Why Do Leaves Change Color?

My trees are late to change color. I have two sugar maples in my backyard that transform to such a gorgeous golden and blush hue, someone takes a photo every year. Normally the peak color is right about now.

Driving out east of the city on the weekend, I saw more changes. From the highway, the mixed deciduous/coniferous forest, almost a uniform green all summer, had become a patchwork of flame colors with random splotches of green-black coniferous trees. Now it's everywhere.

Each species contributes to the autumn palette. The sugar maples (Acer saccharum) are famous for their oranges, reds and golds. The red maple (Acer Rubrum) turns a brilliant red. Oak trees change to yellow and then stubbornly refuse to drop their leaves which are forced out by next spring's growth. Sumacs and dogwoods become purplish reds. They are all deciduous trees. The word literally means "falling off at maturity" to describe these trees that shed their leaves annually. Intermixed with them are the coniferous trees that don't drop their needles and also the tamarack, a coniferous tree that, surprisingly, does, turning golden before dropping its tiny pin-like needles. Altogether, it's beautiful as you can see.

But have you ever wondered
why they change color?

The colors start to change when the night temperatures drop. Dryness can be another factor. At this time, at the point where the leaf joins the tree, a special layer of cells develops, sealing off the veins that have fed the leaf fluids and minerals. The tree seals its side and leaves a tiny scar.

When you realize cold air is a factor, you start noticing how a tree's location influences its transformation; trees exposed to the prevailing wind, at the edge of the forest, or over a path, anywhere where the cold reaches them faster, are often the first to change. The picture, below, is a good example. The trees were red over the path.

The chlorophyll that gave the leaf its green color and provided the vital role of photosynthesis, starts to diminish. It has been there since the spring, using sunlight to convert Carbon Dioxide into sugar and giving off oxygen. As the green subsides, other compounds in the leaves are revealed.

Compounds called Anthocyanins are responsible for the beautiful red colors; Flavenoids produce yellows; and Carotenoids produce yellow and orange. Of course they may be mixed together and in combination with the subsiding chlorophyll.

You might find it interesting that the levels of Carbon Dioxide (CO2) in the world are higher when the northern leaves are no longer providing photosynthesis.

Prolonging the Color

Rainy and overcast days tend to prolong the fall colors. As long as the temperature stays low yet above freezing, anthocyanin production is favored but once the frost comes, it's game over.