Over my life I’ve delighted in the annual pageantry of autumn, watching the green leaves of summer morph into brilliant oranges, reds, and yellows. Even purples. This is one of the perks of living in the Northeast U.S., starting with 24 years in New England before moving to southeastern Pennsylvania.
I confess that when I first came here, I found the season a bit drab; it just didn’t compare to the intense reds and crimsons that are the trademark of a New England fall. It didn’t take me long, though, to come to appreciate the different, but no less vibrant color palette—the brilliant yellow hickories and beeches, the multi-colored sweet gums, and the sometimes deep purple notes in the oaks.
One of the takeaways here is that I’ve experienced plenty of fall seasons where there’s spectacular color, the kind of display that “leaf peepers” (I just stumbled upon that term last month) travel to see. You’d think that by this time I’d know enough about this annual costume change, but I realized that while I knew—or thought I did—how leaves change color, I had never wondered why. Sure, dropping their leaves prepares and protects trees against the impending freezing weather. But why the dramatic color change?
I went looking for information and immediately discovered that I knew far less than I thought I did. What I’d heard and believed for decades is that the colors of fall are always present in the leaves; they’re just masked by chlorophyll. When trees are triggered by dwindling daylight hours and deepening cold, they stop producing the pigment. Voila! With chlorophyll out of the way, we can see those other, previously hidden colors.
An online BBC article by Ally Hirschlag revealed that this explanation falls short. “Leaves that turn yellow were actually always yellow,” says Hirschlag. “The colour comes from a set of biochemical pigments known as carotenoids. These are simply revealed once the green chlorophyll in the leaves breaks down …” That’s the part that I knew, but why single out only leaves that turn yellow?
According to Hirschlag, in leaves that turn red or purple the color change happens differently. Yes, there is chlorophyll loss, but these leaves also begin to produce anthocyanins. These are the blue, red, and purple pigments found in plants, especially flowers, fruits, and tubers.
The second thing I learned is that the chlorophyll doesn’t “disappear.” Its component nutrients—magnesium, carbon, nitrogen—are “reabsorbed by the plants as they prepare to hunker down for the harsh winter weather.” That makes sense; it took energy to acquire those nutrients so why not keep them?
That’s a simple view of the “how.” Now for the “why,” which is still pretty much an assortment of theories.
The annual changing of colors prior to cold winter weather has been happening for a very long time, so it’s not too far-fetched to think that it confers some benefit to the tree. However, as Hirschlag states, “genetic research suggests that trees only began producing the pigments responsible for the autumn colors comparatively late in their evolution, long after they began reabsorbing their chlorophyll. One … theory proposes that red leaf color may have co-evolved alongside pests, acting as a defensive signal to ward off insects.”
Another leading theory is that the pigments act as “a kind of sunscreen for the leaves during a vulnerable stage of metamorphosis.” Anthocyanins—which are potent antioxidants—may protect aging leaves from sunlight damage. Color change is primarily a northern hemisphere phenomenon, and research reveals that sunlight can be more damaging to leaves during autumn in the Northen Hemisphere.
Plenty to ponder in all of this, and it’s really just the tip of the proverbial iceberg. To read Hirschlag’s article, go to https://www.bbc.com/future/article/20251031-the-mystery-of-why-leaves-change-colour-in-the-autumn. And enjoy what’s left of this year’s display.
Pam Baxter is an avid organic vegetable gardener who lives in Kimberton. Direct e-mail to pamelacbaxter@gmail.com, or send mail to P.O. Box 80, Kimberton, PA 19442. Pam’s new book, Listening to Nature’s Voice, is now available on Amazon. For more information or to sign up for her newsletter, visit her website: pamelabaxterbooks.com.
Source: Berkshire mont