September 2020. In my last story posted here, A Decade of Monarchs and Milkweeds, I described how a decade ago, just as I was leaving a job at an international consulting firm where I had worked for about five years and re-starting my independent consulting business, an experiment to attract monarchs to my yard by planting milkweed had succeeded, and I watched the first monarch of the season eclose from its chrysalis and take its first few flaps of flight. It made it across the driveway to my side yard, and stopped to rest on the branch of a small, spindly American chestnut sapling about three feet tall.
What I didn’t mention in that last story is that Bob Bothell, my neighbor, watched with me as that monarch emerged and flew. Bob, who passed away a few years later, was then in his mid-eighties. He had close-cropped white hair, was fond of white T-shirts and faded khaki pants, and in spring and early summer regularly brought me gifts of lettuce and spring onions from his tiny backyard garden patch. Bob had studied (and then worked, I think) at the Art Institute of Chicago, but had returned home to Falls Church to take care of his mother in the simple house next door to mine after his father died. His mother lived to be over a hundred; she was gone by the time the monarch was emerging. Bob was one of the gentlest and kindest souls I’ve known, truly a living buddha I thought.
For some reason I never fully learned, Bob was fascinated by the American chestnut. He was old enough that he might have remembered eating roasted chestnuts as a kid, when there were still some trees that hadn’t succumbed to the chestnut blight. Bob was a regular donor to the American Chestnut Foundation, which was working to restore the species in the wild, and knowing of my interest in things ecological, he would pass their magazine on to me when he was done reading an issue. Earlier in that first year of my monarch-attraction efforts, Bob ordered a bunch of chestnut seedlings from the foundation, and offered me one, which I promptly planted beside my driveway. The little tree was just a switch, with a few branches of a few leaves each. Bob planted the rest in his back yard, imagining a future chestnut forest, I guess. Later neighbors dug them all up to build an addition to the house, except for one tree that still stands opposite mine in Bob’s old yard (a good thing, because the American chestnut is self-sterile and requires two trees for pollination and nut production). Now that tree beside my driveway is as tall as the peak of my roof, thirty-five feet tall, I’d guess. Its base is eight inches in diameter, having grown that big from the diameter of a pencil in a decade. American chestnuts grow fast.
Again this year, as every year for the past several, the tree carried a heavy crop of baseball-sized green burrs, bristling with sharp spikes that can pierce a leather glove if you grasp them too hard. The fruits have grown and ripened all summer, and half a week ago they started to split along their seams and their four spiny petals curled open to reveal the shiny chestnut-brown nuts they had been guarding until this moment. Was it only a coincidence that this happened on the fall equinox? My guess is that the chestnut’s genes knew exactly when to offer themselves up.
The local squirrels were waiting, and worked the tree from dawn to dusk. My guess is that the squirrels’ genes knew exactly how to do this, even though they hadn’t seen wild chestnuts for many squirrel generations. The chestnuts were a squirrel magnet for the whole neighborhood. I tried my best to get “my” share of the chestnut crop, whacking the branches with a long stick, and even climbing onto a ladder, and finally my roof, to reach higher. But I finally resigned myself to the fact that I can’t compete with a squirrel in harvesting chestnuts—these guys have a serious evolutionary advantage. I watched from a treetop-level second-floor window as a squirrel—sometimes there were two or three in the tree at the same time—climbed out on a thin upper branch, hung by its back legs, and reached out to grab a gaping chestnut burr. It peeled off the spiky hull of the burr with its front paws, grabbed a shiny brown nut with its teeth, and descended in a rush.
The tree had obviously overwhelmed the immediate appetites of even the hungriest squirrels, but not their obsession to harvest these rich treasures. I watched one young squirrel, carrying a chestnut in its teeth, frenetically searching all the cracks between the flagstones on my back patio for a place to stash the nut. The cracks were too narrow, apparently, and it explored further toward the edge of my yard. Surely what I was observing is the result of an old evolutionary game: the tree gets a lot of seeds dispersed and buried—and a few eaten on the spot—by avid squirrels during a wild week of chestnutting around the fall equinox; the squirrels cache maybe a few months of food to depend on until January cold forces them into their leafy winter dreys in the bare treetops to keep warm; and when spring comes, a few chestnuts with the luck to be forgotten by the squirrels sprout. That must be the life story of the little chestnut seedling on the west side of my house, now just the same size as the little tree Bob gave me a decade ago. In fact, it must be a natural descendant of Bob’s tree. Next spring, I have a plan to transplant it to my front yard, between house and street—which is called Timber Lane.
The chestnut peak lasted only about a week. I managed to harvest what I guess is about a third of the crop from my tree, and the squirrels got the rest. Curiosity led me to do a little simple research. I counted the burrs I collected, and counted and weighed the chestnuts from those. (Thoreau would be proud of me; he did similar simple science around Walden and Concord so often.) Here are the data:
Burrs: 80 Nuts: 157 Weight of nuts: 4 pounds, 9 ounces
Do the math, and it’s clear that the average burr had two nuts (some had three, some only one, most had two). The average nut weighed just under half an ounce (0.46 ounces, to be exact).
According to the American Chestnut Foundation, American chestnuts are very nutritious, with a protein content of around 10 percent (comparable to wheat or maize), a fat content of around 10 percent—higher on these two components than any other chestnut species in the world. They contain about 70 Calories per ounce. Traditional America recipes seem to relegate chestnuts to additions to stuffings and dressings for turkeys and such. I’ve played around with a few recipes. Chestnut shortbread, anyone? Roasted chestnut soup? Banana chestnut cake? Candied chestnuts? The thing is, chestnuts are so special and delicious just roasted and eaten alone that there isn’t much incentive to do otherwise with them. Maybe in a few years, when the crop in bigger and I manage to steal a few more pounds from the squirrels, I’ll be more tempted to experiment with chestnut cuisine.
Why am I so excited about my tree and tiny chestnut crop? The American chestnut, scientific name Castanea dentata, once ranged from Maine to Mississippi, and from the Atlantic Coast to the Ohio Valley, with the heart of its range in the Appalachian Mountains. It went from being an ecologically dominant forest tree in most of the eastern United States barely more than a century ago to… gone. Disappeared. Nearly extinct in the wild across its native range. The global Red List of threatened species maintained by the International Union for the Conservation of Nature (IUCN) lists Castanea dentata as “critically endangered.”
Depending on the location, this tree sometimes made up between twenty and sixty percent of the forest canopy. It’s nuts, produced faithfully every year (unlike unlike masting oaks and hickories), fed vast flocks of wild turkeys, Native Americans, and Euro-American colonists and their free-range livestock, especially pigs. In case you aren’t familiar with the term “masting,” it refers to the production of large seed crops in certain years while in other years the crop is small or nonexistent. Ecologists interpret this reproductive strategy as a way of overwhelming seed predators like squirrels in some years, but starving them in other years so their populations don’t grow to match—and consume—the available seeds.
What happened to the American chestnut? It’s a case study in how modern humans have, mostly inadvertently, unleashed threats to native species by our relatively recent transportation of plants, animals, and pathogens from one continent to another. Understanding what happened requires a bit of background. There are seven recognized species of chestnuts. All are found in the Northern Hemisphere and classified in the genus Castanea, of the beech family, Fagaceae (to which hickories and oaks also belong). The center of diversity of chestnuts is Asia, where there are four species; North America has two; and Europe a single species, Castanea sativa. The Chinese chestnut, Castanea mollissima, a smaller understory tree than the giant, canopy-dominant American chestnut, was deliberately introduced to the United States in the 1890s. It was thought that its smaller size would make the nuts easier to harvest in chestnut “orchards” than those of their giant American relative. But in 1904 a troubling, unintended consequence was discovered: the introduced Asian chestnut species carried with it a pathogenic fungus. The Chinese chestnut, unlike its American relative, had evolved resistance to this chestnut blight fungus, and typically has only minor damage when infected. From New York City, where blight-infested American chestnuts were first noticed, the airborne fungus spread at approximately 50 miles per year. In the next fifty years, it is estimated that between 3 and 4 billion American chestnut trees were killed by chestnut blight. Almost no mature chestnuts now exist within their historical range, although because of their characteristic of resprouting from roots once the tree dies, small shoots of the former live trees can still be found in some forests “The American chestnut tree survived all adversaries for forty million years, then disappeared within forty” years, according to the American Chestnut Foundation.
And what a loss it was to lose the dominant tree of eastern forests, ecologically of course, but also culturally! After the indigenous Native American tribes, who used chestnuts extensively for food, were overwhelmed and driven out by European colonists, the American chestnut became an important food source and cultural touchstone for those colonists. Old photos and works of art hint at how important the chestnut was to American culture before the blight struck and erased the species from the landscape.
Is there any hope for restoring the American chestnut in our forests? The American Chestnut Foundation, working with a range of scientists, has a science-based strategy for restoring the species. The strategy has three elements: breeding, biotechnology, and biocontrol.
The breeding component, involving backcrossing and selection, is the oldest plank in the strategy. The parasitic fungus that causes the chestnut blight is called Cryphonectria parasitica. It is found naturally in Asia, and Asian chestnut species have coevolved with the fungus and are not very susceptible to it. American and Asian chestnuts are quite closely related, in an evolutionary sense, and hybridize when grown together. My mind flashes to Gregor Mendel, the simple Austrian monk, in his garden with his famous peas. Mendel set the stage for modern genetics, even though it took decades for his experiments to be understood as the foundation of evolution. The American Chestnut Foundation has undertaken a process similar to what Mendel would have used if he wanted to have only purple-flowered, wrinkled peas: backcrossing parental generations to select for the desired genetic traits. Careful selection and backcrossing could breed the blight resistance of Asian chestnuts into the American chestnut’s genotype, retaining the unique characteristics of American chestnuts in succeeding generations of hybrids. Here is how the American Chestnut Foundation summarizes their breeding work:
“During the past 36 years, offspring from blight resistant hybrids have been bred with American chestnuts from across the species’ range. Four generations later, our traditional breeding program has produced a genetically diverse population of American chestnut hybrids with improved blight tolerance from Chinese chestnuts (Castanea mollissima).”
While backcrossing and selection, generation after generation, may eventually produce completely blight resistant American chestnuts that can be restored to the wild, it is a slow process because each generation requires about ten years. Enter biotechnology. It has now been discovered that a single gene may be able to control susceptibility to the blight fungus. Called “OxO,” this gene from wheat codes for production of the enzyme oxalate oxidase; it breaks down oxalic acid, which the blight fungus uses to attack chestnut trees, neutralizing it before it can do its damage. Scientists at the State University of New York’s College of Environmental Science and Forestry are exploring the use of gene-editing technologies, such as CRISPR, to insert the OxO gene into the American chestnut’s genome to enhance blight resistance.
Some people have criticized the possible use of genetic engineering techniques in an ecological restoration context like this. The phrase “playing God” has been used in this critique, suggesting we would be stepping outside of a proper role for our species to mess with the genes of a wild tree. I don’t quite get why trying to undo some serious ecological damage done by humans, albeit inadvertently, qualifies as human hubris. I suspect the people who object to this kind of ecological use of GMO technology would object to almost all genetically-modified organisms, although I am not sure of that. Global agriculture already includes many crops that have been genetically engineered, such as maize and soybeans, into which genes from other species have been inserted, conferring tolerance to herbicides or resistance to insect pests or diseases. Some people seem to oppose all GMO crops, even though they may help prevent the conversion of even more natural ecosystems to agriculture by increasing yields on current farmlands. It’s a complex moral calculation. Weren’t we inadvertently “playing God” when we introduced the alien chestnut blight fungus from Asia? And if so, would it be a sin to again intervene in nature with a fix, using genetic engineering techniques to insert the OxO blight-resistance gene into America chestnuts, and help restore them to American forests?
There’s one more interesting possibility for fighting the chestnut blight that is being explored by the American Chestnut Foundation and other scientists: biocontrol. It turns out that Asian chestnuts, and to a lesser extent the European chestnut, C. sativa, are resistant to the fungus because they somehow seem to have teamed up with a virus that weakens it. How have they “teamed up”? According to research by Choi and Nuss, published in Science in 1992, it happens through a mechanism called “hypovirulence,” in which strains of the chestnut blight fungus that contain viral DNA exhibit reduced virulence. The fungus-weakening viruses belong to a “species” of virus called Chryphonectria hypovirus, according to the International Committee on Viral Taxonomy. But why do Asian and European chestnut species host the low-virulence strains of fungus containing the hypovirus, and American chestnuts do not? That mystery is probably explained by coevolution, with the trees, fungus, and virus all involved in a long, three-way, coevolutionary “arms race.”
In a recent blog, Coronavirus, Human Hubris, and Life in the Coevolving Biosphere, I described how a coevolutionary arms race between a parasite and its host—whether the parasite is a virus and the host a bacterium, or the parasite a virus or bacterium and its host a so-called higher organism like a human—can drive evolution and increase biological diversity. Tantalizing examples show that cooperation between organisms from vastly different levels of the tree of life has evolved through these coevolutionary arms races. Resistance to the blight fungus in Asian chestnuts may be one of such yet-to-be-understood cases, with trees and viruses cooperating to defeat the fungus. But then, why don’t American chestnuts host the Chryphonectria hypovirus? Could it be introduced as a biocontrol agent to fight the chestnut blight and bring chestnuts back to American forests?
November 2020. Here on Timber Lane it’s now Thanksgiving, and the leaves on my chestnut are copper-colored, crinkly dry, still clinging to the branches after all the other leaves have fallen. Monarchs have mostly reached their winter roosts in the mountain of Michoacán, Mexico. Coronavirus is surging again and the transition to a new administration is finally under way as another year winds down. Stashed by chestnut-overwhelmed squirrels in secret places in the neglected corners of my yard and across the neighborhood, a future forest of little chestnuts is waiting to sprout when it’s finally spring again.
- The American Chestnut Foundation.
- Wikipedia. American chestnut (Castanea dentata)
- The American Chestnut Foundation. History of the American Chestnut.
- Freinkel, Susan. 2009. American Chestnut: The Life, Death and Rebirth of a Perfect Tree. University of California Press: Berkeley and Los Angeles.
- American Chestnut Foundation-Kentucky. Chestnuts as Nutritious and Healthy Food.
- Source of Streator photo, “Chestnut trees in North Carolina, 1910, Forest History Society. http://www.rosshowelljr.com/wp-content/uploads/2016/01/american-chestnut-trees.jpg
- Source of Winslow Homer engraving, “Chestnutting,” 1870. https://americanart.si.edu/artwork/chestnutting-every-saturday-illustrated-journal-choice-reading-october-29-1870-37115
- Source of Lauterbach engraving, “Gathering Chestnuts,” 1878. http://permaculturevoices.com/wp-content/uploads/2014/07/HistoricPhotoweb.jpg
- The American Chestnut Foundation. Using Science to Save the American Chestnut Tree.
- The American Chestnut Foundation. Biotechnology.
- Powell, William. 2016. “New genetically engineered American chestnut will help restore the decimated, iconic tree.” The Conversation, January 19, 2016.
- Popkin, Gabriel. 2018. “To save iconic American chestnut, researchers plan introduction of genetically engineered tree into the wild.” Science, August 29, 2018.
- Restoring the American Chestnut. 2020. ESF: SUNY College of Environmental Science and Forestry.
- Popkin, Gabriel. 2020. “Can Genetic Engineering Bring Back the American Chestnut?” The New York Times Magazine, April 30, 2020.
- Kelly, Grace. 2020. “Infighting Over Genetic Engineering Splinters Efforts to Save the American Chestnut.” ecoRI News, July 31, 2020.
- Choi, G.H., and D.L. Nuss (1992). “Hypovirulence of Chestnut Blight Fungus Conferred by an Infectious Viral cDNA”. Science. 257 (5071): 800–803.
- Wikipedia. “Hypovirus.”