Curious Nature: Why do leaves change?
Vail, CO Colorado
The iconic photo of Colorado typically features peaks framed by piercing blue skies and rolling hills of dark evergreen trees spotted with bright red, orange and yellow aspen trees. Aspens are the most prolific deciduous trees in the Eagle Valley, meaning that they are our most common tree that sheds its leaves in the winter. Their changing colors are heralds of our shoulder season and announce the official end of summer. But what makes for a great fall showing of changing leaves? Many people have been asking this question as the colors begin to change. Being unable to answer it myself, I started doing some research.
To begin, we must understand why leaves change in the first place. Deciduous trees, like aspens, are sensitive to the shorter days and longer nights as the fall equinox approaches and they begin their resting period for the winter. During the summer, leaves make sugars and oxygen out of carbon dioxide gas from the air, water taken up by the roots and sunlight. This process is called photosynthesis, and a chemical called chlorophyll is key to building these sugars. Chlorophyll is green and masks the yellow and orange pigments that occur naturally in the leaf. This green chemical gets “used up” by the sun and needs to be constantly regenerated by the tree. As we move closer to winter, the days shorten and there isn’t enough sunlight to power photosynthesis. The trees go into a resting period, using sugars stored up to survive through the winter.
As trees cue into the reduction of sunlight, the cells at the base of a leaf and stem begin to form a corky layer called the abscission layer. This inhibits the transportation of the water, nutrients, sugars and gases between the tree and the leaf. Because these materials are blocked, the green chlorophyll chemical, no longer being regenerated by the tree, fades over time, slowly revealing those yellows and oranges that had been there all along. If sugars are trapped in the leaf, they are responsible for producing anthocyanins, which give us the deep reds of fall colors. Throughout this process, the cells in the abscission layer become more and more dry, eventually becoming very brittle, thus allowing the leaf to be broken off easily in the breeze.
This happens every year. So how and why does the intensity of the fall colors vary from year to year? This depends on the timing of three variables: sunlight, temperature and moisture. Once the abscission layer begins to form, if days are sunny and nights are cool (without frost), the fall colors are most intense. Those sunny days allow for the rapid destruction of chlorophyll and increased production of anthocyanins from the sugars trapped in the leaves. Those cool nights slow the moving of anthocyanins out of the leaf through the abscission layer as it forms. Other weather factors such as an early frost destroy the ability of the leaf to convert those sugars to anthocyanins, so the colors of fall abruptly end, with leaves turning brown and dropping rapidly. Some years, a dry and challenging growing season causes the abscission layer to form early and quickly, and leaves also drop without much change. Each year is different, and the occurrence of rainy days and increased soil moisture also greatly reduces the intensity of colors. We can only hope for the continuation of sunny days and cool nights, which will bring us our intense fall show, but like the snowflakes that will be falling soon, each year is unique.
Rose Delles is an educator for the Walking Mountains Science Center. Fall is her favorite time of year, and you can find her on the new Buck Creek campus in the outdoor Aspen Forest Classroom watching the aspens change.