Autumnal Foliage Can Be Pretty Tree-Killers

Oct. 19, 2005 — -- That maple tree in your front yard may be magnificent this time of year, cloaked in the colorful robes of splendor that herald the transition from one season to the next, but you might want to give it even more respect. It could be a killer.

Researchers at New York's Colgate University have found evidence that the brilliant red hues of autumn aren't just there for our personal enjoyment. They're engaged in a kind of chemical warfare, releasing poisons that could kill off the competition.

Thus maple trees, and probably some other species that turn crimson in the fall, join a growing list of plants that don't just beat around the bush when other plants start intruding into their space. They kill them off. Scientists call it "allelopathy."

For years scientists have known that black walnut trees are lethal when it comes to protecting their turf. And more recently, the mighty chestnut tree that once blanketed the Appalachians has come under suspicion. But the maple tree adds a surprising twist. How could anything that lovely be deadly?

Caught Red-Leafed

It all began when Colgate biology professor Frank Frey and a former student, Maggie Eldridge, started looking into a peculiarity involving plants that turn red in the fall. The predominant colors of autumn break out when chlorophyll in the leaves breaks down and exposes remaining pigments, which are often yellow or orange.

But it takes a different process to produce red. That isn't a pigment that is left over when everything else is gone. Instead, it's produced in the fall, at the very time when the tree is struggling to cope with the energy demands of a changing and challenging season.

Why, Frey and Eldridge wondered, did the maple go to all that trouble at a time when it needed its metabolic energy for other purposes, like stimulating the growth of its root system?

So they collected chemical extracts from red and green maple leaves and yellow and green beech leaves and poured it over lettuce seeds. Some previous research had shown that wood extracts from red maple and red cedar inhibited the growth of lettuce.

But Frey and Eldridge found that red maple is the clear winner. It "dramatically reduced germination and growth compared to all other treatments," they say in a study that is soon to be published.

"When scarlet-tinted autumn leaves are dropped in the fall, it appears that anthocyanins (molecules that produce the red color) leach from the leaves into the soil and protect seedlings and saplings from interspecific competition the following spring," Frey says. In other words, no one but maples allowed.

All of this has a practical purpose. As Frey notes, other research shows that anthocyanins may inhibit the growth of some cancer cells in vertebrates, so eventually there may be a medicinal application. Other researchers see this kind of chemical warfare as a possible pathway to better weed control.

And it has opened a surprising window on just how ruthless some species may be when it comes to protecting their turf.

Combative Chestnuts

For decades scientists thought that the dominance of the chestnut tree in the Appalachian forests was due to its own personal majesty. It was a fast grower, rapidly shading any competing trees. It resisted rot and fire, and was happy in poor soil.

But along comes ecologist Barry Clinton of the U.S. Forest Service's Southern Research Station in North Carolina with some shocking news: The mighty chestnut, it turns out, was engaged in a little skullduggery.

Clinton and colleagues at Clemson University and the University of North Carolina at Chapel Hill tested the effects of fallen chestnut leaves on five tree species that competed for the same space.

They found that an extract from the leaves inhibited germination of the competing trees -- particularly eastern hemlock, a major species along mountain streams in the Appalachians. It also curtailed the growth of a native rhododendron.

But that colorful shrub may have won out in the end. The chestnuts were wiped out by a blight years ago, and since that time the rhododendrons have flourished.

The rapid encroachment of this shrub "may be largely due to the end of the tree's allopathic influence," Clinton says.

Other Botanical Killers

Other trees known to poison their competitors include sycamore, eucalyptus, hackberry, and the notorious black walnut.

Black walnut produces the chemical juglone, which can spell trouble for a wide variety of trees and plants.

Researchers have found, for example, that black walnut can inhibit corn production even if the trees are some distance from corn stalks.

However, lest we be accused of slamming only deciduous trees, it should be noted that even pine trees are not without their chemical weapons.

Pine needles decompose after falling to the ground, releasing an acid that leaches into the soil. That keeps almost anything from sprouting near the tree's roots.

And lastly, the next time someone tells you how broccoli is good for you, tell them that broccoli plants leave a residue in the soil that makes it difficult for some other crops to follow.

It may be a super-food, but it's not without its faults.

Lee Dye's column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.