Autumn Leaves: Just Pretty or Poisonous Too? Most people look at forests in the fall and see a beautiful panorama. But Erik Nilsen looks at them and sees acts of chemical warfare. Nilsen, a plant ecologist at Virginia Tech, is one of a growing number of botanists who think brilliantly colored autumn leaves are full of chemicals that poison other plants and trees.

Autumn Leaves: Just Pretty or Poisonous Too?

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MICHELE NORRIS, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

MELISSA BLOCK, host:

And I'm Melissa Block.

It is autumn, and in much of the country leaves are changing color and falling. But if you think that's a peaceful event, you're not thinking like a tree. In recent years research has shown that the colorful autumn leaves that fall from some trees make life for other trees miserable. That's because these leaves are often full of chemicals that can be deadly to other plants. To find out more, NPR's John Nielsen met a botanist on Virginia's Skyline Drive for a walk in the woods.

JOHN NIELSEN reporting:

Most of the people who drive up into the Shenandoah National Park in the fall to look at the leaves, to look at the beautiful red and yellow changing leaves on the trees, think, `Jeez, what a beautiful, idyllic sight.' Eric Nilsson, a botanist at Virginia State University, sees that, but he sees other things, like, say, chemical warfare.

Mr. ERIC NILSSON (Virginia State University): Oh, yeah, definitely chemical warfare.

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NIELSEN: For example, take that great big maple tree just ahead of this short path into the woods. The leaves are so red that it almost looks like the tree has burst into flames. `Sure, it's pretty,' Nilsson says, but leaves like this can be very bad news for competing trees.

Mr. NILSSON: They have inside of them some chemicals that develop right at the end of the year called anthocyans that may be very toxic to other plants around them.

NIELSEN: As Nilsson talks, he tromps down a path that leads us into a forest full of maple, black walnut and oak, trees that fight endlessly for space, light, nutrients and water. This is nature red in tooth and claw, he says, or at least in leaf and root.

Mr. NILSSON: Oh, it's definitely a struggle, and over time a lot of these trees are going to die out because other trees are going to overtop them and outcompete them below ground and kill them off.

NIELSEN: Nilsson studies the role toxic chemicals play in this struggle to the death; more specifically, he studies the poisons found in colorful autumn leaves. Until recently, this was a field obscure to the point of non-existence, but Nilsson says that's changing now that more and more leaves are turning out to be offensive weapons.

Mr. NILSSON: They release chemicals, terpenes, phenolic acids, anthocyanins, carotenoids. All of them have some potential toxicity.

NIELSEN: They poison each other.

Mr. NILSSON: That's right, they poison each other right--all the way directly to respiration rate itself. They inhibit the growth of other seedlings and stop germination of seeds.

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NIELSEN: Nilsson says research under way in New York has shown that black walnut trees are especially good at poisoning their neighbors; so are the rhododendron trees he studies at Virginia Tech. But it may be that the tree that was the master of this strategy is one that's not even around anymore. It was called the American chestnut, and it was wiped out by an exotic fungus in the early 1900s. Nilsson says the leaves that fell from chestnut trees were exceptionally potent, which is partly why they once completely dominated the forest in southern Appalachia.

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NIELSEN: He says the chestnut's toxic leaves helped build forests that were like cathedrals.

Mr. NILSSON: Would have been much taller, much bigger trees, lower density, less saplings in the understory...

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Mr. NILSSON: ...and more rotting fallen logs and darker, much darker.

NIELSEN: Not long afterwards we marched into a clearing with what Nilsson called a bad tree growing on the edge of it, an invasive Asian species called Ailanthus, or tree of heaven.

Mr. NILSSON: And this one has very strong toxicity in its leaves, producing a compound called ailanthone, named after the plant, that is actually extracted by some people to use as herbicide.

NIELSEN: So if we were to come back to the edge of this open field a couple of generations from now and nobody had done anything, you might expect to see a whole bunch more of these trees.

Mr. NILSSON: I would expect to see many, many more of the trees, a field of Ailanthus.

NIELSEN: Nilsson has been told that tours like this one make forests sound like dismal places and autumn leaves sound like beautiful but nasty falling objects. But he will have none of that. He thinks it's wonderful to see the forest from the point of view of the trees.

Mr. NILSSON: To think about the fact that trees really are stuck; they're in one place, and they're going to have to defend themselves against other trees, other species, even members of their own species that get too close to them. So they have to have their own mechanisms to do that, and it's much more developed in plants than it is in the animal kingdom.

NIELSEN: As we were leaving, I pulled a yellow leaf off a poplar and asked Nilsson how toxic it was. `Probably not toxic at all,' he said. `Sometimes a pretty leaf is just a pretty leaf.' John Nielsen, NPR News.

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