The Science Behind Fall Foliage

Oct 20, 2016

A couple of weekends ago, my oldest son was riding in my truck with me after a drive in the county forest. I saw a beautiful panorama spreading across the hillside we faced while at a stop sign. I remarked about the scene, and my son readily agreed how nice it was. When I looked over at him, though, he was not looking up—he was playing with his smartphone! I chided him gently, and told him that he should enjoy the colorful display now, because he will have to wait for a year to see it again. He gave me the side-eye but didn’t say anything.

It is true, though, at least for me. Every year, I marvel at the brilliance of color, like I had forgotten how amazing it can be. I seek every opportunity to get out in the woods this time of year, be it at work or off the clock. 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.

Every autumn, 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, especially maples, with red colored or red-tipped leaves. Our autumn has been relatively 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. This fall, the weather has been fairly warm, and although we have had nights that were pretty cool, we have not had significant frost or freeze periods so our colors have developed well. Recent winds and rains have knocked down many of our leaves, but there are still plenty of beautiful vistas. 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.