Vines Choking Out Trees in the Tropics

It's a rivalry as old as forests themselves: the ancient battle between trees and their competitors, the vines. But now, ecologists say, the vines are winning. Bill Laurance, of Australia's James Cook University, says increased forest fragmentation and a boost in carbon dioxide may be contributing to the vines' domination.

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JOHN DANKOSKY, HOST:

This is SCIENCE FRIDAY. I'm John Dankosky.

If you've ever walked through the jungle, you'll know it can be surprisingly dark down on the forest floor. You see trees soaring up all around. You're creating a dense canopy overhead. And climbing toward that canopy, snaking up the trees are the vines.

Now it may seem peaceful in there, but what you're witnessing in very slow motion is a fight to the death: a fight between the trees and their old rivals, the vines. It's a battle as old as the forests themselves. Now, scientists say the vines are winning. But they're not certain why. Is that a bad thing? Is there anything we can do about it?

Hopefully, Bill Laurance can tell us. He's a distinguished research professor at the Centre for Tropical Environmental and Sustainability Science at James Cook University in Cairns, Australia. Welcome back to SCIENCE FRIDAY, Dr. Laurance.

DR. BILL LAURANCE: Thanks, John.

DANKOSKY: So the vines are winning, huh? The tables have turned in this old fight.

LAURANCE: It seems so. There's an increasing amount of evidence which really does seem to indicate that the war, this sort of ancient war, is shifting more and more in favor of the vines for a few reasons that we think.

DANKOSKY: OK. So what are some of those reasons? Any theories as to why the vines are starting to take over?

LAURANCE: Some of the things we're pretty sure. For instance, we know that when we log forests, when we fragment forests, this really favors vines. They love disturbance. They love having forest edges. They love having small trees that they can use as trellises to climb up into the canopy. And, of course, everywhere in the world, practically we're fragmenting and degrading and logging forests. On top of that, we're introducing vines into places where they don't belong.

So there's things like kudzu, which is, you know, expanding rapidly across the southeastern U.S., increasing at a rate of about 150,000 acres a year. There's rubber vine here in Australia. There's all kinds of foreign vines. And when you introduce one of these species into a new place, it often escapes its natural enemies, its natural parasites and diseases and competitors, and it can just sort of go nuts. So that's one thing - another thing that's happening.

And then perhaps most mysteriously, we're seeing vines increasing in completely intact, undisturbed forests, and that's one of the big mysteries out there as to just why we're seeing vines increasing in places where we really - they really shouldn't be increasing.

DANKOSKY: Hmm. If you have questions about this battle between the vines and the trees, 1-800-989-8255. That's 1-800-989-TALK.

You talked about the vines obviously climbing up the trees toward the canopy. Don't the vines need the trees? If they kill the trees, they can't climb up to the canopy. I mean, it seems as though there should be a balance.

LAURANCE: Well, the vines really do - they're really serious parasites on trees so that they suppress tree growth. They reduce the fecundity of trees. And trees that are infested with vines die a lot more rapidly. In fact, what some of the vines can do - like the lianas, the woody vines in rainforests, is they'll actually move from one tree to the next, and when one tree falls, it'll oftentimes drag other trees down with it.

In fact, the loggers hate these vines because - they call them widow makers because they can cut one tree down, and then the vines will actually yank a tree right down on top of their heads. In areas where you get heavy vine infestations, you can get very suppressed forests. I mean, the forest is stunted. It's low. It can be completely, in some cases, overgrown by vines. And, you know, it's not a healthy forest when you get that kind of situation.

DANKOSKY: But I guess that's my point. I mean, most parasites are relying on a host, right? If the host dies, the parasite maybe wins that little war. But it's got no place to live. If the vines kill all the trees, I don't know, what happens to the vines?

LAURANCE: Yeah. It's probably not a question of them completely killing all the trees that's sort of shifting the forest towards a degraded, stunted forest where the natural regeneration is suppressed, where the, you know, the biology of the forest is shifted into some unnatural and unusual direction. From our human perspective, there's a few reasons why we'd be worried about that. One, of course, is that we do want these forests to have some kind of natural function to maintain their normal species diversity and that type of thing.

But also, the trees in the forests lock up incredible quantities of carbon, billions of tons of carbon. And when you start killing the trees and suppressing their growth, in effect, what happens over time is quite a lot of that carbon can be released into the atmosphere. One of the prevailing theories about why we're seeing vines increasing in the intact forest is, in fact, the rising levels of carbon dioxide in the atmosphere, of course as a result of humans spewing tons of greenhouse gas, billions of tons of greenhouse gases every year.

Vines, it turns out, really seem to like the high levels of CO2. It basically fertilizes them. It gives them a competitive advantage over trees. Another thing that's happening is that the dynamics of even intact forests seem to be changing when higher levels - with higher levels of CO2. Trees themselves are also growing faster. Not as fast as vines, but when there's faster tree growth, you get more competition among trees.

LAURANCE: More trees have to die because they're competing for light and water and nutrients. And so the whole forest becomes more dynamic. It starts turning over more rapidly. And vines love that. They love forest disturbance. They love the situation where there's lots of trees dying and new treefall gaps being created. So it seems like the global phenomena that we're creating right now are sort of shifting this ancient competitive balance in the favor of vines.

DANKOSKY: The vines like the carbon dioxide, but they're not as efficient in storing that carbon dioxide as the trees.

LAURANCE: No. That's exactly right. So in fact, it's - the vines themselves store just a tiny fraction of the carbon that's - that would be stored in the trees that they're killing or suppressing. So we've been working on vines in the Amazon for quite a long time. And what happens to the forest when it's fragmented? The vines go crazy. Many of the trees die. And there's only - the vines themselves replace only about one or two percent of the carbon that's lost as a result of this large-scale tree death that we see in fragmented forests.

DANKOSKY: Aside from what it means for the trees, what does this rise of the vines mean for other species in the forests?

LAURANCE: Yeah. That's a good question. We really don't know - yet. I mean vines are a natural part of the forest ecosystem, but what we're seeing is this kind of shift in this, you know, archaic competitive balance. And that really is a good question. We know that when we walk around and we see these disturbed forests that the vines can take over. And you see this, you know, it's something that, to an ecologist, would not be considered a healthy forest. But there are some things that feed on vines, and vines produce fruits and that type of thing. So there certainly would be some species that would be favored.

But remember also, you know, these trees, they're the ones that are investing in the wood, and they're the ones that are creating the tree hollows and huge flushes of fruit and leaves that are producing a lot of the food and food resources for the forest. So basically, we're taking away or we're reducing the effects of those trees. And really, those are the ones that are probably housing and sustaining the majority of the forest biodiversity.

DANKOSKY: Jason is calling from Washington, D.C. Hi, Jason.

JASON: Hey. How are you, guys today?

DANKOSKY: Good. What's on your mind?

JASON: So I'm curious, you mentioned that when the vines are moved from their natural habitat they lose predators. So could you intentionally introduce them either to combat the sort of parasitic vines or even maybe vines that cause rashes for humans, that are more pest-related?

LAURANCE: Yes. There are cases where people have introduced a species that would - for instance, the Opuntia cactus, which is going crazy here in Australia. They've actually introduced a moth that, in its natural range, feeds on that cactus, and it has helped to control it. But it's a double-edged sword, and you have to be pretty careful with this kind of biological control. Another dismal example is the cane toad, which has gone crazy here in Australia. It was introduced to help control a cane beetle.

And, in fact, the toad doesn't eat the beetle, but it's gone completely crazy. The toad is poisonous, and it's spread all over Australia, so - and killing a lot of native wildlife as a result. So it's kind of a dangerous tool. One can potentially do this type of thing, but it's considered a fairly high-risk strategy in a lot of cases.

DANKOSKY: Yeah. That - it didn't sound like that went too well. I'm wondering, are there any thoughts, though, about how to stop this, how to get rid of all these vines before they do choke out the forests?

LAURANCE: Well, for the exotic things, it's tough. I mean the problem with the exotics, and for instance in the United States there's about 150 species of foreign vines that are invading the U.S. right now. People often don't realize that. The problem is, once the genie is out of the bottle, it really is difficult to control them. In most of these cases, when you start to get an exotic species becoming established, you really have to jump in and almost nuke it immediately. And if you don't, if you let it get to the point where it's spreading, you can just invest millions and millions of dollars, and oftentimes it's impossible to control it at that point.

DANKOSKY: John's in Glastonbury, Connecticut. John, you get rid of vines for a living?

JOHN: Yeah. Well, yeah. That's something that I've been working on for a while. I'm a member of the Connecticut Invasive Plant Working Group, loosely affiliated with them. And there was - for a time, I was quite active and had put a lot of thought into fighting a problem that we have here in Connecticut, right along the Putnam Bridge in particular, all northeast - northeastern, southeastern Connecticut is bittersweet - Oriental bittersweet. And people that don't realize it look and say, oh, look how green, look how lush the side of the road is.

But they don't realize that if you look closer, those are not maple leaves, those are not normal fauna that is indigenous to the area. And, yeah, bittersweet is a real problem. In fact, I had a couple of mock business cards put up trying to - how to combat it better than the Saturday Cub Scout working groups that sometimes get together, and I had some clever phrases on business cards. I'm like, what do you think of this (unintelligible)...

DANKOSKY: Hey, John, I'm going to let you go 'cause we're breaking up your line there. But it seems as though, Bill, this is happening in America. It's happening in the tropical forests as well. In America we have all these invasive species that, again, people maybe aren't taking all that seriously. But they are taking over roadsides all across the United States.

LAURANCE: Yeah. I mean there's, you know, if you go online and you look at pictures of kudzu, it's an amazing - it's incredible, the rate at which it grows. And there's pictures of houses and entire forests. And I mean, I practically think in some cases if, you know, you tie up your pet and forget to check on it right away, you may come back and find it completely colonized by kudzu. There's a photo going online, viral right now, of a car in China that somebody parked and it was completely overtaken by a vine. And they're - somehow now they're actually driving it around. The vines are still alive on this car. So these vines can be amazingly aggressive and fast growers.

DANKOSKY: Let's go to Michael in Detroit. Hi, Michael, what's on your mind?

MICHAEL: Hi. Thanks for taking my call. I had a question about defense mechanisms for trees against vines. I was in New Zealand last year and actually heard about a tree that will shed its bark to actually combat vines. Are there any other defense mechanisms that trees have?

LAURANCE: The bark shedding is one of the most common strategies. Some of the trees will also prune the limbs. When a vine starts to wrap around the limb, they'll actually shed those limbs. There's some ideas that there may be some kinds of toxic interactions going on in certain species of trees. And then, also, if you get into some of the epiphytes, some of the plants that grow up on tops of trees, like some of the orchids and basket ferns and things, it turns out then, in some places in the world, these epiphytes have cooperative or mutualistic relationships with ant species, with very aggressive ants.

And if a vine or a liana tries to grow up, you know, close to that epiphyte, the ants will attack it and kill it. So there are some mechanisms there, and there's probably things that we don't really understand very well, particularly in the tropical rainforest, where there's still many, many mysteries to be solved. But it's clear that this is a war that's been being waged for millions and millions of years. And so I'm sure the trees have an array of different kinds of defenses. But as we say, the balance seems to be shifting right now in favor of the vines.

DANKOSKY: We're talking about the battle between the vines and the trees. I'm John Dankosky and this is SCIENCE FRIDAY from NPR. So aside from vines, in the big picture how are things looking for the tropical forests that you study? We've been hearing about tropical rainforests being degraded for years. How's it going right now?

LAURANCE: Well, it's still happening, unfortunately. The rate of forest destruction's probably dropped down a little bit in the last 10 years, and there's been some shifts. For instance, in places like Brazil, rates of forest destruction have come down a bit. But in other places like Indonesia and the Congo Basin, we're seeing big increases. The bottom line is the current rate of forest destruction is about 10 million hectares a year, which is about 40 football fields a minute. So we're losing a lot of tropical forest. And also a lot of what remains is oftentimes fragmented and degraded in various ways. So we're really changing the world's tropical forests very rapidly.

DANKOSKY: And as you said before in talking about why the vines are rising, that breaking up of the forest into little tiny sections, the more of that that happens, the more forest edges, the more the vines can take over and the more other problems there are.

LAURANCE: Absolutely. The fragmentation is just an enormous environmental threat to forests. And absolutely, the vines just love that. These edges that are created, they just become smothered in vines in many cases. And yes, it's one of the most dramatic - it's certainly not the only manifestation of forest fragmentation, but it's one of the most obvious things to an ecologist.

DANKOSKY: Now, you had a paper in the Journal of Science a few weeks ago about habitat loss and species diversity. Tell us what you found.

LAURANCE: Well, we were studying forest fragments in Thailand, and actually these were little islands that were created when a reservoir was made there about 25 years ago. And some of the islands were up to about 150 acres in size. So we had - several of my colleagues studied this process of what was happening to the native fauna as these islands were being created. And it was really striking. We expected to see things like the large animals, like the tigers and elephants and some of the deer, disappear. But there was actually a very rich native, smaller mammal fauna there, and it included all kinds of (unintelligible) cool animals.

There's a very diverse assemblage of squirrels and rodents. There's tree shrews. And these very unusual animals called moonrats, which are not rats at all, but they're sort of like a gigantic hedgehog that doesn't have spines. Anyway, so you have this very rich and diverse small mammal fauna, and we actually thought they would probably be able to persist in these fragments. But in fact, what we really saw was just a scenario of kind of ecological death. One colleague went out there and in the first five years found that most of the native species disappeared within the first - within smaller forest fragments up to about 20 or 25 acres in size.

And then another colleague went out there about 20 years later and found just everything, virtually, had disappeared. There were almost no natives left at all. And what had taken over was this exotic invader which was the Malayan field rat, which is sort of a lot like the black rat, which has been introduced to different parts of the world, and has killed off a lot of birds on islands, things like that. So this opportunistic invader, which is kind of a walking garbage can that will eat anything, had absolutely proliferated in incredible numbers on these islands. And all the natives, virtually, had completely disappeared. It was sort of an ecological Armageddon. Really, some of the worst fragmentation impacts that we've seen.

DANKOSKY: We just have a few seconds left, but I guess I'm wondering, is there anything that people in developed countries can do, as they hear your story, to avoid being part of this problem, to avoid contributing to this deforestation?

LAURANCE: I think you can join some of the good conservation organizations out there. There's a number of good ones that are really working to do things. Of course you can try to support legislation and laws, and try not to buy products that are contributing to forest destruction, like some of the timbers that are coming out of tropical regions, that type of thing. But I think the best way is to join some of the good conservation groups. They will keep you informed about the best strategies and the best things you can contribute as an individual.

DANKOSKY: Bill Laurance is a distinguished research professor at the Centre for Tropical Environmental and Sustainability Science at James Cook University in Cairns, Australia. Thank you so much for joining me today.

LAURANCE: Thanks, John.

DANKOSKY: B.J. Leiderman composed our theme music and we had help today from NPR librarian Kee Malesky. If you missed any part of this program, or would like to hear it again, just subscribe to our podcast, audio and video, on iTunes and Android apps. You can point your tablet or smartphone to our website, sciencefriday.com, or you can join our mailing list. If you like our show, well, then like us on Facebook and continue a week-long conversation @SCIFRI. You can also email us, the address is scifri@sciencefriday.com.

In New York, I'm John Dankosky.

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