American Chestnut; The Life, Death, and Rebirth of a Perfect Tree by Susan Freinkel. University of California Press, 2009, paper.
Back in 1904 an urban forester noticed that American chestnuts
(Casatunea dentata) in the New York Zoological Park, now the Bronx Zoo, were dying of a blight caused by a fungus. The fungus would soon spread to other chestnut trees in the city, then to the forests of the eastern United States and Appalachia. By the end of the sad story that Susan Freinkel tells, billions of trees were dead and the American chestnut was all but wiped out everywhere in its range.
The American chestnut had been an important species in its ecosystem, dominating the upper canopy. Never the preferred wood for the furniture and building industries, it was ‘second best’ but a good source of tannin. Its ample nut harvest was one of fall’s pleasures; we sang about “chestnuts roasting on an open fire.”
Foresters were divided about how to respond to the blight. Some believed that it was hopeless to fight the fungus. Might as well harvest the trees, dead and alive. There was one brief effort to stop the blight, financed by the state of Pennsylvania, to no effect.
The blight fungus kills the trunks and branches of the tree; it doesn’t destroy the tree’s resilient root system. New growth sprouting from its roots will continue for years, only to be killed off by the fungus before the young trees have matured enough to reproduce. Eventually the root system wears out and the tree dies.
The U.S. Forest Service’s advice to take down even healthy specimens complicated, Freinkel contends, efforts to find disease-resistant trees. Individual trees with a slightly different genetic makeup might have survived and propagated. Occasionally even groves of chestnuts did survive, mostly outside the normal range of the tree.
There were two approaches to fighting the demise of the species; fix the tree so that it couldn’t be hurt by the fungus, or fix the fungus so that it couldn’t damage the tree. The European chestnuts were saved by the second strategy. A virus was found that sapped the fungus’s virulence. That approach was also tried here with little success. That because there are different strains of the fungus and because the fungus can develop immunity to any virus, breeders had to contend with an ever-changing target.
The more successful strategy in this country has been to cross the breed with the resistant Chinese chestnut. But Chinese chestnuts are short understory trees not capable of dominating the upper story of American forests. Worried about its form, American breeders backcrossed the hybrid Chinese/American Chestnut with a surviving C. dentata, and repeated the process until they had a tree that retained the resistant trait of the Chinese chestnut but now has the form of the American tree.
Freinkel reminds us, however, that there will be problems when reintroducing the ‘tame’ chestnut into the ‘wild.’ Our forests have had one hundred years to adjust to the demise of the chestnut. Oaks and poplars now dominate their upper canopy.
Freinkel also discusses an option that biotechnologists have proposed. They would alter the genetic structure of the tree by inserting a gene into chestnut embryos that will increase its resistance enough for it to survive. That scares a lot of folks, however, just like genetically-modified food crops do. A genetically modified tree would essentially be a new species, posing the problems of invasive species and genetic drift. Biogeneticists assure us that Mother Nature usually takes care of the first problem, and they deny the second.
Then there is the issue of the ‘wild’ to which eastern forests should be returned. A fire tolerant species, the American chestnut’s dominance was likely the result of Native Americans’ use of fire to maintain their woods.
The book mourns the loss of this wonderful tree. It is also an absorbing account of our continuing interaction with a much damaged forest environment.