What Causes the Leaves to Change Color?

When you don’t live in a tropical climate, one of your rationalizations—aside from not having a job in a tropical climate—is that you like the four seasons. And no season captures the essence of built-in change better than fall. The crisp air, the apple picking, the pumpkin patches, and, of course, the leaves. The brilliant leaves. Now you might be wondering, what causes the leaves to change color? Even if you’re not, I’m going to go ahead and answer the question.

 

Background

Not to bring back bad memories of middle school science class, but it’s worth starting with a super quick review of photosynthesis. Plants (a category that includes trees and shrubs) are known for this process, which in its most bare-bones form involves the following: water, carbon dioxide, and the energy from sunlight combine to form simple sugars (nutrition for the plant) and oxygen (key for other life forms). The process occurs in leaves, explaining why leaves are important for a plant’s “food generation.”

A key player in photosynthesis is the chloroplast, a specialized subunit located within cells of the leaves. And in the chloroplast is a pigment called chlorophyll that has the all-important function of capturing energy from sunlight and converting it into forms that allow for the biochemistry described above to carry on properly. Chlorophyll is, of course, a green pigment, explaining the green color of a leaf.

During the summer, chlorophyll production is a priority, ensuring that other pigments in leaves that also participate in photosynthesis are masked by the green. What are these other pigments? Carotenoids, which can be divided into two broad categories, namely xanthophylls (yellow) and carotenes (orange).

 

So What Causes the Leaves to Change Color?

It should be mentioned that we’re referring to deciduous trees and shrubs, as in the ones that shed their leaves. Contrast these to evergreen species that either live in gentler climates or have found different ways to adapt to the winter.

When deciduous trees and shrubs note a decrease in the availability of sunlight, they begin a process of winterization. A centerpiece of that process is to ditch the leaves. Why? Because leaves can lose water, something that can be harder to replenish in times of lower rainfall and/or frozen ground water. Furthermore, generating chlorophyll takes energy, an expenditure that starts to have a limited return on investment. Finally, having leaves during windy and snowy conditions can be hazardous.

A series of biologic changes occurs with the goal of sending less resources to the leaves and ultimately giving them a sendoff. As chlorophyll production decreases—it is also broken down along with other things for its nutrients—xanthophylls and carotenes get a brief opportunity to shine. Another group of pigments called anthocyanins can be made, adding red and purple to the mix. The visuals can be rather striking (until the leaves fall and you have to rake them).

If you’re wondering about nutrition in the winter, the trees and shrubs typically store sugars that were made in the leaves in the form of starch, thus providing nourishment during the otherwise lean months.

As a final point, while September, October, and November are synonymous with fall in much of the world, don’t tell that to your friends in places like Argentina, Brazil, and Australia. In the Southern Hemisphere, leaf peeping is a March-through-May activity.

 

When I previously wrote about the Great Lakes, I suggested a trip to my home in Rochester, New York, to try the world-class tap water from Lake Ontario. Perhaps you can come in October and also experience the vivid fall colors. Just make sure you’re in Florida by November.