How American should an American Chestnut be?

The American Chestnut used to rule the forests of North America. However in the early 20^th century a blight almost wiped them out. Billions of trees were killed and all that remain are a few thousand in isolated stands. Scientists have found possible defences against the blight, but both methods will change what the American Chestnut will be in the 22^nd century.

It’s hard to be certain how many American Chestnut, Castanea dentata, trees there were in the USA before the 20^th century. There was no urgent need to count them until it was obvious they were dying en masse, and by then it was too late. The killer was Cryphonectria parasitica.

C. parasitica arrived some time after 1900. It’s fungus that likes to enter via wounds in a tree and grow in or under the bark. There it grows as a canker around the stem. As it grows it releases oxalic acid, killing plant cells. When the canker surrounds the stem there’s no way for food to pass the stem and everything upwards dies.

It’s thought that the fungus arrived with Asian chestnuts. The Asian chestnuts have evolved along with the blight over millions of years, so they’re much less susceptible to it. The American Chestnut had about forty years to adapt. It’s now functionally extinct. As it stands it has no future. However, it might return with some help. Asian chestnuts have some genes that give it resistance to the blight. If the few remaining American chestnuts could be cross-bred with them, then they too might have resistance.

If you cross and American and Asian chestnut then you end up with something half-American and half-Asian. Cross that with another American tree and you get something three-quarters American. At the moment the hybrid trees are one-sixteenth Asian and fifteen-sixteenths American. Is that American enough? Who gets to say how good is good enough?

Blythe et al. have a paper recently published in Restoration Ecology: Selection, caching, and consumption of hardwood seeds by forest rodents: implications for restoration of American chestnut. As the title suggests, she and her team have put the hybrid trees to the test, by seeing how rodents react to the chestnuts.

They compared the hybrid chestnuts with American chestnuts and other nuts and seeds found in American forests. The hybrid chestnuts might look convincing to humans, but there was a preference for the American chestnuts over the hybrids from the rodents. Another puzzling feature was that they scattered the hybrid chestnuts over a wider range than the American chestnuts. The hybrids were eaten, but they’re not a straight substitute for American chestnuts. Blythe et al. conclude that the hybrid chestnut trees are not the ecological equivalent of the American chestnut trees.

But hybrids are not the only hope for the American Chestnut.

Another solution is genetic engineering. The problem is the tree cannot cope with oxalic acid. If they could get the genes to cope, then the tree could be almost entirely as it was before. Wheat has those genes. If they could be pasted into the chestnut it wouldn’t kill the fungus, but it would enable the tree to live with it. Blythe et al. say that a GM chestnut would be more likely to be an ecological equivalent, but they also warn that for social reasons there could be resistance to planting GM American chestnuts instead of the hybrids.

Another possibility would be that there isn’t just one solution, but both hybrids and GM trees could be used as mixed plantations to restore forests.

It raises a question that follows a lot of de-extinction proposals, who is the ecological restoration for? Is it to restore the damage done to an ecosystem, or is it more a statement about how people feel about an ecosystem? The hybrid chestnuts might be different, but if de-extinction is a social process and less a matter of ecology, then maybe the public will decide that hybrid chestnuts are American enough.

Blythe, R. M., Lichti, N. I., Smyser, T. J., & Swihart, R. K. (2015). Selection, caching, and consumption of hardwood seeds by forest rodents: implications for restoration of American chestnut. Restoration Ecology. http://dx.doi.org/10.1111/rec.12204