North Country expert explains fall leaf color change
Saturday, September 21, 2013 - 8:25 am

By PAUL HETZLER

As a very young lad I was told that the summer sun bleached pigment from clothes hung on the line, and saved up the colors to paint on autumn leaves. Or maybe it was used for rainbows—it was quite some time ago.

It occurs to me that solar dryers (a.k.a. laundry lines) and fall leaf colors are similar in that they operate free of charge, but their performance depends on the weather. The same clear-sky conditions that produce dry, good-smelling (and a teensy bit faded) laundry also make for the best leaf color. While the former process is well understood, the latter is a story fraught with murder and intrigue, and requires some explanation.

Chlorophyll, the green molecule at the center of the photosynthesis miracle, is what makes the world go 'round. Some say money is, but without chlorophyll the sole life on Earth would be bacteria, whereas without money we'd only have to barter. (Given that both items are green, it's easy to understand the mistake.) Green gives way to fall colors, though, when trees start killing their own chlorophyll, revealing the yellow xanthophylls and orange carotenenoids that were in the leaves all along.

How could a tree be so heartless as to slay its chlorophyll? Aside from the obvious—it doesn't have a heart muscle—the answer is to keep from drying to a crisp in the winter. Each leaf is jacked into the tree's circulatory system: water and nutrients enter; sugars exit. In autumn these connections have to be sealed, or the open vascular tissue would allow moisture to get out and pathogens to get in. When the days shorten to a certain point, trees start to make a plug, or abscission layer, between leaf and twig, thus choking chlorophyll and rolling out the new color scheme.

Yellow and orange are hidden under green, but whence comes red? This is where the mystery begins. We know that warm sunny days and cool nights result in more red color, and that relatively few tree species can produce red. In case anyone asks you, which I realize is unlikely, the chemicals responsible for the red and purple range are called anthocyanins. These large, complex molecules take a lot of energy to create, and most plants invest in them in springtime to protect young emerging leaves from UV damage. After a leaf hardens off, anthocyanins break down and the plant stops making them.

Because botanists—aside from yours truly of course—are smarty-pants types who hate to say “I don't know,” we'll tell you trees make anthocyanins in the fall to protect leaves from the sun. Some of us can even say this with a straight face.

Renowned as frugal and pragmatic creatures, trees don't spend savings without a dang good reason. It seems pretty far-fetched that trees would use precious energy to protect dying chlorophyll while they're busy making the abscission layers that are killing said chlorophyll. If the 'fall suntan lotion' explanation is correct, maples should turn red at roughly the same time, with all leaves coloring evenly through the crown, and in any weather conditions (except freezing, which puts an abrupt end to color change).

If you call up and ask me to explain fall color, I'll admit that I don't—actually, I may tell you about faded laundry on the line.

Paul Hetzler is a forester and a horticulture and natural resources educator for Cornell Cooperative Extension of St. Lawrence County.