“This is where the will to grapple with our hard and pressing environmental problems begins: in relationship to something other that you love beyond any utility, beyond any logic.” —Susan Freinkel
It was said that at one point in history, the forests of the eastern United States were so dense that a squirrel could travel from Maine to Missouri on the tops of trees, never to touch the ground. In those forests of old, Castanea dentata, or the American Chestnut, would have been abundant. The species once numbered nearly four billion individuals and was the largest, fastest-growing, and tallest tree in the forest. The tree was essential for the ecological food web. Each year, chestnuts fed insects, birds, and mammals of all kinds, including people. Nut-ripening coincided with the holiday season, and hence we have the song lyric “chestnuts roasting on an open fire” sung in Nat King Cole’s soothing baritone. The American chestnut’s legacy today is not one of abundance, but loss. In the early 1900s, a fungus called Cryphonectria parasitica killed the tree by the billions, and reduced it from its former prominence to small, shrub-like seedlings in its old range. Now, scientists at the State University of New York College of Environment Science and Forestry (SUNY-ESF) have created a genetically modified American chestnut that resists the blight. If approved by the U.S. Government, it would be the first genetically modified tree approved for environmental conservation use in the U.S. As with most genetically modified organisms, it is not without controversy. A look back at the history of the chestnut might provide some context to inform the debate around releasing a genetically modified tree into our forests.
The Rise
Chestnut pollen data shows three distinct events which paint a picture of how the American Chestnut rose to prominence. In the late Holocene (2,500 years ago) there was an increase in the abundance of chestnut pollen everywhere within the New England range. This could be a result of general climate cooling after the mid-Holocene thermal maximum. It could also include competitive interaction with other species or be attributed to changes in the prevailing disturbance regime.
The second event is a near-doubling of chestnut pollen as a percentage of total tree pollen associated with the onset of European agricultural practices in 17th century New England. The dense distribution of chestnut sprouts at many New England locations could be an artifact of land use history. Abandoned fields previously used for agricultural practices may have formed ideal locations for chestnut seedlings.
The third event was an abrupt disappearance of chestnut pollen between 1900–1925, which can be directly attributed to the arrival of the blight.
The Fall
Cryphonectria parasitica likely arrived in New York City in the early 1900s via the wood of Chinese chestnuts destined for fancy gardens in the United States. Once released, the blight spread like wildfire to American chestnuts. The fungus originally infected the Chinese chestnut (Castanea mollissima) and the Japanese chestnut (Castanea crenata), but since these trees co-evolved with the pathogen, they are resistant to its damaging effects.
Chestnut blight has transformed the American chestnut tree in that it has destroyed nearly all large chestnut trees and eliminated their ability to sexually reproduce on any significant scale. Since the blight can’t kill the underground root system, many of the seedlings remaining in the original distribution are very old—some over a hundred years old. These seedlings are clones that originated before the appearance of the blight. The few exceptions are isolated stands of chestnuts beyond the natural range (for example, in Wisconsin, Northern Michigan, and northern Illinois). This reproductive strategy allows chestnuts to survive indefinitely without sexual reproduction. It also confers the inadvertent advantage of minimizing blight infection by lower available bark surface.
The chestnut is adapted for survival in the forest understory as part of a natural process of gaining position in the canopy. A similar species, Castanea pumila, commonly referred to as dwarf chestnut or Allegheny Chinquapin, is similar to the American chestnuts shoots in two main ways—both show success in competition with other understory shrubs, and both show an ability to respond to release. Release is a break in the canopy as a result of disease or disturbance that allows more sunlight to reach the understory. However, these two plants differ in the way they respond to a release. Allegheny Chinquapin uses the release to produce a mast crop while American Chestnut uses the release to generate a long-lived canopy-dominant tree. One could speculate if the chestnut had adopted the Chinquapin’s methods, things may be different.
The Possible Resurgence
There is hope for the American Chestnut via a genetically modified (GM) cultivar developed by scientists at SUNY ESF. The blight-resistant cultivar, called Darling 58, contains a gene from wheat that detoxifies oxalic acid, which is the acid that the fungus produces to form a canker after colonizing a wound on the bark. The canker eventually proves lethal by girdling the trunk. The wheat gene produces an oxalate oxidase enzyme that is found in all grain crops and many other familiar foods. Though the enzyme does not kill the fungus, it causes it to change its lifestyle—instead of killing the tree, it can survive on the bark as a harmless saprophyte. If approved, the cultivar would be available to the public. It would be the first GM tree approved for environmental conservation use in the U.S. The U.S. Government previously approved virus-resistant papaya and plum trees for agricultural purposes. All regulatory decisions from the Government (including the USDA, FDA, and EPA) are projected to be completed by fall 2023.
Few species are as beloved as the American Chestnut was in its prime. This makes the loss sustained even more sobering. A genetically modified tree has the potential to fundamentally change North American forests. If approved, this could pave the way for other endangered species such as the American elm. In the new era of environmental degradation and climate change, this may be the best answer to losing beloved and ecologically important species. However, altering any ecosystem comes with risks, many unforeseen.
What are your thoughts on a the GM American Chestnut cultivar? Should it be available for public use and released into nature? Should GM organisms be only for agricultural purposes, or conservation purposes as well? Or neither? Why?