Baker: A great year for fall colors

“What a glorious autumn for colors this has been. More breathtaking red, copper, yellow and orange than I can recall having seen since we moved to Northwest Ohio 35 years ago. Makes you wonder what was so different about this year.”

Although those words would surely apply to this year’s gorgeous display of fall colors, they actually come from an essay I wrote in praise of the brilliant foliage of 2020. And just a couple of days ago Deb and I were commenting on how neither of us could recall such a blazingly beautiful fall. How quickly we forget.

And since we do forget quickly, and because it has indeed been a glorious autumn for color, I thought it might be helpful to review some of what botanists have learned about why fall hues are brilliant in some years and regions of the country, and not so much in others.

Drought can lead to dull fall colors

While Ohio has had a lovely autumn, the Washington Post ran a piece in October pointing out that portions of New England, the Midwest and West “may be duller than usual in many places because of an exceptionally hot and dry summer, which put the trees under stress.”

In areas of prolonged drought, fall colors tend toward drab yellows and browns. However, there’s more to Northwest Ohio’s brilliant fall colors this year than our having received plenty of summer rain. The recipe entails just the right mix of pigment chemistry, genetics, and weather.

Although most chemicals found in a leaf are transparent, there are a variety of compounds — the pigments — that do add color to the leaf. However, an important point here is that, unlike the pigments the guy at a Sherwin-Williams adds to a gallon of paint, coloring a leaf is not a leaf-pigment’s primary job.

Each type of pigment has one or more important functions within a leaf. Its color is largely a side effect of the pigment’s chemical structure; attractive to us perhaps, but typically not of much importance to the plant itself.Most of us have at least some fuzzy memory from high school biology that the most abundant plant pigment, chlorophyll, initiates photosynthesis by capturing some of the energy in sunlight. But because of its chemical structure, chlorophyll is only good at absorbing the blue and red components of sunlight, and very poor at absorbing green, which explains the lush color of your front lawn. Since the green component of sunlight isn’t of much use to chlorophyll, most of it just bounces off leaf surfaces.

Two other common groups of plant pigments are the carotenoids and the flavenoids. Carotenoids add yellow, orange or brown hues to a leaf. Of their various functions within a leaf, one of the most important involves transferring energy to chlorophyll molecules. Although carotenoids are found within a leaf throughout the growing season, their contribution to a leaf’s color is largely masked by the much greater abundance of green chlorophyll pigments.

But chlorophyll is not a very stable molecule, degrading very quickly in the process of simply doing its job. During spring and summer, the production of new chlorphyll balances the destruction of the old. But heading into September and October, chlorophyll production slows to a trickle and the yellows and oranges of carotenoids finally show through.

There are many forms of flavonoid pigments, but in thinking about the bold reds of autumn foliage, we need to focus on one subgroup, the anthocyanins. Unlike chlorophyll and carotenoids, anthocyanins are only produced in a leaf with the dropping temperatures and shorter days of fall. While not directly involved in photosynthesis, they do play a role in protecting chlorophyll from decaying too quickly as the season advances.

Perfect weather combination produces brilliant colors

The chemistry of anthocyanin production is a bit tricky, but a succession of sunny, early fall days with cold but not freezing nights enhances their manufacture … at least in those plants whose genetics allows for the production of any anthocyanin at all.

Some trees, such as cottonwoods, elms and hickories, produce very little of it regardless of the weather, and their leaves typically turn a uniform yellow. On the other hand, with their genetically based ability to ramp up anthocyanin production during the kind of weather we’ve had this fall, area sugar, silver and especially red maples were able to put on quite a show.

So overall, the U.S. Forest Service notes that “a warm wet spring, favorable summer weather and warm sunny fall days with cool nights should produce the most brilliant colors.”

One of the last groups of chemicals in leaves to break down are the tannins — sturdy, astringent compounds that protect a living leaf from insects and other herbivores. As suggested by their name, tannins are brown, the terminal color of decaying leaves now heaped in thick, aromatic masses across the forest floor.

Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.

This article originally appeared on Fremont News-Messenger: Baker: A great year for fall colors