The Threats Facing Trees — and How to Save Them
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Lucy Corhoulis is standing at the base of a giant redwood tree on her property in Northern California. I always say, like, you know, you don't climb a ladder with your arms. You climb a ladder with your legs. So we're going to be using our legs quite a bit.

She's a professor of tree physiology at Cal Poly Humboldt University. One of her specialties is the giant coastal redwood. And she's convinced ecologist Chris Morgan that one of the best ways to learn about these massive trees is to climb one. Chris has secured his harness, and they are ready to go. Okay. Okay.

And then the motion is sort of like inchworm-like. It's a jug. So you slide your lower ascender up, and then you sit down. And then bring this up, and then stand with your legs and move your upper ascender up. The tree they're about to climb is about 10 stories high. And getting up there is much harder for Chris than Lucy, who's had a lot of practice.

Like if you need to go for dinner and stuff and then wake up for breakfast while I still make it up there, just... It's going to be a journey. You're doing great. Chris is the host of The Wild Podcast. His climb was part of their episode, Redwood National Park, Saving the Tallest Trees on Earth.

In the lower part of the tree, the bark is spongy and wet. So the spongy bark definitely, like sometimes you press up against a redwood tree and you're like soaked. You know, your pant has this big wet mark on it. Redwoods are also known for their wet bark.

Redwoods take advantage of the foggy air that rolls in from the ocean, and they absorb the fog and the rain into their bark. It's a water supply, and it helps them survive drought and fire. A mature redwood can drink over 150 gallons of water a day, and about 40% of that comes from the fog. My friend Alana called it sipping from the clouds. Cool. Should we go a bit higher? Yeah. Yeah.

As they climb, they encounter different ecosystems. The lower parts of the tree are covered in mosses and lichens. Higher up, they can see little round scars where the tree has pruned its own branches because they weren't getting enough light. They basically...

were kicked off the island, you know, they weren't making enough sugar essentially to pay their way. And the bark will eventually kind of grow over them. But for decades, you can still see kind of the remnants of where the base of the branch used to connect to the tree. 40 minutes into the climb, six or seven stories up, and we enter an important layer of this upward ecosystem. Oh yeah, we made it up here quite a way. Look at that.

So we're officially at the base of the live crown. This is where the redwood starts to really have branches and needles. They're covered in lichens. Squirrels and tree voles build their nests up here. Chris and Lucy keep making their way up. It looks like you're getting the hang of jugging. Yeah, about time. Now that we're at the top. Whoa, there's the Pacific. Yeah. Holy cow.

Wow, it is overwhelming. So how high do you think we are now? Holy smokes, that's a long way down.

It really brings it into perspective being able to see the ocean from the top of a redwood. How do you feel when you're up here at the top of a redwood? I guess I feel reverent. Just seeing these trees that have been in one place for 2,000 years is really inspiring. Whatever problems have come their way during that long period of time, they've had to stand their ground and kind of endure and get through it.

Trees can't pick up and move when problems come their way. They have to weather storms, droughts, or massive rainfalls, and increasingly changing temperatures. We don't just love trees because they're beautiful. We need them. They are an indispensable part of our ecosystem, but many of them face a host of new challenges. On this episode, what will help our trees survive and thrive?

Let's hear more from The Wild Podcast with Chris Morgan.

Redwood National and State Parks in California is home to some of the oldest trees on earth. Redwoods can live for over 2,000 years. These majestic trees had been in California for millions of years, but most of them were cut down starting in the 19th century when the logging industry was booming. Protecting what's left and restoring some of these redwood forests is a major effort.

And in a surprising twist, it involves cutting some of them down. Here's Chris. Wow. Well, this is where it's all happening then. You know, it's mixed emotions. I'm a tree lover. It takes a little getting used to. I'm the same way. I'm certainly a tree hugger at heart. I'm wearing a red heart hat, walking up a steep, muddy road with Lathrop Leonard, a forester with California State Parks.

He's brought me to a key spot for the Redwood National and State Parks, which is weird because everywhere I look are felled trees, fresh-cut logs. Four guys are working this section with three-foot chainsaws. They stop work to wait for us to walk by. One big tree, two, three big trees across the road that have just been felled. OK, so just ignore it. It's fortunate.

They used to be redwoods as wide as a three-car garage here, until they were logged 100-plus years ago. Now it's packed with much smaller redwoods and fir trees that replaced them back then. Some of them grew naturally, some were planted.

And you see this clump of trees here, this is classic, those are all growing from one stump, right? So there was one big tree and now there's, what, ten trees coming up from that one stump. When the loggers cut down the old-growth redwoods way back when, it triggered that clever redwood response. They sprouted new growth.

Those shoots themselves became trees all at once. And the theory is that too many of these growing from all those stumps, they're crowding each other out. They're competing for water and light and soil. And instead of thinking about, oh, I hate to kill this tree, I think a little bit more about,

oh, this tree next to the tree, how happy is that tree going to be that it's got all this extra space and this extra light and it's going to be able to grow better for it. So I think shifting the focus away from what you're killing to what you're helping. And of course, not only helping the individual tree, but the ecosystem as a whole. Left to nature, this situation of too many young trees all growing at once wouldn't happen.

There'd be big old trees and some middle-aged trees and younger ones too. In other words, a healthy, messy mixture of it all at different stages. And the goal is to get back to that messy mix. And so what we're really doing here is we're speeding up a natural process and taking down some of these trees out of the clump so that the ones that remain grow faster and become the big old-growth redwood trees. There's one coming down right now.

Holy smokes! I've never been that close to a tree coming down. That is unbelievable. Wow! I tried to picture what it must have looked and sounded like in here in the logging heyday. Imagine a 500 tonne redwood hitting the ground. Must have been like an earthquake. Some of the loggers who are working on these thinning operations now had family members who actually logged old growth trees.

As we're walking back down the logging road, one of the loggers upslope is watching us. He's leaning with his foot on a freshly cut stump like a scene straight from the 1800s. His name is Derek Ford. He's the owner of the logging company the park has hired to do some of this second growth thinning. Back in the day, this was much different. You'd travel from camp to camp. My dad's dad was a powder monkey.

These big logs, they would fall these big trees and their equipment wasn't strong enough to move them. Imagine a 320-foot tree felled and lying in the forest, almost impossible to move, so a powder monkey would use dynamite to literally blast them into more manageable halves or quarters. So actually the talent was...

If you didn't put enough in, it didn't do anything. And if you put too much in, you ended up with a bunch of toothpicks. Of course, the timber company, they don't pay for toothpicks. That's a good way to get fired. For the most part, I think that they were working to support their families because of the old stories that I heard my dad tell. They were really poor. My dad remembers as a kid, he lived in someone's chicken house.

He said you could see light through the walls and they were happy with that as long as they had a job. I can tell Derek's proud of his logging heritage but that he's perhaps even more proud of what he's doing today. Nowadays things are a lot different. You actually want to feel like you're doing a good deed at the end of the day and that's how we feel here.

It's not clear if this thinning is the best solution. It's hard to picture the results. But Lathrop says he can help with that. On our way out, he takes me to a forest just a couple of miles away where they tested to see if this thinning would actually work. An experiment from 20 years ago. So we're walking into a big forest experiment. That's a laboratory, a living laboratory almost here. Exactly. And what are we looking at? So what I'm seeing is that...

There's a whole bunch of trees in here really closely spaced together. And they're all kind of small and skinny. And there's not much live foliage on the trees. When the loggers clear-cut the old growth in this area, they reseeded it with new trees, which created this crop of second-growth trees, small redwoods, all growing close together, crowded.

This first section of the experiment that we're walking through right now is the control. Lathrop and his team haven't touched it since it was seeded over 30 years ago. You see the trees are so close together that

that the only live branches are at the very tip tops of the trees and all the other branches are dead because there's just not enough sunlight getting to the forest floor down here. Just like an empty carpet of needles, basically, isn't it? Exactly. We're in almost a desert, like there's just zero biodiversity here. So there are trees here, yes, but not much else. It's lifeless and barren.

Clean, almost clinical. Nothing like the messy and thriving look of nature in the layers of an old-growth forest. Yeah, yeah. So now we're going to walk just next to this control area where we did nothing. Yeah. We're going to walk into an area that we thinned 20 years ago. It's literally right next to it, a few paces away from the dead zone into this, like...

— A lush Jurassic Park. — And that's exactly the idea. We're standing on the line between these two treatment blocks. Twenty years ago, both of these areas looked exactly the same. Here, where the trees were thinned, there are shafts of sunlight coming through, illuminating the green plants on the forest floor.

It even smells different, damp and alive. There's a lot of different plants growing here. It's so different to the dead zone back there. What are you seeing? Which species? Yeah, so I think in the control area where we did no thinning, I think a few sword ferns was about the only thing that I saw there. But look at this. We've got wild ginger growing. We've got trailing blackberry. We've got salmonberry. We've got cascara's.

We've got grandfurs. A dozen different kinds of moss. Oh, so many types of mosses. There's some bracken fern. Oh, this is vanilla grass here. And of course, when you get this variety of berries, plants and trees, you get a boost in wild animal and bird biodiversity too. I think about all the different animals that are reliant on old growth forests that cannot survive.

without old-growth forests, and to be able to see them move back into this area and be able to use it the way they have been since time immemorial, like, how special would that be? It's not a perfect solution. Some scientists say the cut redwoods will just sprout again, or that opening the forest floor up to more light will let other species take over, and perhaps even increase forest temperatures.

It's complicated. But with so much conservation work, it's trial and error. And the results of this experiment won't be known for decades, left to future generations to understand. If we can put the pieces out there and let nature kind of take over and make that happen, then I feel like, you know, we've done our part to get things back to where they should be.

That was an excerpt from the podcast The Wild with Chris Morgan. It's a production of KUOW in Seattle with support from Wildlife Media. It's produced by Lucy Suchak and Matt Martin, edited by Jim Gates. We'll link to their episodes on our website, whyy.org slash The Pulse. We're talking about trees and what will help them survive and thrive.

Forests soak up and store away billions of tons of carbon dioxide every year, about one quarter of what we produce. Scientists believe that tropical forests like the Amazon are responsible for the largest part of this carbon sink. And within those tropical forests, it's the biggest trees that do most of that work. So when these giants are ailing or dying, it has major implications.

But how many are we losing and why? Reporter Daniel Grossman traveled to the Amazon in Brazil to find out what researchers are learning about this. I meet Evan Gora in a clearing in the Duque Forest Reserve in the central Amazon. After a hearty dinner of rice and beans, he explains that the forest itself is having a meal of carbon dioxide. The plants all around us, they are actively pollinating.

consuming that carbon. And the trees with the largest appetites are the biggest ones, the giants, roughly the ones with trunks too big for you to wrap your arms around. Only about one tree in a hundred is this big. And it's these trees that hold around half of the carbon in the trees of a tropical rainforest. Adriani Esquivel-Muhlberg, Evan's research partner, says that makes their health especially important.

These big trees have been there for hundreds of years and when one big tree dies, all the carbon is gone and it will take hundreds of years to be replaced. Recent studies show that the tropical carbon sink is declining.

Researchers like Adriani and Evan think that one of the reasons is the trees might be dying younger. You need to understand how these trees die, and we don't. Evan is a staff scientist at the Cary Institute of Ecosystem Studies.

Adriani is a professor at the University of Birmingham in England. Their Higante project is an investigation into what's killing the world's largest tropical trees here and in several other forests around the world. Without knowing what killed these giant trees, we can't understand the vulnerability of these forests in the future. The Amazon is changing, and it's not for the better. It's getting hotter.

The dry season is growing longer in places, and storms appear to be growing stronger. More lightning, more wind, more water stress, they all seem to be occurring. And there's expectations that that's going to increase into the future.

Giant and small trees will likely respond differently to these changes. That's because they live in different worlds. If you're a small tree... You live largely in this dark, damp, non-exposed space on the forest floor. On the other hand... If you're a giant tree, if you're sticking up in the canopy, your crown's fully exposed. So we expect that they are more affected by...

Wind, lightning, drought. Scientists can't be sure because they haven't yet collected enough data on giant trees.

Research on the growth and death of tropical trees usually takes place in square, one-hectare plots about the size of a baseball diamond. But to actually be able to understand the rates of giant tree mortality, how they vary over space and over time, you need hundreds, hundreds and hundreds of hectares. Because in a hectare, only about one giant tree dies every year.

And that's why Evan has brought along a drone to help him gather better intel. Our proposed solution is switching our approach from looking at the ground level to using a drone. So going from one hectare to 1,500 hectares. So 1,500 times increase in our sampling area.

Vanessa Rubio, a postdoc researcher with the project, hunches over a small plastic airplane sitting on the ground. With its six-foot wingspan, it looks like an oversized toy.

But it's a serious research tool. The drone's deadpan voice responds to commands from her laptop. Once the team is ready, the drone will fly autonomously in a grid pattern over the team's 1,500-hectare jungle plot, snapping thousands of overlapping pictures every month.

taking stock of giant trees when they're standing and capturing their absence. The drone wiggles its tiny rudder and elevator, but she doesn't launch it. They're only practicing today since they don't yet have the permits to fly it. When they begin flights, the team will scour the monthly hall of photos for holes that appear in the canopy between flights.

They'll hike out to each one to see if one or more giants fell over and investigate why. It's dawn and it's pouring. After breakfast, Evan and Adriani put on their long-sleeved shirts and pants to avoid mosquitoes and high boots to block snake bites and set off for the day into the forest.

We pass a giant tree that would take several people holding hands in a chain to wrap around its trunk. Most of the others are no thicker than my thigh. We descend into a valley, ford a stream, and ascend a mud-slick trail on the other side. I kind of thought that it wouldn't be quite as rainy as this. Right now we're going around. Just fell down. And now my pants are all covered with mud. But my equipment's okay.

The rain tapers off. Then they spy a giant trunk lying on the ground. Dozens of trees were crushed when it fell, cutting a gash in the otherwise continuous canopy. So what do you think? Wow, this tree with a lot of burned leaves on the top. Today they're honing the detective skills for determining why trees die. The signs of death by high wind, drought or fungal infection aren't always clear.

But the clue to the killer of this tree couldn't be more obvious. Right above their heads. So all the evidence suggests this tree was directly struck by lightning. The crown of the downed tree is defoliated. Its leaves have fallen off. He says that when a bolt of lightning strikes a tree, electricity leaps between branches to other crowns nearby.

The high voltage current damages neighboring trees, but only when they face and come in contact with the tree that was struck. Evan easily recognizes in the canopy here the pattern left by searing flows of electricity released by a stroke of lightning. The end of this branch is dead, only where it's near this tree.

completely defoliatd tree and the rest of the crown is healthy. Some forecasts predict climate change will increase lightning globally by 50% by the end of the century. Evan and Adriani say that within a few years their study should help to predict how lightning and other causes of death of giants might influence how much CO2 the Amazon might absorb in the future.

Their study could also help efforts to choose tree species most likely to survive when degraded forests are replanted. That story was reported by Daniel Grossman. His trip to the Amazon was made possible by the Pendleton-Mazer Family Fund and Abby Rockefeller and Lee Halprin. Patrick Veneer provided additional audio recordings.

We're talking about trees and what will help them survive and thrive. Coming up, scientific research turns into detective work to figure out why some ash trees seem to be immune to a devastating pest. That's an ash. It's definitely an ash. It's healthy. It's full of green leaves. It looks great. That's next on The Pulse.

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With so much global trade and travel, trees in the U.S. are frequently exposed to imported pests and pathogens, hundreds of them over the past century. And some of these pests have decimated forests and wiped out entire tree species. But in one instance, a destructive pest led researchers to a surprising discovery, one that could save many future trees.

Here is Dan Wanchora with a story from the Points North podcast.

One day, Kathleen Knight is walking through a forest in northwest Ohio. She's a research ecologist for the United States Forest Service. So she likes to say she gets paid to take walks like this. I have a moment almost every day in the field where I'm like, trees are awesome and amazing. They do so many amazing things. It's 2006, and all around her are these big, beautiful ash trees. You know, tall trunks with lush green canopies at the top. These trees are thriving.

She collects some data and heads out. Then, three years later, Kathleen is back at this same forest, but it's a completely different scene. And that was kind of our moment of shock, saying, wow, they're all dead. Hundreds and hundreds of dead ash trees, as far as the eye could see. Back then, this was happening to ash trees all over the Great Lakes region, all because of a small invasive bug called the emerald ash borer.

It was some really depressing research early on because we had all these beautiful ash forests, floodplains with huge ash trees, and then we watched all of these trees die. We watched those ecosystems change.

There are billions of ash trees all over North America. Ash trees filter groundwater. They give habitat to all sorts of plants, animals, and insects. Indigenous people depend on ash to make baskets. Ash is used for things like baseball bats and handles for tools. And to see it disappear so quickly and change so quickly, it is, it's heartbreaking. ♪

But Kathleen and her colleagues have a job to do. They gather a bunch of data from these dead ash trees. They measure their trunks, look for emerald ash borer exit holes, and then they pack up their gear and head back to their cars.

We were pretty quiet on the walk back out. And as we're walking across the bridge, I think it was one of our interns who spotted the tree. He was like, hey, is that an ash? And we all stop and look at this tree. And we're like, that's an ash. It's definitely an ash. It's healthy. It's full of green leaves. It looks great. It was really this moment of confusion. How is this tree possibly existing after what we've seen all day long here?

We knew it was big. We knew it was big to find a surviving tree. Now, they had to figure out why it survived. Because that answer might save an entire species from extinction. The emerald ash borer, or EAB for short, was first found in North America just outside Detroit, Michigan. It was more than 20 years ago, in 2002. This is a story of death and destruction.

It's about an invasion. That's a clip from a Detroit public television program. As you'll see at the end of this program, these trees are doomed. The little beetle arrived on wood packing material in cargo ships or airplanes from China. From there, it spread quickly. The bugs have wings and can fly short distances, but people are the main way they get around. Do you know what's in your firewood? Stop the spread of emerald ash borer. Don't move firewood.

Now, EAB is found in 36 states and 5 Canadian provinces. Its population hits ash trees in waves. Parts of the Great Lakes region are in a second wave right now. Kathleen has seen the effects of that here in central Ohio. She's got a tan Forest Service baseball hat on and a backpack with a water bottle and a can of bug spray sticking out of it.

She spots an ash tree and walks up to it. So this is one of our larger ones. It died too, and you can actually see the canopy of the dead tree. That's a very recently killed ash that died during the second wave. And I'm going to see if I can find some galleries here for you under the bark, if I can peel back. The real damage done by the emerald ash borer is done by its larvae. They bore just under the bark and make these squiggly-looking tunnels.

Kathleen pulls back a little bit of the bark. Oh, you can already see a few up here. So as the emerald ash borers create these tunnels, if you get enough emerald ash borers, they basically cut off that circulatory system and girdle the tree, and it can't get water and nutrients transported up to the canopy. And that kills the tree fairly quickly.

As EAB devastated ash trees in North America, researchers scrambled to find a solution. Early on, they tried cutting down large swaths of trees to create a sort of fire break to stop the spread. Then they tried insecticides. Those can work to save a small number of trees or maybe one in your yard, but not a whole forest. Later, the focus shifted to tiny wasps from Asia, which kill emerald ash borer eggs and larvae. They help, but don't completely get rid of EAB.

Overall, the outlook for ash trees in North America was still really dire. Which brings us back to that moment when Kathleen finds that lone surviving ash tree. She looks at it and wonders if she's staring at another possible solution. But for her to figure out how this tree survived, Kathleen has to find more of them. So in 2010, she puts together this team, and they go on a mission to see if they can find more of these surviving ash trees.

And they do. One of the people Kathleen is most excited to tell is Jennifer Cook, a geneticist with the U.S. Forest Service. Jennifer has been working with ash trees for so long, she's now allergic to them. It's like poison ivy to me. Oh, wow. That bad, huh? Yeah. Jennifer is based out of a research station in central Ohio.

When she hears about these surviving ash trees that Kathleen found, she's pretty skeptical. Because of the data that was being reported, we didn't believe, we were buying into the, yeah, nothing is going to survive, there's no resistance, because so many other scientists were saying that until we saw Kathleen's field data and the pictures that she took of the healthy trees that she found. That's when Jennifer starts thinking, they might actually be onto something here.

The emerald ash borer is really good at finding mature ash. It's not like the bugs are just missing certain trees. So Jennifer and her team come up with a hypothesis. And that is, some ash trees in North America have a genetic resistance to the emerald ash borer. Basically, the ability to fight off EAB. But Jennifer says that's just the hypothesis. They need more data. We're scientists, so we're skeptical.

They come up with a term for these trees that's cautious but hopeful. Lingering ash. It's kind of like, you know, that last person that leaves the party. They just linger. And you're just like, well, what are you doing here still? Kathleen Knight keeps looking for even more of these lingering ash trees. She finds them in parts of Ohio and Michigan and takes branches from these trees back to Jennifer Cook's lab.

There, Jennifer clones them and runs tests to see if there's genetic resistance. And she discovers something amazing. These trees not only have resistance to the emerald ash borer, they actually kill EAB larvae. Scientists really didn't see that coming. The mantra at the time was no co-evolution, no resistance. Meaning that since this insect was from a whole other continent and our trees didn't

grow up exposed to them that they didn't evolve any sort of mechanisms to defend themselves. Scientists don't know exactly what genetic trait gives these ash trees resistance to EAB, but they do know the resistance is on a spectrum. Some trees have more of it than others.

But in order to prove their hypothesis, Jennifer still has to find out if the genetic resistance is passed down from parent to offspring. Because that is the key piece of information you have to have to know that breeding is actually going to work. So Jennifer and her team clone a bunch of different lingering ash. They crossbreed them in the lab and then wait for them to produce their own seedlings.

Not only do they get seedlings that are resistant to this bug, they get seedlings that are more resistant than their parents. So now we're really starting to get excited. I shouldn't say starting to get excited, but now we're convinced. We're close enough to being convinced. And that means their hypothesis is right. Resistance to the emerald ash borer is genetic in some ash trees in North America.

Then comes the big task: actually saving the trees. Jennifer needs to create an entire orchard of resistant ash to prove her lab results out in the field. Her team will harvest the seeds from the most resistant trees and grow them into seedlings. Eventually, those seedlings will be planted in forests across the region. Jennifer says that could happen in the next decade.

This entire process can be replicated, helping spread resistant ash all across the continent. We're working with trees from Ohio and Michigan, so I can't take those trees and the resistance seed they produce and plant them all the way down to Mississippi. They won't be adapted to grow there. So we have to do what we're doing over and over to make seed orchards for each separate region.

Back in a forest in central Ohio, Kathleen Knight, the ecologist, continues to look for lingering ash today. She spots a couple trees and walks up to them. More than half of the canopy looks great. And then there's another tree right here to our left that also has a good healthy canopy. So these are two that I'm hoping will make it through the second wave and continue to survive.

Kathleen is quick to point out, in order to save the species across North America, they can't search for lingering ash alone. We need other people out there who are in the forests, keeping an eye open, looking for large, surviving, healthy ash trees. Because there's not enough of us to survey every forest. And there are a lot of important trees out there that could be missed if people aren't watching for them.

One way to do that is with an app called TreeSnap. Anyone can use it to report what they think might be a surviving ash. Scientists like Kathleen can then see that info and use it in their research. Despite it sometimes feeling like an uphill battle, Kathleen is optimistic about the future of ash trees. I mean, that's what gets me out of bed every day is that the work is really important. We're literally working to save trees. And not just the ash tree. Other species, too.

Because more catastrophic pests are going to come along. And now scientists will know. Look for genetic resistance. And those trees could also be saved. That was Dan Ventura, host of the Points North podcast from Interlochen Public Radio. Points North is about the land, water, and inhabitants of the Great Lakes. We'll put a link to their episodes on our website.

Coming up, a changing climate means a new cast of characters when it comes to tree planting. We've got some interesting things like hardy citrus. We've got a pineapple guava. We've planted a hardy variety of banana called the dwarf orinoco. That's still to come on The Pulse.

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This is The Pulse. I'm Maiken Scott. We're talking about trees and what helps them survive and thrive. There's been a run on planting trees for years now as one potential solution to climate change, to re-green the Earth, and to sequester more carbon. This idea took off especially after a study found that the Earth used to support 6 trillion trees before humans arrived.

And now, we're down to about half of that. Planting trees feels hopeful. It's a tangible, doable solution to what can otherwise seem like an overwhelming problem.

But as many failed tree planting efforts around the globe have shown, it's not as simple as it seems. Planting a tree is the first step and it really requires communities and local communities to support these trees into the future and for keeping trees to be, you know, economically viable. That's conservationist Lauren E. Oaks.

To understand the hard work that goes into planting and growing trees, Lauren interviewed the scientists and workers behind forest restoration projects. She features them in her new book, Treekeepers, The Race for a Forested Future. I have hope that we can do this work better and we can deliver projects that offer a diversity of benefits that will benefit the climate system.

but they'll also help people and all life on this planet into the future. Lauren says it's one thing to plant fast-growing trees to sequester carbon. But that's really different than if you're targeting holistic restoration and trying to bring back a diversity of species and let things, you know, rewild, if you will, or go wild again.

And those kinds of things also take a lot more time then. For sure, yes. Which is some of the critiques, you know, particularly for those that are interested in sequestering carbon on a quick basis, which we all are, whether it's through trees or other mechanisms. Those are some of the critiques that some of the old growth, you know, slower growing native species might get. So, for example, you can look at an oak species. Yes, it will sequester carbon quite well over a long life.

but it's going to grow slower than a spruce or a polonia for sure. You know, the best planting is supported locally, is tailored to the local context and

is its own model. And that's not appealing, you know, if you're thinking at a global scale, how do we green or how do we increase our forest cover? People want one solution that can be applied in many different places. Lauren saw a successful forest restoration project in action when she visited the Azuero Peninsula in Panama.

The land had been used for cattle ranching for a long time. But decades of deforestation brought challenges like drought, soil degradation and biodiversity loss. And the productivity of the land has gone down. So they're just not getting the return on cattle that they have in the past. And so people in the area wanted and needed another route. And I

I was just amazed to see the level of engagement. There were women growing, they called them backyard nurseries, growing seedlings in their backyards and taking care of the little seedlings that they would bring back to the mother nursery to be planted. They were targeting massive reforestation across the peninsula. They had private landowners, so it's not just one big tract of land. It was many landowners coming together to participate. So there was this aspect of...

community creating a corridor. I was just really amazed at the diversity of species they were growing. They were collecting seed from mother trees throughout the region that they could find in the existing forest.

And it felt to me like a project that would, yes, deliver on carbon and carbon credits into the future, but, you know, was also bringing in employment and opportunities to women in the communities. And what kind of changes could you see in the landscape? We went out to a number of places that had planted recently, and you could see a diversity of tree species growing, and they were young and, yes, hopeful, and, you know, you wonder, will they all survive? But the places we visited, they were doing really well, and...

showing a promise of a connected habitat in the future. Lauren says it will be really important to collect and save seeds from trees around the world to ensure that we have the right trees to plant in the right places. We have a history of planting 100 or so common species often used in timber plantations, but if you're thinking about restoring diversity across the planet, we need to be collecting seed from trees

wild sources and from this other 70,000 plus tree species that are around the world. Lauren E. Oaks is a conservation scientist and science writer. Her new book is Treekeepers, The Race for a Forested Future. Whether you're picking a tree to grow in your yard or taking part in a whole restoration project, you have to choose the right tree for your climate zone.

On a map, the North American climate zones look like wide bands, ranging in colors from red for hot and humid to yellow, green and blue for cooler temperatures. But these zones are changing as temperatures are getting warmer, which is causing a shift in what kinds of trees can grow where. And now some growers have a whole lot of new options. Susan Phillips reports.

Sitting along the Schuylkill River in West Philadelphia is the Woodlands, an arboretum-turned-cemetery that dates back to colonial times. Some of the trees here are the largest of their species in Pennsylvania, including a London Plain and a Black Oak. Other centenarian giants have succumbed to illness, like Dutch Elm Disease.

But in a far corner of the property, an area where there are no gravestones, urban farmers are working to climate-proof the city for a warmer future. One that could see bananas growing in Philadelphia.

Phil Forsyth takes me inside what is called a high tunnel. It's an unheated greenhouse. It's only using the sun's energy. It's really a very simple structure. It's a series of metal hoops with some plastic over it. And so in the winter months, sun comes through the plastic, captures that heat and keeps it a little bit warmer. And so that's how we're just experimenting to go one zone further south. One zone further south refers to zone H. One

one of more than a dozen plant hardiness zones determined by the U.S. Department of Agriculture. Philadelphia currently lies in 7B. But Phil says that's changing.

Zone 8 could be right around the corner. It currently spans from coastal Virginia to central Texas and includes South Carolina, Georgia, and Mississippi. It's got hot summers, mild winters, and a much longer growing season than Philadelphia has traditionally had. So this is our Zone 8 food forest growing inside our high tunnel greenhouse structure here. And the idea is to

grow plants that are not quite viable outdoors, at least not yet. We've got some interesting things like hardy citrus, yuzu here. Behind you is an olive tree. Believe it or not, those are also zone 8 crop.

So we can grow both Mediterranean and somewhat subtropical things in these conditions. A Korean tea plant over here. This is a loquat tree. It's beautiful. We've got a pineapple guava. We've planted a hardy variety of banana called the dwarf orinoco. In addition to being in the structure, we'll wrap in the winter like we would a fig tree. So it'll have...

fencing and fall leaves wrapped around it for insulation and see if we can get some bananas growing here. You heard that right. Bananas growing in Philadelphia.

And then behind us here, we've got some more hardy citrus, kumquat, citramello. New options are coming in, but some of the trees that have traditionally flourished in the region may no longer make it. Phil says, for example, when it comes to apples and pears, they may not get the long, cold winters they need.

That's a big problem for a state like Pennsylvania, which ranks fourth in terms of apple production. Because we're all struggling with these very extreme changes in the weather. Daniel Weber is a tree fruit expert and educator with Penn State Extension Service in the south central part of the state. And it goes way beyond just the chilling requirements and whether or not the trees actually have enough fruit.

It's as subtle as apple trees being grafted. The rootstock is actually a completely separate apple tree. The roots are a completely different tree versus what you have on top, which produces the varieties we actually enjoy and eat, like a honey crisp. Daniel says all fruit trees have a top part that produces the fruit we like to eat and is grafted onto a rootstock that is sturdy and adapted to grow best in that particular soil and environment.

In the past, this didn't cause a problem. But with warming winters or an earlier spring, the rootstock may not be in sync with the top of the tree. The variety may come out of dormancy, may have met its chilling requirements long before the rootstock does. And that tree is now active. It's growing. It's attempting to flower. It's attempting to push out its first leaves.

And the roots aren't responding because it is still dormant. He says some believe this is what has led to what they are calling apple decline. Where entire blocks, entire orchards of trees just melt down.

After flowering, leaves wither, flowers turn brown, the fruit drops off and the tree dies. Or at best, it's just extremely stressed. Daniel says he's excited to learn how the Philly Orchard Project's experiments work out. And he says those who experiment are likely to be the most successful in the future. But he does have a warning.

The pests and diseases that thrive in warmer areas will also move north. So there will be new challenges. I think I'm pretty safe in saying there's nobody in Pennsylvania who knows how to deal with diseases of kumquats. For now, growers are trying to figure out which species will survive and which will not in this warming world. For The Pulse, I'm Susan Phillips.

That's our show for this week. The Pulse is a production of WHYY in Philadelphia, made possible with support from our founding sponsor, the Sutherland Family, and the Commonwealth Fund. You can follow us wherever you get your podcasts. Our health and science reporters are Alan Yu and Liz Tong. Our intern is Julianne Koch. Charlie Kyer is our engineer. Our producers are Nicole Curry and Lindsay Lazarski. I

I'm Maiken Scott. Thank you for listening.

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