Beech Leaf Disease Sweeps Across Princeton

Princeton is losing its beech trees.

We were feeling celebratory, having just completed a successful corporate workday in Herrontown Woods, when I happened to pass by this small branch of a beech tree along the red trail. The leaves were strangely contorted, with dark green stripes. I had heard distant rumblings about a disease of beech trees, but had managed to keep my head in the sand until that moment. 

Back home, diagnosis was but a google's search away. Similar images popped up on the screen, along with the name: Beech Leaf Disease. Tree maladies typically come with an acronym. Emerald ash borer is EAB. The dreaded asian longhorned beetle, which they've had some success keeping from spreading across the eastern U.S., is ALB. The Bacterial Leaf Scorch that afflicts pin and red oaks is BLS. Now there was a new one: BLD. 

For those unfamiliar with the American beech (Fagus grandifolia), it's a native tree related to oaks and chestnuts, with beautiful smooth gray bark. They can get very big and live for centuries. Thousands of them grow in Princeton, in the preserved forests along the Princeton ridge and on slopes above the Stony Brook. 

The "grandifolia" in the latin name refers to the leaves, which are larger than the leaves of European beeches. This photo shows some healthy leaves (on top) and the curled, darker green leaves that have been contorted by nematodes overwintering in the buds. Beech leaf disease is caused by these nematodes--tiny worms spread by birds or the wind. 

Viewed from beneath, the infected leaves show a curious striping of dark and light green. 

During a subsequent hike in Autumn Hill Reservation, I was astonished to find nearly all the beech trees affected--their leaves contorted, their crowns beginning to thin. Beech in Rogers Refuge are showing symptoms, and Mountain Lakes preserve is reportedly also affected. According to online sources, essentially all of our beech trees will be dead within ten years. The news comes exactly ten years after the first emerald ash borer was found in New Jersey, with the skeletons of ash trees still haunting our woodlands.

According to this map, on the Holden Arboretum website, the disease was first spotted near that arboretum in Ohio in 2012, and has spread in all directions, most rapidly eastward.

According to the Maryland Extension website, the microbe causing the disease is Litylenchus crenatae mccannii, a subspecies of a nematode found in Japan. As one would expect, the only beeches resistant to this particular nematode are those that coevolved with it in Japan. 

The Holden Arboretum website mentions a chemical treatment that is being tested. It is a compound that is sprayed on the tree in the fall just as the nematodes are moving from the leaves down into next year's buds. Unfortunately it is highly toxic. The snail's pace of tree research compared to the rapid development of Covid vaccines caused one friend to ask, "Where is science when we need it?" 

The loss of a tree species from the canopy has all sorts of impacts on wildlife. Ash, elm, and maples bear abundant seeds early in the season to feed on. Two of those three have been largely lost. Nut-bearing trees provide food in fall and winter. Gone from wildlife diets are chestnuts, bacterial leaf scorch is reducing oak production of acorns, and it now looks like beech nuts will become very rare. Websites detail the ecological web of connection and dependence that is unraveled by the loss of a tree species. 

A post last year by the Brandywine Conservancy in Pennsylvania provides a particularly chilling description of what is in store for eastern forests:

"As the disease progresses, leaves will become smaller in subsequent years, and it will seem like autumn in the summer as infected leaves brown and fall from the tree, resulting in thinned crowns and branch dieback. Eventually, BLD will cause beech trees to abort their buds, leading to the death of the tree. Young beech tree saplings die within 2–5 years of infection, while mature trees live a bit longer. Death from BLD is likely accelerated in beech trees stressed by drought or Beech Bark Disease, which is a different infection that involves scale insects and fungi."

Here's a writeup I found on beech bark disease, which also poses a mortal threat. 

I encourage people to visit favorite beech forests in the area sooner rather than later, to appreciate the now threatened beauty of this singular tree. Over the next few years, if you are fortunate enough to find one that remains healthy while others around it succumb, you should let people know. The Holden Arboretum site provides someone to contact.

Yesterday evening, I visited the fabulous congregation of European beech off of Elm Lane on Constitution Hill in western Princeton. The many trunks appear to all come from the original massive trunk in the middle. 

Seen from a distance, they appear to be separate trees, but more likely were either branches that touched the ground and took root, or sprouts from the original tree's massive root system.

You can see how some of the trunks still have a sort of navel, where the original branch from the "mother tree" was cut off.

Its leaves, smaller than those of the native beech, were  showing early signs of the disease.

Some of Princeton's most spectacular native beech trees grow in the Institute Woods. That will be my next stop--that and a hidden valley between the Princeton University chemistry building and Washington Road, where I found a mixed forest of 200 year old trees, part of the great American forest cathedral that, in unspeakable sadness, loses its towering pillars, one by one.

Here is how I concluded a recent letter to the editor in the Town Topics: 

Outrage is often triggered by the intentional cutting of trees. The highly visible spotted lanternfly caused a stir, yet has proven relatively innocuous. The biggest threats we face are neither visible nor intentional. The emerald ash borer is hidden behind bark. Nematodes are microscopic. Our machines’ climate-radicalizing carbon dioxide? Unintended and invisible.

There is so much joy still to experience, for me particularly in Herrontown Woods, and yet in the larger workings of the world, so much to grieve.

A Winsome Bugloss-Wood Poppy Combo

There's a winsome duo--one yellow, the other blue--that I first saw blooming together in my friend Gail's garden. I don't think my having two degrees from the University of Michigan has anything to do with their blue and gold appeal, though the blue one I first encountered in Ann Arbor. Though one is native and one is not, I think of them both in the same breath. They both thrive in moist soils and some shade, and combine attractive flowers with attractive foliage. Both spread by seed, not rampantly but just enough to create a sense of delight at the sight of a new one having popped up here and there. Both have common names that can get in the way of fully enjoying them. Neither grows in the wilds of Princeton that I have ever seen. 

The first is Siberian bugloss (sounds best when pronounced "BOO-gloss", a perennial that, as its name suggests, is native to Siberia, south to the Caucusus. It has tiny blue flowers and,

as its latin name Brunnera macrophylla suggests, large leaves that remain attractive through the growing season. The name "bugloss" comes from Greek, meaning 'ox's tongue', referring to the leaf's shape and rough texture.

The large leaves and clouds of blue flowers mix well with other garden plants, in this case a largely random congregation of hostas, mayapples, and day lilies.

They also mix well with a native yellow flower called wood poppy. I'd call it by its other common name, celandine poppy, but that creates confusion with the super invasive lesser celandine. 

The diphyllum in its intimidating genus-species name, Stylophorum diphyllum, also refers to the leaves. After the flowers are gone, the foliage forms a nice mound through the summer 

that fits in well with other plants like Christmas fern.

Wood poppy has a nonnative invasive look-alike called greater celandine (Chelidonium majus) that I've only found a couple small patches of in the Princeton area. 

As this comparison shows, wood poppy has larger flowers. 

And wood poppy has hairy seedpods like these, while greater celandine's seedpods are smooth.

Siberian bugloss also has a look-alike, this one with a much more appealing name: forget-me-not, which has tiny leaves and lighter blue flowers. Its genus, Myosotis, also draws from animal anatomy: it's Greek for "mouse's ear."

A few times I've had to break the news to gardeners that the lovely flower they've been growing and admiring in their gardens is not the endearingly named forget-me-not, but instead Siberian bugloss. 

Siberia, bugs, loss, ox's tongue--these are not the associations people wish to conjure when admiring such a pretty plant. The sometimes used "false forget-me-not" doesn't land well either.

Other common names for the plant that are far less common but far more palatable are "great forget-me-not" (they're both in the borage family, after all) and "heartleaf Brunnera."

The plants are easy on the eyes and easy on the garden; but the names can get in the way.

Campus Grounds Sprout Local Flora

Some areas of Princeton University's intensely landscaped campus are starting to sprout local native flora. Using native plants doesn't necessarily mean those species will be found in local nature preserves. Many native plants popular in landscaping--purple coneflower, red buckeye, bottlebrush buckeye, oak-leaved hydrangia, witch-alder, Virginia sweetspire--are seldom or never seen growing on their own in the local wild. 

But recently, the University has been planting the actual species frequently encountered in local nature preserves like Herrontown Woods and Mountain Lakes. This spicebush shrub in front of the Lewis Center for the Arts, 185 Nassau Street, resonates with the spicebush  so common in our woodlands.  

 

Turf has been replaced with a sedge meadow, probably the same Pennsylvania sedge that can be encountered in remnant meadows in local woods.

Masses of Christmas fern now grow on campus as they do along the slopes of the Princeton ridge.

A few species, like the foamflower making this mass of white, are rare to nonexistent in local woodlands, but the overall trend seems to be to treasure what is authentically local. 

Though some of the University's early efforts to plant native landscapes became overrun with weeds, this 2-3 year old planting down near Robert's Stadium is thriving. Better soil prep and thick mulch (and more knowledgeable gardeners?) have, at least thus far, conquered the weeds. The native species chosen are again those one finds in the local wild: cutleaf coneflower, arrowwood Viburnum, and wild rye grass. 

People naturally want to plant things that are special, and special used to be defined by distance--as in exotic plants imported from distant continents. Distance made the botanical heart grow fonder. It's heartening to see a shift in what is viewed as special, towards a valuing of what is truly local.  

Chimney Swifts Converge on a Tower Near You

Updated 5.18: The back side of an abandoned school building doesn't seem an auspicious place to spot endangered wildlife, but check out that chimney. It changes everything.

In a recent letter to Town Topics, Princeton ecology professor Andy Dobson invited readers to witness one of nature's more remarkable annual phenomena, playing out in and above the tower of the old Valley Road School. From his letter:

In the half hour after sunset, several hundred swifts will be “turning and turning in a narrowing gyre” centered around the tower of old Valley Road School building behind Conte’s Pizza. It is quite a spectacular sight as the rapidly spinning circle of birds “know exactly where it leads, and you can watch them go ‘round and ‘round each time.” Suddenly, they will begin to drop down and disappear into the tower to roost for the night. “Wait ‘til you see half the things that haven’t happened yet.”

Andy encourages us to "come to the playing field on Valley Road and enjoy a truly remarkable local wildlife spectacle." 

"They will probably be there at dusk for the next couple of weeks while they pair up and locate nest sites on local tall buildings. The site is Princeton’s equivalent of the Serengeti wildebeest crossing the Mara River on their annual great migration."

His letter was published on May 8, and in typical fashion I didn't get myself over there to have a look until 9 days later. When I arrived, right at sundown, there was nothing to be seen--a golden opportunity for self-rebuke--but just as I began to leave, a high-pitched cheeping somewhere above pulled me back. They came, a few at first, then many, swooping down on the chimney only to veer away at the last possible moment. There is no clear choreography to their acrobatic flight, as they head off in all directions at great speed, sometimes in pairs but mostly on their own, each one's acrobatic flight describing a broad circle out across fields and rooftops, always to return to play yet again with the magnetic pull of the chimney. As the light fades, their gleeful independence ultimately yields to the collective impulse, drawing them down into the chimney's depths to spend the night.

   

Here's a link to Andy's wonderful letter. And here is a link to local writer Carolyn Jones' well researched article on our local chimney swifts and the longterm threat redevelopment of the Valley Road School site poses to their very specialized habitat. 

Andy's comparison of chimney swifts to the migration of the wildebeests that he studies in Africa has added meaning for those of us who have seen the university as often detached from the community of which it is a part. He recently began teaching a course called "Woods and Rivers of Princeton." The course gets students out exploring local nature, and has become so popular that this coming fall's course is triply over-subscribed. This valuing of the local is gratifying to see, and I like to think is part of a larger trend. 

Update, 5.31.24 Andy reports that "the swifts are still there, numbers declining as they pair up and locate nest sites. They should be back by the middle of August in even larger numbers with young of the year."

Helping Herrontown's Beauty Express Itself

There's a lot of built-in beauty at Herrontown Woods. Rocks, wood, and water serve as the basic infrastructure upon which other beauties are overlaid. 

This time of year, it's the understory that gets to shine, just before the tree canopy envelopes the woods in shade. Much of the habitat restoration work we do at Herrontown Woods involves bringing back the beauty and functionality of the native flora. By removing nonnative invasive shrubs that clog the understory, we open up vistas and release the existing native flora from stifling competition. In a sense, we are filling in for the deer, which chow down on native shrubs while leaving the nonnative shrubs uneaten.

Walk up the new boardwalk from the main parking lot to witness a corridor brightened by flowering dogwoods, 

and hundreds of blackhaw Viburnum shrubs adding clusters of white flowers extending deep into the forest.

Redbuds can't survive in the deep shade of the forest, but they proliferate on the more open Veblen House grounds.

This year, we spotted two wild azaleas blooming along Herrontown Road. Fifty years ago, it would have sounded strange to be excited about a couple wild azaleas in the preserve. They were numerous back then, but have been literally laid low by increasing deer numbers and deepening shade. 

It's taken more than a decade of ramblings in the preserve to realize that some kinds of native shrubs we thought long gone in fact remain numerous on the forest floor in miniaturized form, browsed before they can grow sufficiently to bloom. The town's deer culling program has helped native shrubs like spicebush to rebound, but for some species, additional effort is needed.

Protected by cages and given some sun, pinxter azaleas, serviceberries, and hearts-a-bustin' are making a comeback in the Botanical Art Garden (Barden) next to the main parking lot. New plantings of native buttonbush, silky dogwood, pussy willow and elderberry are also being protected until they can grow and flower beyond the deer's reach.

In these ways, we help another layer of beauty in Herrontown Woods to express itself.

Tent Caterpillars and the History of Black Cherry Trees in Herrontown Woods

Black cherry trees draw attention in early spring because of the "tents" that tent caterpillars weave on them. I was surprised to find out that these tents are sometimes mistaken for gypsy moth infestations. There's also some disagreement as to how damaging tent caterpillars are to the trees they feast upon, so I decided to do some investigation. 

First, some distinctions between tent caterpillars and gypsy moths. Tent caterpillars are native, feed primarily on cherry trees, build conspicuous tents, and do their feeding on the fresh, tender leaves just beginning to emerge in April. Gypsy moths are a nonnative species imported from Europe, start feeding in May on a very wide spectrum of hardwoods and even some conifers, and do not build tents. 

Gypsy Moths

The story of gypsy moths in our area is most easily grasped by looking at how many articles about them have been archived in the Papers of Princeton through the decades. Outbreaks of gypsy moths remained minor in New Jersey until the 60s, then grew into massive defoliations of forests in the 1970s. By the early 1980s, gypsy moth populations were beginning to drop, thanks to a natural virus, introduced parasites, and aerial sprayings. A naturally occurring bacteria called Btk proved safe and effective when sprayed on trees where gypsy moths were feeding. There was a recurrence from 2007-8, but numbers have dropped off since then. Though the forests largely healed, the trauma of past gypsy moth infestations lives on in people's memories. 

Tent Caterpillars

What we have this spring, and springs extending back through millenia, are native tent caterpillars making their tents. 

Some sources on the web suggest that tent caterpillars, despite the powerful visual of the tents and defoliated branches, don't do enough damage to a tree to worry about. I'd really like to believe that, but this young black cherry tree, now bearing eleven tents from which the caterpillars make forays, is almost completely defoliated. 

They say a tree can survive one complete defoliation, but if defoliated several years in a row, it becomes increasingly susceptible to disease and insect attack.

Meanwhile, our very hungry tent caterpillars have even followed a branch over to a neighboring pin oak, which now, too, is getting chowed down upon.

We pause to note a couple recurring themes in nature. One is that the tent caterpillars will shift to a less desirable food source (a pin oak tree) if their favored cherry tree leaves run out. Deer, too, will begin eating less desirable foliage if they run out of their favorites. Thus, an overabundance of deer in the 1990s almost wiped out spicebush in our Princeton woodlands, despite it being low on the list of deer's preferred foods.

The other example of a recurring theme in nature is that the tent caterpillar eats only one crop of leaves, then is done for the year, allowing the tree to recover. The worst thing a predator could do is be so effective as to eliminate its prey. 

The introduction of a new insect, though, could throw off this balance of predator and prey. If another insect, say, a gypsy moth, came along and defoliated the same tree yet again in the same growing season, that tree would be in big trouble, having twice committed energy to manufacturing a whole crop of leaves, only to have them eaten. One question is whether the gypsy moth outbreaks in the '70s and '80s killed more of one tree species than another, causing changes in forest composition still noticeable today.

There's a lot of caterpillar behavior whose purpose is not immediately obvious. A week ago, caterpillars were crawling about on the outside of the tent, turning this way and that. My best guess was that they were expanding the tent by adding another layer of silk, but no strands could be seen coming from their bodies as they moved about. 

And why are these caterpillars clustered on the side of the tree, outside of their protective tents? Wouldn't they be easy picking for the birds that are said to consume them? 

As their spring residency has continued at the Barden in Herrontown Woods, the tent caterpillars have spun not only isolated tents but also enveloped the trunk and limbs in a silken web reminiscent of the webbing people drape on their shrubs for Halloween. A closer look reveals that the caterpillars have spun silken highways upon which they commute from tent to the "pasture" of the canopy. These highways are only one lane wide, requiring a caterpillar to temporarily step aside if it meets another coming the opposite direction. Some silken highways are suspended in air, like overpasses--a great way to smooth out the rough terrain of a black cherry tree's "black potato chip" bark. 

A tree colonized by tent caterpillars, then, has elements of occupancy, transport, and exploitation not unlike the human footprint on the land, with our homes, highways, and farm fields. A big difference being that the tent caterpillar's settlement is seasonal--more like the impact of nomadic tribes than our permanent villages--giving the tree a chance to recover.

Another big difference between tent caterpillars and other builders of shelters--bees, ants, birds, mice, people--is that the caterpillars don't seem to bring anything back to the shelter other than their bigger, well-fed selves. They aren't adults bringing food back to the young. The caterpillars, like super resourceful children, work collectively to raise themselves, then leave the tree on their own to pupate and turn into adult moths. 

A nice writeup about tent caterpillars (Malacosoma americanum) found at ThoughtCo.com states that the broad side of the tents faces the sun, and that the caterpillars make three forays per day, returning to their tents inbetween. That will be something to check out next spring.

Surprisingly, the subject of wild cherry trees in what is now the Barden (formerly a pine plantation) came up more than 50 years ago, in Richard J. Kramer's book about Herrontown Woods. 

"Wild black cherry, which grows to magnificent size in the Allegheny Mountains, is a poorly formed tree in Herrontown Woods, occurring mostly in areas which were recently open fields. Its best growth has been in the pine plantation, where specimens are 30 to 40 feet tall and possibly may develop into good-sized trees. Apparently these black cherries were able to develop along with the young pines after these were planted in the open field. Although the birds do bring seeds of the cherry into the forest, the many seedlings and the few saplings that occur there grow poorly and remain shrub-like."

Gone now are most of the pine trees in the pine plantation, and those larger cherry trees are nowhere to be found. Our 12 little black cherry trees in the Barden, all saddled with tent caterpillars, must be the descendants of the larger cherry trees Kramer describes. Only one large black cherry tree is known to exist now in Herrontown Woods, almost completely free of tent caterpillars, growing next to Veblen House. Have tent caterpillars contributed to keeping the black cherry trees of Herrontown Woods from achieving full size, in the past as well as in the present?

If we wanted to relieve the cherry trees of the spring burden of hungry caterpillars, we could remove the tents and remember to crush the egg masses laid by adult moths on small branches in late summer, as suggested in this useful post about the insect. 

But as insect numbers continue to decline, the role of trees as food for native insects grows in importance. And if the cherry trees remain small, that will allow more sunlight to reach the many wildflowers growing in the Barden. Leaving the tent caterpillars to grow undisturbed can serve as an experiment, to see if they continue to flourish year after year, or if nature's array of predators, pathogens, and parasitoids finally up their game and reduce the burden these trees now bear.