JOHN DANKOSKY: This is Science Friday. I’m John Dankosky, I’m in for Ira Flatow this week. Big trees are incredibly important to forest ecosystems, capturing carbon, housing animals, propelling new tree growth, and providing much-needed shade. But when I say big trees you might be thinking about the biggest trees, giant sequoias or redwoods, trees that can grow to roughly 25 stories tall. Well, big trees are an essential part of every forest. These are considered the top 1% of trees in each forest ecosystem, and they play an integral role in preserving forests all over the world.

To help us better understand the wonderful world of big trees is ecologist Jim Lutz. He’s an associate professor of forest ecology at Utah State University in Logan, Utah. He also manages three Forest Dynamics plots in the American West through the Smithsonian Network. Jim Lutz, welcome to Science Friday. Thanks so much for being here. And thanks for talking about big trees with us.

JIM LUTZ: Well, John, thanks for having me.

JOHN DANKOSKY: Let’s start off with some of the basics here, when we’re talking about a big tree, what exactly are we talking about from your perspective?

JIM LUTZ: Well, when we look at forests around the world, or even around the country, we have trees of different sizes. Every forest actually has big trees. A good guideline for where a big tree starts would be about two feet in diameter but that’s not really always a great comparison. The best comparison is with the trees that are around it. And really a big tree might be the ones that are the biggest 1% in any forest.

JOHN DANKOSKY: So how old are the oldest big trees or does it vary widely by the type of tree?

JIM LUTZ: Some of the very long-lived species, like the giant sequoia or Douglas fir, or even some oaks, can live hundreds or thousands of years, whereas some big trees perhaps in Eastern US forests might only be 100 years old or 150.

JOHN DANKOSKY: Why is it that some trees get much, much bigger, while their neighbors of the same species stay relatively small?

JIM LUTZ: Well, this is what’s really important and why big trees are important in every forest. So when trees start to grow, when trees are very small, maybe they have sort of equal access to sunlight, or to water, or to resources but over time, some trees become winners and some trees become losers. And once trees have fallen behind in the competition game, they sort of never can really catch up.

JOHN DANKOSKY: So the bigger trees just keep getting bigger and they’re crowding out the smaller trees around them, they’re able to take in more of the stuff that allows them to just keep surviving and thriving?

JIM LUTZ: Right. In some forests, the limiting resource is light and so trees that are bigger are also taller. In some forests, the limitation is water or nutrients, and big trees have bigger root systems which allow them to compete both above ground and below ground.

JOHN DANKOSKY: We talked earlier about why big trees are so important to forest ecosystems. Maybe you can just flesh that out a little bit more for us Jim, I mean, what is it that makes them so important to ecosystems?

JIM LUTZ: So in terms of the tree population, big trees are proven. They’re proven successful. So in their genes is an adaptation for conditions that we see now and in the recent past. Also, big trees, they’re very important to the future of the forest because they’re producing many, many more seeds than those around them. One function that is really getting a lot of attention these days is carbon sequestration, how much carbon the trees can hold. And big trees really hold disproportionately more of the carbon, primarily because well, the carbon is mostly in wood but a big tree is often also a tall tree, so it’s occupying space in the forest that small trees just cannot be in.

JOHN DANKOSKY: So from a standpoint of carbon sequestration, if you have one really big oak tree, it’s going to perform better in terms of sequestering carbon than 10 or 15 little tiny oak trees?

JIM LUTZ: Absolutely. If we look at big trees in forests all around the world, from California to Cameroon, Michigan, to Malaysia, the biggest 1% of trees hold 50% of the biomass in those forests.

JOHN DANKOSKY: Another really interesting thing about big trees is what an excellent habitat they are for animals. And I know that you’ve looked at any number of birds and different types of mammals that live inside of trees. There are actually bats that live inside the bark of Douglas fir trees. I mean, there’s some amazing things the trees are able to do in terms of providing housing for animals.

JIM LUTZ: Big trees when they’re living, provide a lot of habitat. And then after big trees die, they become big snags, dead trees, and then when they fall they become big logs. Now, this is maybe oversimplistic but to make a big log on the forest floor it takes a big tree first. And sometimes the habitat just can’t be equaled by small trees. For example, if we take a large bird like an eagle, and eagle nests are very, very large, it needs a tremendous tree just to be able to support a nest like that.

JOHN DANKOSKY: So much of the world’s ecosystems are in peril right now because of climate change and other human-caused factors. What is the state of the big tree in America or globally right now, Jim?

JIM LUTZ: Well, it’s a mixed bag. The important thing to remember about a big tree is it takes a long time for a tree to get big. So what that means is over that long period of time, one century, two century, or three centuries, conditions need to be conducive to tree survival and growth.

So if we’re in a situation where conditions are changing very, very rapidly, the trees that are big right now might be having a harder time living in new conditions. And if trees can establish in those new conditions, it might take them centuries to, in turn, get really big. So around the world, we have been seeing decreases in the amount of big trees. A lot of that’s because of human activity, either logging or land use conversion but some of it is also due to changes in climate that are just making it a little harder for big trees to survive.

JOHN DANKOSKY: Yeah, those changes in climate include everything from a severe drought to increased risk of fire, if you have more wind events, you end up knocking over branches. It seems as though it’s a pretty hard time to be a big tree in a forest right now because of all of these factors.

JIM LUTZ: It is. Especially in the West where we’ve been having a lot of very large, high severity fires, it’s been a tough time for the big trees. In the past, when forests were a little less dense and fires were smaller and less severe, big trees could survive those fires. Now in the West, we’re seeing a lot of fires that are so hot that even the very largest trees are dying.

JOHN DANKOSKY: And you’ve been studying the effect of big wildfires on some of the big trees in places like Yosemite National Park. Maybe you can tell us a bit about how the aftermath of these fires have compared to how big trees have fared in the past?

JIM LUTZ: Now we had a fire in our large Forest Dynamics plot in Yosemite. When I say large, about 70 acres with 35,000 trees that we’re tracking. And the fire that burned through that forest in 2013, it wasn’t a big severe patch that’s killing a lot of trees, and in fact, when fire came through that forest most of the big trees survived. Unfortunately, after 2013 we had a very, very severe drought people say, a millennial drought for California, one we haven’t had for 500 or 1,000 years. And that drought itself weakened the trees, especially coming after the fire, and the drought or the drought in conjunction with beetle attacks did kill a lot of the larger trees.

JOHN DANKOSKY: So much of our conversation, and this isn’t really surprising at a time of climate change, has been about the threats to big trees, but this is something that you study and you’ve made your life’s work, I’d love to talk to you for the last couple of minutes about just why you love these trees so much, what is it that draws you to the biggest of the big trees?

JIM LUTZ: It’s perhaps a little bit more than just purely scientific. When we walk out into a forest we feel maybe awe in the presence of these large trees, and we can find a connection with the world at large. And I think the large trees, because they’re so majestic and because there’s so much history. We know that they’re so old, they’ve seen so much.

JOHN DANKOSKY: I love that idea too of just the amount of history that is imbued in these trees. When I’ve been to redwood forests and you put your hand on a redwood that you know that thousands of people over maybe a thousand of years have seen and touched, that is really a remarkable thing for something that is living, it’s just a different time scale than us kind of puny humans are used to.

JIM LUTZ: People all around the world have found this connection with big old trees. And it is the case that trees are one of the few things, the few living things that live so much longer than we do. For centuries, even millennia in some cases, and that’s what makes it very, very hard to study forests, because our lifespan, our lifespan as people, or as researchers is only such a small fraction of the lifespan of these big trees, that to first approximation it looks like they never change.

If we go to the forest and we go next year, the tree may still be there and appearing exactly the same. But really there is change. And if we watch enough trees, tens of thousands of trees, over long enough periods of time, maybe we can understand these long-term trends and where the forests are going.

JOHN DANKOSKY: I appreciate you spending some time with us today just talking about these majestic trees. I want to thank Jim Lutz, associate professor of forest ecology at Utah State University. He’s based in Logan, Utah. Thanks so much, Jim.

JIM LUTZ: And Thank you, John.

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