Autumnal Foliage Can Be Pretty Tree-Killers

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 A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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