Why are those leaves falling?
What is red, orange and yellow and doesn’t get hurt when it falls? Autumn leaves!
Whether it is called “fall” or “autumn,” it is my favorite season. I have always found it sort of ironic that the season I am most thankful for happens the same time of year as Thanksgiving.
There are some things I can count on to be a constant that no matter what is happening (like a pandemic) and autumn is one of those things.
I am not sure what it is about fall ... the swish-swish of the leaves as I deliberately kick my feet through them, the sweaters or the campfires or geese honking overhead.
To me the leaves transforming from the verdancy of summer to the blaze of fall is magical. The color-changing process is simply magical. That’s the only way to describe it from my personal point of view. But there is also a thorough scientific explanation of this magic.
While this change is beautiful, what’s really happening is I am watching the leaves starve themselves and die.
So to understand the why of the change colors come fall, I had to work backward and figure out why they’re green in the first place.
When school students come to the center, one of the topics they learn about is plant life and of course the whole process of photosynthesis. Photosynthesis means “putting together with light.”
Plants need three things to keep them alive - water, carbon dioxide and sunlight. Water is absorbed by the roots. Carbon dioxide is absorbed through tiny holes in plants’ leaves, flowers, branches, stems, as well as its roots. So how is the sunlight absorbed?
Sunlight is absorbed by a chemical in the tree’s leaves known as chlorophyll. Chlorophyll absorbs different wavelengths of light, which is why it looks green.
When the sunlight reacts with the water and carbon dioxide, this combination becomes a type of food for the plant in the form of sugars. Those sugars are then transported throughout the plant as fuel.
Because chlorophyll needs sunlight to be produced, it makes sense that the chlorophyll production begins to slow down with the colder temperatures and shorter days. And this is why leaves change color come fall.
To make sense of all the scientific explanations, I connect this idea of concealing colors this way: I wear a few layers in the fall and each layer I put on covers up the layer underneath. The layers I have on are concealed by the layers on top.
Leaves have colors hidden under the green pigments of chlorophyll. Plant leaves also have yellow and orange pigments in them all the time which are also covered up. Yellow and orange colors are due to pigments called carotenoids, which are also responsible for the color in carrots and in corn. It is only when the temperatures start to drop that these other colors are revealed.
When leaves change color, another pigment becomes visible: flavonoids, which are responsible for the reds. These colors are particular to fall because their pigment is created only when the temperature drops.
I have heard so many “recipes” for brilliant fall foliage. Dry, sunny days with cooler temperatures at night are perfect conditions. The dry weather leads to more sugars in the leaves, which results in brighter reds. The colder temperatures seem to signal to the tree to start shutting down for the winter.
Interestingly enough, a sudden cold snap that lasts for a few days will reduce the intensity of the colors. When leaves start to fall off the trees, a layer of cells form along the base of its stalk, sealing off the leaf from the food supply. This stops the movement of sugar from leaf to tree, and when that leaf is blown off, it leaves behind a leaf scar. The remaining sugars are stored in the tree.
All these chemical changes along with the acidity of the soil create this burst of color that “leaf peepers” travel far and wide to witness. All of these combined elements also explain why the intensity of colors varies from place to place as well as from state to state.
Some scientists also think that why leaves change color has something to do with their evolution and is in fact not even that useful to the tree anymore. They believe that the color may once have been used to attract certain insects, some of which are now extinct.
“Because plants evolve very slowly, we still see the colors. So leaf color is a fossil memory, something that existed for a reason millions of years ago but that serves no purpose now,” said Bryan A. Hanson, professor of chemistry and biochemistry at DePauw University.
Science can only explain so much, and I respect that science, but to me the rest is just pure magic.
Jeannie Carl is a naturalist at the Carbon County Environmental Education Center in Summit Hill. The center rehabilitates injured animals and educates the public on a variety of wildlife found in the area. For information on the Carbon County Environmental Center, visit www.carboneec.org.