Color me enlightened — Pretty leaves a sign of weather changes

Why do leaves change color?

A highlight of fall is the appearance of bright colors in trees and other plants throughout Alaska. Most common in Alaska are the bright yellow leaves of the birch and aspen trees.

During the summer, birch leaves were green because the leaves were continually producing chlorophylls that reflect the green light wavelengths. There is a constant production of new chlorophylls throughout the summer that replace “worn-out” chlorophylls. Yellow-colored carotenes are a more stable photosynthetic pigment that are also present within those same leaves. However, they are hidden because of the green-reflecting chlorophylls.

In fall, temperatures and quantities of light decrease. This triggers changes in the leaves as less water and nutrients arrive at the leaves. With less nutrients and water, less chlorophyll is produced. Carotenes are more stable molecules than chlorophylls, so they remain after the chlorophylls are no longer present. Thus, birch leaves change from chlorophyll green to carotene yellow.

Another plant pigment group called anthrocyanins, which are red, can be formed in some leaves and on some colored fruits, like apples. These are formed in conjunction with the presence of sugars. As less and less water arrives in the leaves, the concentration of sugars in the leaves rises. With more sugars, more anthrocyanins are formed and the colors become red or even dark purple. Bright light destroys chlorophyll and increases formation of anthrocyanins. Cooler temperatures cause chlorophylls to break down, too. The loss of chlorophylls allows carotenes and anthrocyanins to become more visible and we see the red colors in leaves of fireweed and some ornamental plants.

So, the brightest autumn colors appear when there is bright light (causing a breakdown of chlorophylls), dry conditions (causing higher sugar concentrations), and cool temperatures (causing increased formation of anthrocyanins). When an absiccion layer forms between the branch and the leaf, it falls to the ground.

David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus.E-mail your science questions to redoubtreporter @alaska.net.