Natural Connections: The science behind fall colors
Fall colors are all about death. We’ll start with a sort of obituary then — an explanation of the leaf lives we lose every fall.
We’re lucky to live in this swath of the continent known as the Northwoods, where fall colors are spectacular.
Go a little south, or head farther north, and the forest changes. According to John Pastor, professor emeritus from the University of Minnesota Duluth, we have the extremes of our climate to thank for this autumn show. There’s something about the contrast of hot summers followed by frigid winters that encourages diversity in the shapes, colors and lifespans of our trees’ leaves.
At the risk of being a killjoy, I’d like to remind you that fall colors are all about death. We’ll start with a sort of obituary then — an explanation of the leaf lives we lose every fall.
As soon as leaves emerge in early summer, the trees begin again to form new buds. The basic cells for leaves, as well as shoots and flowers, are neatly organized and packed tightly within protective scales or thick fur. When warmth and light return next spring, those buds burst and the leaves expand.
It takes a lot of resources to grow a new leaf, and then maintenance costs continue as long as the leaf is photosynthesizing. I envision a worksheet full of math problems. Each species-specific equation contains different negative numbers representing the costs of tissue construction, maintenance and reproduction. They also include various positive numbers of productivity. The answers, though, would all be the same: something just above zero.
Leaf shape is part of those equations. In his book, "What Should a Clever Moose Eat?" Pastor put a number on it: 80% of deciduous species in the Northwoods have leaves that are either toothed or lobed. The correlation between leaf teeth and cool climates is so pronounced that paleoecologists use the percentage of toothed leaves in fossil plant communities to back-calculate prehistoric temperatures.
On the surface, it seems like a pretty random correlation. But scientists have determined that toothed leaf margins rev up their sugar factories faster in early spring than smooth leaf margins, thereby extending the growing season. The teeth also lose water at a higher rate, but water isn’t hard to come by during a Northwoods mud season, now is it?
Leaf lifespan is another variable in the equation. The more it costs to build a leaf, the longer it takes to pay off the overhead. Evergreen trees invest significant energy into building needles that can withstand harsh weather, photosynthesize at lower temperatures and ward off pests. Pine needles live two to three years, and spruce and hemlock needles last four to 10 years. In contrast, cheap, flimsy birch leaves last just a few months. Neither way is necessarily better. Each strategy finds success.
As sunlight wanes in late summer, it’s time to close down the photosynthesis factories of leaves. Now the trees must try to salvage what they can. Nitrogen and phosphorus are sucked back out of the leaves and into the twigs, where they’ll remain on-deck to fuel next spring’s leaf growth. The tree benefits from storing N and P above ground, because the twigs thaw long before the frozen earth releases her nutrients.
Finally, we’ve arrived at the onset of fall colors. Of course, they were there the whole time. All summer long, vibrant green chlorophyll was the star of the show, and outshone all the rest. As chlorophyll breaks down and is resorbed, though, its trusty sidekicks are revealed. Orange carotene pigments captured wavelengths of light that the green chlorophyll could not, and then transferred that energy over to help fuel photosynthesis. Yellow xanthophyll pigments absorbed dangerous excess energy in the leaf and dissipated it as heat. This prevented cell damage, and warmed the surrounding environment.
Red comes next. Once the phosphates have been reabsorbed into the twigs, sugars in the leaves break down and form anthocyanins. Anthocyanins absorb UV light, especially at low temperatures. Like sunscreen, the pigments protect the leaf cells while they finish sending their nutrients back to the twigs. They can be blue or purple, but their most conspicuous form is red. Red is the only fall color that is created and not just revealed.
To create anthocyanins you need lots of sugars, which means a summer with dependable soil moisture. Once summer’s over, though, cool, dry weather is the ticket. Rain literally washes the color out of the leaves — leaching pigments and sugars from dying cells.
While the forest becomes a rainbow, the deciduous trees are quietly growing several layers of cells across the base of the leaf’s stem where it is attached to the twig. Jack Frost doesn’t kill the leaves, because the trees themselves do. Finally, the abscission layer weakens the leaf’s hold on the tree just enough that a stiff breeze can whisk it away.
That’s it. The Northwoods slides into the gray depths of winter, brightened only by the highly invested evergreens. Summer can’t last forever, but neither can fall, winter, or spring. Next year, we’ll get to watch the kaleidoscope of colors play across the landscape all over again.
Portions of this article originally appeared in Northern Wilds Magazine.
Emily Stone is a naturalist and education director at the Cable Natural History Museum.