Resurrecting The Great American Chestnut Tree

Dan Herms, vice president of research and development at the Davey Institute, talks about the American Chesnut tree - the history, chestnut blight and how scientists are trying to bring the species back to the United States. 

In this episode we cover:  

• History of the American Chestnut (0:38)
• Mature specimens of the tree (2:44)
• Bringing the tree back (4:19)
• GMOs (7:57)
• Back crossing (11:27)
• How are the hybrid forests doing? (12:56)
• The future (15:14)

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Transcript

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Doug Oster: Welcome to the Davey Tree Expert Company's podcast, Talking Trees. I'm your host, Doug Oster. Each week, our expert arborists share advice on seasonal tree care, how to make your trees thrive, arborists' favorite trees, and much more. Tune in every Thursday to learn more, because here at the Talking Trees podcast, we know trees are the answer.

I'm joined again by Dan Herms. He's Vice President of Research and Development for the Davey Tree Expert Company in Kent, and Dan has been on the show many times. Today, we're talking about something that I'm fascinated with, the American chestnut tree. Dan, tell us a little bit about the history of the American chestnut.

Dan Herms: Good morning, Doug. It's great to be with you again. The American chestnut, at pre-settlement times, was one of the most dominant trees in eastern North America, ranging from eastern Texas, Mississippi, all the way up through the Appalachia, into New England. One of the largest trees in eastern North America. Fast-growing, the wood very resistant to decay, so it was prized for timber, and also ecologically very important for the nut crop, a very predictable mast of chestnuts that were produced every year that would support wildlife, and consumed by humans, and so forth.

Chestnut was a very important tree economically and ecologically in eastern North America. Then, chestnut blight was accidentally introduced to North America. Fungus disease was first noticed in 1904 in New York City, at what's now the Bronx Zoo, and it was introduced with Japanese chestnuts from Japan. This fungus disease spread rapidly through eastern North America, and by mid-century, that eliminated mature chestnuts from the forest.

Interestingly, the blight doesn't typically kill the root system, and so you get these sprouts, and these can be fairly common today, these scrubby little sprouty chestnut trees in the eastern forest, but once they start to get some diameter, the fungus then kills those sprouts again, and so there's this perpetual state of shrubby sprouts that die back, and then re-sprout in the forest.

Doug: I've seen the little sprouts in the forest, but I've been told, there are some mature specimens somewhere that have survived. Have you ever seen one? Have you ever seen one of the big ones anywhere?

Dan: I have. I've seen large trees in Michigan. I think my understanding is, most of the large surviving trees were planted, and are growing outside the natural range of chestnut, and so they have escaped the disease, and so there's trees that I've seen in the upper peninsula of Michigan. It's a pretty famous stand in Wisconsin, but there are some trees, large trees, within the native range of chestnut that have somehow escaped, and it may be that they're naturally resistant.

There is work underway with those trees to evaluate if they are, in fact, naturally resistant, and use those in breeding programs for breeding North American chestnuts to develop resistant varieties to use in restoration of the forest.

Doug: I mean, it's such a beautiful wood. When you see, like here in Pittsburgh, I will go into older buildings, and I will see it used all over. My understanding was, it was an easy wood to work. It was a straight wood, and oftentimes, a very beautiful wood. What are some of the things that are being done to bring this tree back? Do you think it's going to be able to come back? [chuckles]

Dan: Yes, [chuckles] well, good question. Yes, it is beautiful. You find these old antique chest of drawers, and things made out of chestnut, and so forth, just beautiful grain. Yes, so there's work been going underway for a while to develop trees with resistance, with the goal of restoring chestnut to the Eastern forests of the United States.

The American Chestnut Foundation has played a central, perhaps, the central role in this effort. One approach that they've used with some success is, crossing or hybridizing American chestnut with Asian species, like the Chinese chestnut, which has natural resistance. The chestnut species in Asia are naturally resistant, because they co-evolved with the fungus, and they have natural resistance.

Just as an aside, I'll say, this is a reoccurring phenomenon. We get an invasive species that is introduced to North America, be it a disease, or an insect. The North American species have no history with it, no resistance, and they experience mortality across their range, and so, chestnut blight was probably the first. Then we had Dutch elm disease, hemlock woolly adelgid in the hemlock trees, most recently, emerald ash borer in the ash trees.

It's the same phenomenon. Yes, so going back to the American Chestnut Foundation and the hybridization project, so crossing the resistant Asian species with the susceptible North American species, and then they back cross the offspring, so that they get a genotype that's almost pure North American, with the resistance genes from the Asian species just captured in a small part of the genome.

They can get a species that's well over 90% North American with just a small percentage of the Asian genome provides the resistance. The resistance is due to just a few genes. These have been planted, thousands of these have been planted, these hybrids. They've used them in strip mine reclamation. I've read even that one was planted at the White House.

This is an ongoing effort. I mentioned the crossing of the American chestnuts, the surviving American chestnuts, the American Chestnut Cooperative Foundation, I believe it's called the ACCF is leading that effort. Then, there's a third effort, which is creating genetically modified chestnuts that have resistance to the chestnut blight.

Doug: Let's talk a little bit about that. Whenever you bring up GMO, people get freaked out. They worry about it. Is there a reason to worry about a GMO chestnut tree?

Dan: GMO plants are definitely controversial, no doubt about it. The concern is that the genes will escape into the wild type populations, and change the ecology. Now, GMO plants are widely planted as crops. Almost all the corn, for example, and soybean in the United States is genetically modified, but they don't have the native relatives for the genes to escape to.

In the case with chestnut, if chestnut is restored, and the genes would escape, that could cause some unintended ecological consequences. Now, the gene that they're using is a gene from wheat, and the chestnut blight produces an acid. The fungus produces an acid, and this acid is damaging to the cells of the chestnut tree. The vascular system causes a cankering. That's how the tree is damaged by the fungus.

This wheat gene produces an enzyme that degrades that acid, and makes the tree tolerant of the fungus by disarming it, and so that's the approach. They've been working, the State University of New York, College of Environmental Science and Forestry has been working on these genetically modified chestnuts for some time. There's a cultivar called Darling 58 that they have been working on, and the American Chestnut Foundation have been evaluating this cultivar, but recently, just late last year, the American Chestnut Foundation decided to no longer support that research, or the petition to the USDA to plant this tree into the wild, and it was also about that time, it was found out that there was some kind of a mix-up in the identification of the cultivar, and that the cultivar that the American Chestnut Foundation had been evaluating, was actually one called Darling 54, and not Darling 58.

It wasn't performing well, and for various reasons, they decided to no longer support that project. My understanding is, the State University of New York College of Environmental Science and Forestry is still pursuing a permit with USDA APHIS, the Animal and Plant Health Inspection Service, that regulates genetically modified plants among other things, so if it's approved, it would be the first native tree to be released into the forests of North America.

Doug: I want to go back to the hybridization, because I don't understand what backcrossing is. Is there a way to explain it to just to a regular homeowner how they create that, how they do that back-crossing?

Dan: Yes, I'll try, and I am not a geneticist.

Doug: Okay.

Dan: Basically, and the same thing is done with other crop plants, hybrid corn, and you take two parents, you mate them, take their offspring, and then, if you have an American chestnut and a Chinese chestnut, that offspring will be 50% American, 50% Chinese, right? They take that offspring, and then mate that with another American chestnut. Then, you'll have an offspring that can be 75% American and 25%.

Then, you take that tree that's 25% Chinese, and mate that with another American chestnut, and you keep diluting the Asian genes.

Doug: Yes, I get it. You explained it very well. [chuckles]

Dan: Yes, so that's how that works, and then they screen those offspring, and select the superior ones for the continued breeding.

Doug: I've seen some of the forests with the hybrids. How are they doing? I mean, they're making progress, right? They're getting some decent sized trees?

Dan: I have not actually ever seen them in the wild, but my understanding is that they're performing well.

Doug: The ones I've seen, they were actually up on Chestnut Ridge, believe it or not, [chuckles] in PA, and they had a serious fencing around that, because the deer and the bears actually want to go after the trees as soon as they put on their fruit, and so--

Dan: Oh, that makes sense. The nuts are delectable.

Doug: It just must have been so tragic in so many ways when that chestnut blight came in, because so many-- So much of the economy revolved around chestnut trees. They say that the wood that took you from cradle to grave. [chuckles] I've read these stories where people, when it would-- The chestnut would bloom, they would look up on the mountains, and it blooms late, it blooms in the summer, and you look up there, they'd say it looks like snow on the ridges, that's how prevalent they are.

Then, I've read one of the story where George Washington was crossing Chestnut Ridge, and he had to get off his horse, because there were so many leftover chestnuts, and the shells, and all that, that the horse couldn't walk on it, so those are the fables I've heard anyway.

Dan: Yes, well it was the dominant species for sure, especially, in particular habitats like the moist well-drained slopey, acidic soils. Yes, spectacular tree. I've just seen like the old black-and-white photographs of these huge, majestic chestnuts with people standing in front of them for scale just, they're just-- They're huge, they're like something you'd see in the Pacific Northwest.

Doug: What do you think for the future? From a scientist point of view, from my point of view, I'm just like, I'm just so hopeful, and I figure with all this great work being done, that probably not in our lifetime, but eventually, that tree might come back from a scientist point of view, what do you what do you think?

Dan: I'm hopeful, and I'm hopeful that it can be a model, because we have these increasingly devastating invasive species, and I mentioned a few, but there's others too. The redbay ambrosia beetle, and its associated disease, beech bark disease, beech bark scale, balsam woolly adelgid that are-- Have killed their host plants on a range-wide basis, and I think host plant resistance, because that the lack of host plant resistance is the problem.

Biological control like what they're trying with emerald ash borer, I think has a place, but when you plant susceptible North American species in Asia, they get wiped out by these organisms, even though the biological control agents are present. If we can have a successful resistance breeding program that leads to restoration of a dominant species in the American forest, that would be a great thing. It hasn't happened yet, but I'm hopeful that it will.

Doug: We're both hopeful, and as always, thanks for explaining it to us from a scientist point of view. Always great to talk to you, and I know we'll be talking again soon. [chuckles]

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Dan: Look forward to it, Doug, as always.

Doug: Thanks, Dan. I'd love to see one of those mature chestnut trees. I have never seen one. Now, tune in every Thursday to the Talking Trees podcast from the Davey Tree Expert Company. I'm your host Doug Oster, and do me a big favor, subscribe to the podcast, so you'll never miss an episode.

Do you have an idea for a show? A comment? Have you seen a chestnut tree? [chuckles] Send us an email to podcasts@davey.com. That's P-O-D-C-A-S-T-S @ D-A-V-E-Y.com, and as always we'd like to remind you on the Talking Trees podcast, trees are the answer.

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