That old line on why leaves change color

As a wee lad I was told a story wherein the bright summer sun would bleach pigment from clothes hung on the line, and save up the colors to paint on autumn leaves. Thinking back on that yarn it occurs to me that solar dryers, a.k.a. laundry lines, and fall leaf color change are similar in how they operate. They are both elegant and cost-free, but their performance depends on the weather. The same clear-sky conditions that produce dry, fresh-smelling, and just a teensy bit faded laundry, also make for the best leaf color. Although 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 does, but they need a reality, um, check. Without chlorophyll the sole life on Earth would be bacteria, whereas without money we’d merely have to adjust to a barter system. Given that chlorophyll and currency are both green, it’s easy to understand the mistake.

Green gives way to fall colors, though, when trees start to kill 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 seep out and pathogens to get in. When the days shorten to a certain point, trees start to make a waxy plug, or abscission layer, between leaf and twig, thus choking chlorophyll and rolling out the new color scheme.

Yellow and orange, as we learned in high-school Biology, 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 produce red fall color. In case anyone asks you, which I realize is unlikely, you can tell them the chemicals responsible for the red and purple range are called anthocyanins. These large, complex molecules take a lot of energy to create, and many 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.

Like medical doctors, botanists sometimes find it hard to make their mouths form the phrase “I don’t know.” This temporary and selective facial paralysis has afflicted me far too often, and to my shame I’ve pitched a reasonable but untested reply. Many authorities have said trees make anthocyanins in the fall to protect leaves from the sun. With practice, some of them have even been able to say it without giggling. This explanation is far too simplistic and fraught with problems.

Renowned as frugal and pragmatic creatures, trees don’t spend savings without a dang good reason. It seems far-fetched that trees would use precious energy to protect dying chlorophyll at the same time they are 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 leaves coloring uniformly through the crown, and in any weather conditions except freezing, which puts an abrupt end to color change.

If you call me up to inquire why some trees use red and purple on their autumn leaves, I’ll admit that I don’t really—well, actually, it depends on the day. I may just tell you the story about about faded laundry on the line.

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

Tags: botany, fall foliage, leaves