see how these younger ash trees react to emerald ash borer outside of a lab or greenhouse.
For the program to thrive, more resistant ash trees must be identified. In the early days of the emerald ash borer invasion, a common practice was to cut down all the ash trees where the pest had been detected, to stop the spread. While research found this increases the spread (the beetles just fly farther afield to find new ash trees), Rosenthal and Wildova say they still encounter people who preach cut first.
“People want to do something,” Rosenthal says. “And what’s more American than cutting trees down? That’s a headwind that we face.”
If all the lingering ash trees are cut down, those that will remain will be the ones that had been deemed “significant” enough to warrant the labor and expense of constant insecticide injections. The state parks department has designated one stand near the entrance to Minnewaska State Park to receive treatment; the trees may become a living museum where New Yorkers can show their children what ash trees are.
For the Mohawk people, a living museum is not enough. Rosenthal and Wildova have spent time on the reservations along the New York-Canadian border, speaking with elders and craftspeople about the cultural importance of black ash trees.
“For them it’s an existential problem,” said Rosenthal. “Some say it would be the end of their language, because so much of their language is tied to ash. And they’ve made huge efforts to maintain their language.”
The Mohawk and Iroquois people also weave baskets from black ash for ceremonies. It’s tradition that, during weddings, the families of the newly wedded couple exchange black ash baskets. One basket weaver told Rosenthal that if the black ash tree dies out, for Mohawk people it would mean the end of marriage.
A forest full of secrets
“You name the species,” says Taro Ietaka, “and there’s something that’s coming for it.”
Ietaka, a Cold Spring resident who serves on the village’s Tree Advisory Board, is a parks supervisor for the Westchester County Department of Parks, Recreation and Conservation. During his 20 years working with forests, he’s watched the threats in the Highlands steadily increase.
“There’s a lot of stressors, especially in our area,” he says. “There’s the changing climate, the big summer droughts, the air pollution. But then you throw on top of that some of these exotic things coming over, like emerald ash borer and the like…” He looks at the beech tree behind us. “They’re still not 100 percent sure what’s going on with this.”
It’s late October at Cranberry Lake Preserve, a park north of White Plains that Ietaka used to manage. The leaves are a riot of autumn color, although the beech tree is still green — deeply green. Ietaka plucks a leaf and points to the bands across it. “The stripes, that’s pretty unmistakable,” he says.
For years, beech trees have contended with a bark disease that has been lethal for stands in the Adirondacks and Maine but less so in the Highlands and south. “It’s ugly, but down here it usually doesn’t kill the tree,” says Ietaka. “They scar over and go on living happily.”
In 2012, researchers in Ohio noticed something new happening with beech leaves: First came the deep green banding, then the leaves turned leathery, then crinkly, and then they dropped. Beech usually keep their leaves throughout the winter; the faded yellow of winter beech is one of the few predictable pops of color in the Highlands’ forests during the colder months.
But once the leaves fall, the disease prevents new buds from forming. Brent Boscarino, who coordinates the Invasive Species Citizen Science Program for the New York-New Jersey Trail Conference, notes that once beech leaf disease afflicts a tree, it will typically be dead within three years. Parts of Ohio have reported a 100 percent mortality rate. “That’s what we’re scared of,” says Boscarino.
It’s not clear what is causing the disease, much less what can be done to combat it, but there are clues. Its rapid spread eastward indicates a high probability of an invasive component, since the disease clearly isn’t encountering any natural resistance. So far it’s been found in Ohio, Pennsylvania, Ontario, New York and Japan, which might indicate that it originated in Asia.
The most important clue may be the presence of nematodes, a microscopic worm, found in afflicted leaves. It’s not known if the nematodes are responsible for the disease or are a symptom, but Ietaka isn’t waiting to find out. He’s received clearance to inject a few beech trees with an insecticide similar to the one used against emerald ash borer.
“I was just in parts of Fahnestock, and it looks like every beech tree has had this.”
So far, beech leaf disease has not been discovered in the Highlands, although with the disease encroaching from the west and the south, it appears to be only a matter of time. Or perhaps it is here already, in a different form.
This past fall, beech tree leaves here have been turning deep green, then shriveling and falling off, but without the characteristic banding of beech leaf disease. “I was just in parts of Fahnestock” State Park in Philipstown, Boscarino said at the end of October, “and it looks like every beech tree has had this. The leaves are staying this dark green” and look diseased. “But we were not able to see banding. So is it beech leaf disease?”
The disease — so new it is unnamed — has so far only shown itself in the Highlands where the familiar beech leaf disease hasn’t made an appearance. Across the river in the western Highlands, Schuster, the Black Rock Forest executive director, and Matthew Brady, its manager, have been on the lookout for beech leaf disease.
They haven’t seen it yet, but they have found examples of the newer leaf disease, which both believe is likely a deformity caused by a late frost (May 15) this year. “We had 2 inches of snow,” notes Brady. If that’s true, the trees should recover come spring. If not, the mystery, and the devastation, will deepen.