The Science Behind Fall Colors

Oct 15, 2018

When you ask people what they like most about autumn, usually the beautiful fall colors are near the top of the list.

In this episode of Wildlife Matters, the Masked Biologist examines the science behind the beauty of autumn leaf colors.

Autumn has arrived. The weather has been holding out, stretching the feel of summer, but the fall colors are developing brilliantly. I always regret that fall colors just don’t look as good in a photograph as they do to my eye. Every year, I marvel at the brilliance of color, like I had forgotten how amazing it can be. Grouse hunting right now is not necessarily ideal; the leaves assist the birds in their hasty getaway. I still like to get out, even taking the family along, because then I get to see how each individual tree puts on its own show. My favorites are trees that have multiple colors, like yellow fading to red. This year it seemed to me that we had a lot more of those kinds of combinations than usual.

Every fall, I hear from people that colors are triggered by cold temperatures. This is not exactly true. Temperature has its role to play, but it is not the primary factor. In fact, leaf turn is the result of a change in chemical activity, and is triggered by a change in length of daylight.

Tree leaves are green because they are filled with chlorophyll, a crucial pigment that helps the tree in manufacturing its own food from a combination of water, nutrients, and sunlight.  When the days start to shorten, the trees are spending less time making food from sunlight. They produce less chlorophyll, the dominant green coloring in leaves. As the chlorophyll reduces, the carotenoids become more prominent. Carotenoids are also in use during the growing season; you may recognize carotene as that helpful nutrient in healthy foods like carrots, corn, squash, and peppers. Yellows, oranges, and browns all derive from this pigment.

How about the reds? The third pigment comes into play only at the end of the growing season. Anthocyanins add red coloring to plants; apples, cherries, cranberries, raspberries, basically any red fruit contains this pigment. Anthocyanins have one task to perform in tree leaves. When food production, photosynthesis, drops off in the fall, the tree’s productivity drops off—especially when we have warm, sunny days and cool (but not freezing) overnight low temperatures. It can produce a lot of valuable sugary sap during the day, but without continued transport the sap stays in the leaves where the tree risks losing it with the next strong breeze. The red anthocyanins are pumped into the leaves to help the tree ensure it gets as much sap out of the leaf before it shuts down. This explains why, this year, we seem to have an abundance of trees with red colored or red-tipped leaves. Our autumn has been warm and sunny.

Although daylight length is the primary trigger of leaf change, weather does play a role in the ultimate quality of our fall color show. If we get a really hard frost early, trees can lose leaves without having them change to anything but brown. In very dry times, color can be delayed by weeks. Fall colors can be muted by a significant long wet stretch of weather. If you think about the growing season we have had, our biggest setback was the delay of spring. We had cold and snow well into May. This definitely delayed the start of our growing season. Otherwise, we had a summer that was a bit warm, but not too hot and not terribly dry. This fall, the weather has been quite warm, and although we have had nights that were pretty cool, we have not had significant frost or freeze periods. This means we had just about ideal conditions for the greatest variety and intensity of fall colors.

Get out and enjoy them for yourself. A photo may not do it justice; in my opinion, though, fall colors in the Northwoods are something to be experienced, not merely seen or photographed.

Striving to make new things familiar and familiar things new, this is the Masked Biologist coming to you from the heart of Wisconsin’s great Northwoods.