The resurrection of the American chestnut

Contrary to popular belief, when European settlers landed in North America, the continent wasn't a primeval wilderness. It was a cultivated garden, the handiwork of Native American civilizations that were decimated by disease before white colonization began in earnest.

One method of indigenous cultivation is agroforestry, or forest gardens: planting fruit and nut trees that yield a harvest of food and other products year after year. One of these trees that indigenous people relied on, and that colonizers learned to take advantage of, was the American chestnut.

The heyday of the chestnut

The American chestnut, scientific name Castanea dentata, was once ubiquitous in North America. In its heyday, an estimated four billion chestnut trees grew in a temperate forest band stretching from the Deep South to New England. Chestnut trees have been present on the continent since the last retreat of the glaciers. For thousands of years, they were a dominant species in the eastern deciduous woodland.

Chestnut trees were woven into the fabric of early American life. They yield an ample annual crop of nuts, which could be roasted and eaten (hence "chestnuts roasting on an open fire"), or ground into meal to bake with, or fed to livestock to fatten them up. The nuts also fed wild game like deer, turkeys and hogs that settlers could hunt. Chestnut trees grow rapidly, and their wood is straight-grained and rich in natural tannins that make it resistant to decay. The timber was used for homes, fences, furniture, ship masts, telegraph poles, railroad ties, and other wood products. Tannic acid extracted from the bark was used to make leather. Americans called the chestnut "the perfect tree" and "the redwood of the east".

Then, in the early 1900s, disaster struck. Asian chestnut trees brought to the U.S. by breeders carried a parasitic fungus, Cryphonectria parasitica—better known as chestnut blight.

The blight fungus infiltrates the tree and grows underneath the bark. It secretes toxins that kill the living cambium layer, forming a patch of dead tissue called a canker. Eventually, the canker becomes a ring around the trunk, cutting off the flow of nutrients and killing the tree.

Asian chestnuts are resistant to blight, but American chestnuts weren't. In a tragic echo of Native Americans dying from introduced European diseases, the alien fungus spread rapidly. As soon as the catastrophe was discovered, foresters tried to cut down and burn dead trees to stop the blight from spreading, but it was to no avail. Within a few decades, the blight had wiped out nearly every chestnut tree in North America.

In the most technical sense, the American chestnut isn't extinct. The fungus doesn't harm the root system, which can remain alive underground even when the tree above is dead. When they have an opportunity, chestnuts will try to regenerate, sending up sprouts from the stump. But as soon as they poke their heads above the surface, the blight kills them again.

Seeking a cure for blight

Almost since the wipeout of the chestnut tree, Americans have mourned its loss and wondered if it might be possible to bring it back.

To restore the chestnut, we'd have to make it resistant to blight. There are two approaches that have been tried to achieve this.

One is traditional hybridization, the same method used by farmers for millennia to breed plants with desirable characteristics. Plant breeders cross the blight-resistant Chinese chestnut with its American cousin, then backcross the hybrid with American chestnuts. They repeat this process over multiple generations, with the goal of creating a tree that has the blight resistance genes of the Chinese chestnut, but is otherwise almost identical to the American chestnut.

The disadvantage of this method is how much time it requires. It's essentially a form of guided evolution, and evolution doesn't run on human timescales. It takes about seven years to breed each new generation of trees, plus years more to grow them and evaluate whether they can fend off the blight. Plant scientists have been working on breeding blight-resistant chestnut trees since 1922, and their efforts are still ongoing.

The other method that's been tried is modern genetic engineering. Scientists at the State University of New York College of Environmental Science and Forestry (SUNY-ESF) created a transgenic chestnut variety called Darling 58. It contains a gene taken from wheat that codes for an enzyme called oxalate oxidase, which breaks down the tree-killing oxalic acid compound produced by the blight.

However, this method has suffered setbacks. In some field trials, the wrong transgenic variety was planted by mistake, spoiling the experiment and rendering the data unusable. It also appears that, even when it works, the energy drain of producing the enzyme stunts the trees' growth and makes them more likely to die prematurely. The SUNY-ESF scientists are continuing to work on the problem, more recently developing a chestnut variety that only produces oxalate oxidase when under fungal attack.

The Thousand Chestnut Challenge

In the competition to create a blight-resistant chestnut, the backcrossing method currently holds the lead. Since 2017, the New York Restoration Project, in partnership with the American Chestnut Foundation, has planted several hundred hybrid trees in Manhattan's Highbridge Park. So far, they seem to be resisting the blight, although it could take up to twenty years to make sure they're immune.

Now they're moving to the next level with the "1,000 Chestnut Challenge". The plan is to give away a thousand hybrid chestnut seedlings, for free, to anyone in New York City who wants to plant them and care for them. The hope is to establish a stable, blight-resistant chestnut population that can be used for further research and for breeding future generations of trees.

The biggest unanswered question is: assuming these efforts bear fruit, what comes next? Would the reborn American chestnut live only in urban parks and gardens? Or should we reintroduce it to the wild? Would it successfully take root and grow in its former niche, picking up where it left off? Even if it thrives, it will take decades, if not centuries, to fill out its previous range.

This is a type of de-extinction—the belief that humans should use science to resurrect species that we wiped out through our own carelessness—and brings with it all the ethical debates that accompany this idea. If the chestnut comes back, the world it returns to won't be the same world it departed. Do we have the right to do this, let alone the obligation? Would it be a means of atonement, a way of repairing the damage we've done? Or is it just another manifestation of the hubristic belief that we're wise enough to control nature?