Why Leaves Change Color in the Fall
How do they do that? How do leaves that have been deep green all summer transform to red, yellow, gold or orange in the autumn? For that matter, why do some trees take weeks to shed their leaves while others defoliate overnight?
Actually, the answers lie both in a plant’s genetics and its environment.
Living in the north has its advantages this time of year when the trees and shrubs conclude their growing season with delightfully colorful displays. Fall color starts in September with poison ivy, ashes and walnuts and doesn’t end until November with larches and willows.
The yellow pigments, carotene and xanthophyll, are present all summer along with green chlorophyll. Autumn’s short days and cooler temperatures inhibit the production of new chlorophyll. What is already there starts to break down and disappear, thus allowing the yellow to shine through. Ginkgo, redbud, larch, hickory, birch and witch hazel are trees that turn yellow and gold.
Other trees produce the red or purple pigment, anthocyanin. It too can mask the yellow. The sunny days and cool nights of October cause the production of sugars, which then cause the production of anthocyanin. This explains the two-tone effect when interior leaves turn yellow while the outer leaves, exposed to more sun, turn purple. Dogwood, black tupelo, oaks, winged euonymus and some maples have anthocyanin and turn a fiery red or purple.
The third pigment, tan or brown, is caused by tannins, the culprit responsible for temporarily staining your sidewalk and patio. Finally, it all comes to a stop as freezing temperatures in late fall stop the coloration process and turn any remaining leaves black.
You may have noticed that some trees, like maples, shed their leaves over a several-week period while the leaves of other trees, like gingkoes, seemingly jump off in one day. Why?
When a maple gets ready to shed its canopy, a scar forms under the petiole (leaf stem) where it attaches to the branch. The scar protects the branch during the harsh winter months. The scars start forming first on branches at the top of the tree and the ends of outer branches, because they have been exposed to the coldest air. As those leaves fall, the interior leaves become the exterior leaves thus causing more scar formation and more leaf drop.
On ginkgo trees, all the protective scars form at the same time, consequently all the petioles are severed from the branches simultaneously. Along comes a windy night, and in the morning you discover a bare tree.