Science Friday
Hosted by Ira Flatow
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Genetic research could speed the restoration of the American chestnut tree. Plus, "rewilding" small spaces with fast-growing miniforests.
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Hi, I'm Ira Flatow, and you're listening to Science Friday. Today on the podcast, to paraphrase Sondheim, "We go into the woods." First, an update on efforts to restore the American chestnut. These trees once towered over the landscape, dominating forests in parts of the eastern U.S. They provided food for people and animals, and they were a hugely valuable source of lumber. But in the late 1800s, a fungal blight in the U.S. virtually wiped out the American chestnut plants across the country. Streets once lined with beautiful trees went bare. And due to the blight, the species now exists mainly as a shrubby plant,
not as a towering tree. But new genetic work could help speed up breeding efforts to add fungal resistance to create a heartier chestnut population. Joining me now is Jared Westbrook. He's Director of Science for the American Chestnut Foundation. Welcome to Science Friday.
Hello, Ira.
Nice to have you. We've talked about chestnut restoration efforts. How long has it been going on? Where does the project now stand?
So, chestnut restoration in the U.S. has been going on since the 1920s, and people have tried a lot of different things. First, the trees got the blight originally from imported Chinese and Japanese chestnuts that were brought over, and they had the blight pathogen on these trees. And the Japanese and the Chinese chestnuts actually are interfertile when you cross them and breed them with American chestnuts. So, the early efforts were in hybridization between the Asian species and the American species. With the American Chestnut Foundation breeding program, we've been doing this for 40 years.
Uh, we have been doing this program called back crossing where we take the Chinese and the American hybrids and we cross them back to a lot of different surviving wild trees. Um, people tell us, oh, there's a flowering tree near the side of the road. And uh, we then give them pollen from some of our hybrid trees that we've selected for having improved blight resistance. And then they have bred those hybrids with diverse American chestnut trees all across the range. So we now have hundreds of orchards, um, all up and down the east coast of the US. And you know, some of the things that we've done recently is just look at all those trees. They've been growing and they've been living with the blight the last 15 years.
And we've sequenced their genomes. Um, we've looked at how resistant they are. And so now what we're doing is breeding those best trees together. And we can speed up this process of doing the breeding and selection um, by way of doing DNA testing on their kids and being able to sort out which of the kids have improved resistance relative to parents. So then we can kind of ratchet up the resistance levels over multiple generations of breeding.
Now, are you doing that breeding by hand pollinating or how is that happening?
Yeah, so we have 16 state chapters and these are folks that have been doing this breeding for decades and they care for each individual orchard and mow the grass and keep the trees alive. And so we um, then exchange pollen, you know, so they, we tell them, okay, this tree is likely to have resistance.
Right.
And they send us the pollen and then we coordinate shipping that pollen to the location where we want to do the breeding. So it's a very decentralized effort.
So you're never going to have a purebred American chestnut again?
There are actually some rare surviving American chestnuts, um, and we have bred some of those trees together. When we look at their kids, you know, actually, some of their kids also have partially enhanced resistance. So we are continuing to do breeding with those trees. There's a relatively few number of those trees, like um, when we actually looked at all of these surviving American chestnuts and we looked at their kids, like only seven trees, I would say, out of like hundreds that we assessed have, you know, appreciable improvement in resistance. So the hybrids help us capture more of the diversity because the American chestnuts that we breed with don't have to have the resistance.
Well, let's, let's talk about that because I would like to know how a Chinese chestnut differs.
Yeah, so the Chinese chestnuts, they've been bred in China for like orchard production of chestnuts. So they tend to have a shorter stature. The American chestnut grows more like a forest tree. The Chinese chestnut tends to grow more like an orchard tree. And what we're trying to do is combine the tall growth of the American chestnut with the blight resistance of the Chinese chestnut.
If you know the genetics involved, could you just engineer the trait into the American chestnut? And say, we'll get a giant tree and have the resistance.
Yeah, so like one of the mistakes that we've made over time with this project is we assumed simplicity. Like with our backcross program, we assumed two to three genes contribute to resistance. And what we learned when we were doing the DNA sequencing of our hybrids is that actually hundreds of different parts of the genome contributes to that resistance. So it's a complex trait. And when you do, I mean, for complex traits, like the standard, the global standard for improving those traits, like let's say milk yield in cows, growth in trees, you want to use recurrent selection, which is basically select the best parents, select better kids, keep going over multiple generations. With genetic engineering, it can be kind of challenging, like because
we, you know, we put in a gene from wheat that seemed to have some improvements in resistance when you looked at the seedlings and we inoculated the little baby chestnuts. But then when we put them out in the field and we looked at the resistance over time, it didn't hold up very well. And the genetically modified trees tended to grow slower. So it's a complex system and using this breeding strategy of multiple generations of breeding for improved blight resistance and growth is for sure going to work.
And so you're going back to the future.
Going back to the future. And one of the things I just want to mention is that we can make this go a lot faster. You know, trees are very slow. Doing multiple generations of breeding in trees is a multi-decade long process. And with genomic tools, basically what that enables us to do is we can do this crossing between these best trees and then do some genotyping on their kids and be able to like maybe plant the 10% best together from our DNA test without even having to do the inoculations on the trees. You plant those trees together, they start to breed with each other and then we can have a lot of seed for forest restoration.
Hmm. So, so then how long do you think it's going to take for this restoration?
So, you know, there were four billion, estimated four billion trees. So planting four billion trees is a huge task. But I think like in the next decade with doing this in faster selection, we can have the next generation of trees. It takes about like seven years for the chestnuts in an orchard to start producing flowers and nuts. So in the next seven years, from the crosses that we're making now, we should start to have seed available for some of this forest restoration trials. And I think that with the improved breeding techniques that we're using, we should be able to double the level of resistance that we have now to the next generation.
So tell me then what success looks like to you?
Success looks like trees that have mostly American chestnut genetics, that are blight resistant, that grow tall, that are competitive, that capture the diversity that remains in the wild population. We want to put like maybe a few hundred to a thousand together in sites like in the forest where there's been a clear cut or there's been a fire in a disturbance area. We plant these trees together and maybe we plant a few hundred to a thousand and 20% of those trees maybe carry enough resistance to where they will survive long term and start to fruit on their own.
And if we get a critical mass in all of these smaller planting locations distributed across the East Coast, now what we want is the tree to start to reproduce on its own in the forest. So we don't have to have perfection for that. We need to have some subset of the population, like 10 or 20% of the trees that we plant have resistance and they grow to the canopy. Then that enables the population to continue to kind of self perpetuate on its own in the forest.
You know, our listeners are going to be listening and saying, gee, I wonder if there's something I can do to help. Is there something an individual can can do to help you?
Yeah. So, I mean, we have already a large network of people that have been planting and maintaining these orchards. And going forward, we need sites for doing some of our field testing. So we want to be able to plant like 200 trees at a site. And over time, we will give them the chestnut fungus, we'll inoculate them, and we'll evaluate their resistance and make selections and continue the breeding. So that's, that's one way to help. Of course, we're a nonprofit organization. And so just becoming a member of the American Chestnut Foundation can help sustain the effort. We need to continue this effort over decades.
I hear you saying money.
Money. Money or land.
Well, if you have land, how much land? I mean, seriously, if I have a, how many acres do I want to give you? What do I need to give you? And how do, how do I give it to you?
Well, some of these plantings are like anywhere from like a half acre up to multiple acres. The key is to maintain the orchard over time. So, you know, we partner with all these people and they, they like put up deer fences or, you know, mow around this, the trees. So it is a commitment to do that. And then we, we go in and we inoculate. So just understand that this, this is research and breeding. So we are going to give them the disease and then we're going to make selections from there.
This is multi generational, it sounds like.
Yeah, I mean, I'm the third, I'm in the third generation of scientists working on this. So, and we need to continue to bring younger people into this effort.
Well, Jared, this sounds really interesting. I think you're going to get some response. Thank you for being with us today.
Thank you, Ira. Appreciate it.
Dr. Jared Westbrook is Director of Science for the American Chestnut Foundation in Asheville, North Carolina. After the break, what if you could build a forest in the center of your town? Stay with us.
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Continuing our forest excursion, maybe your town doesn't seem well-suited for miles of woods, or maybe the forests around you were cut down years ago. Well, how about this option, a mini forest? Hannah Lewis is the author of the book Mini Forest Revolution: Using the Miyawaki Method to Rapidly Rewild the World. Welcome to Science Friday.
Thank you. Thank you for having me here.
What is a mini forest?
A mini forest is basically a human attempt to regrow a native forest that's as natural and ecologically functional as possible. It's a native forest that is perfectly suited to the soil and the climate and the topographical conditions where you're planting it, and to do that in the small spaces around where we live and work.
So this is different from people just planting some trees. What is the Miyawaki method all about?
The Miyawaki method was developed by a Japanese botanist and professor named Akira Miyawaki. His work spanned the second half of the 20th century and into the 21st century. It involves identifying the native climax community for a given spot and planting the whole community, so not just the canopy trees, but also the understory species as well, and planting them densely. The general rule is to plant about three plants per square meter or per square yard. The plants are often pretty small when you put them in, you know, just a few feet tall, just a couple of years old.
And so they're they're not making a lot of their own shade or litter yet. So the soil is exposed. And so the other part of the method is just applying a dense layer of mulch to protect that soil in the first couple of years while it's still exposed to sunlight.
So can anybody do this? Or do you need special skills or equipment or knowledge?
The method was designed to really welcome community members of all ages, from all different backgrounds, to come together and plant together. The plants are small enough that it's like putting a tomato transplant in the ground almost because the trees are still small. So it's very, it's very welcoming for people to plant together. The planning process does take a lot of thought and consideration and consultation with local forestry experts, local ecologists, people that understand the ecosystem and can help figure out what that native climax community is.
And also, you know, the ground preparation, sometimes a landscaping company or a professional is it's helpful for somebody with those skills and understanding to help with the land prep too.
So you need, you need somebody to come down and help you prep the land. You just don't pick out a vacant lot and start doing this.
I mean, a vacant lot is a perfect place to start. If the folks in charge of that vacant lot are open to it. But there's definitely some planning and collaboration and partnership that goes into it, you know, ahead of time before the planting.
And what kinds of trees are we talking about here? Are you looking for rapidly growing trees?
So really what you're looking for is, is the native climax community. They're the species that grow in at the end of a sort of a natural period of ecological succession. Where what we all notice when we see a vacant lot is that it quickly gets taken over by small, fast-growing, uh, short-lived plants and that are taking advantage of lots of sun and sunlight and space. But then over time, you get bigger plants, slower-growing plants, and the site changes. So eventually you'll get woody species, you'll get some fast-growing pioneer trees, like maybe pine or birch. But then you have a shadier, moister site with, you know,
the roots have loosened up the soil there and the conditions are different than when it was a vacant lot. And so eventually shade tolerant species then germinate there. And when they take over the canopy, they shade out everything else. Everything else that needed sun to grow there can no longer grow there. And so it becomes sort of a self-perpetuating community. And so, yeah, so you're not actually picking the fast-growing species. You're picking the shade tolerant ones that will form that community, that sort of self-perpetuating climax community for a forest.
So then do you need a minimum size for your forest if it's, if it's creating all the shade and whatever for it to be successful?
Yeah, sort of a, a Goldilocks kind of size is like the size of a tennis court, which is easy to picture. For it to work as well as possible, yeah, you want to have a certain depth, like, um, people talk about at least four meters or four yards deep. And what that does is that helps to create a microclimate where sunlight, the wind, the external temperatures cannot penetrate as well as they could if it was narrower. And so you get that cooler, moister microclimate that forest species appreciate.
Now, how long does all of this take to get to a successful, self-sustaining stage?
It takes about two to three years for it to become self-sustaining. And the way that happens is you're planting species that are going to do well there because they're native, they're co-evolved with each other, they're adapted to the local conditions, they grow well. And since they're planted close together, as they grow, they branch out and they start to touch each other and form a canopy. And so then they're shading out weeds and they're creating that microclimate that holds humidity inside. So you no longer need to water a weed after two or three years.
Wow, this is fascinating, Hannah. Thank you for taking time to be with us today.
Yes, thank you. Thank you so much.
Bring is coming up. Hannah Lewis, author of the book Mini Forest Revolution: Using the Mi'waukee Method to Rapidly Rewild the World. She works for a nonprofit Renewing the Countryside in Minnesota. And if you want to learn more about the idea of mini forests, there's an article on this topic on our website at sciencefriday.com/mini forest. This episode was produced by Charles Burquist. And you know what would help this podcast grow tall and strong? Send this episode to your most forestry friends and rate and review it wherever you get your podcasts. See you soon. I'm Ira Flatow.
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