Every year, thousands of leaf peepers take to the roads to see the brilliant hues of fall foliage. So where does that color come from?
For the most part, it's already there, hiding behind that endless sea of green. That green, generated by a chemical called chlorophyll, overwhelms other pigments present in a plant's leaves. While visually unimpressive, chlorophyll is how plants use the energy of the sun to convert carbon dioxide and water into sugars.
As the weather gets colder, chlorophyll begins to break down and doesn't work as well; plants stop producing it and, slowly, their leaves die while drawing out all the important nutrients for its rebirth in the spring. As the chlorophyll disappears, the other pigments come to the front and those oranges and yellows become evident.
Called carotenoids, these compounds are responsible for the color of things like carrots and bananas. In plants, these chemicals make themselves useful by helping the process of photosynthesis along and protecting the chlorophyll from harmful molecules that might interfere with the process.
The bright reds aren't simply waiting inside of the plant's cells for their chance to shine, though; they are produced in the late summer and early fall.
During the year, a deciduous, or leaf-bearing, tree goes through a number of phases based on the impact of its environment on its biology.
Imagine that a tree is a traveling salesman. Of course, instead of selling vacuums it's making sugar. Making a sale takes a lot of effort; the salesman has to drive out to someone's home and spend time making small talk and delivering his pitch. So the salesman has to weigh the costs against the potential benefits if he wants to make a living; it's not going to be worth it if he has to drive four hours if he's not going to sell the vacuum.
The tree has to make those same decisions about when to produce leaves and when to let them die. There's a certain range of sunlight that's ideal for manufacturing sugar where a plant can gain a lot more than it invests - just like there's an ideal route for selling vacuums - and that range falls in a predictable order. When the amount of sunlight begins to increase, a plant starts to put out leaves, and, as the amount of sunlight wanes, it stops investing in them because the returns are no longer worth it.
Of course, the plant doesn't make these decisions consciously; they've been dictated by millions of years of evolution and adaptation to a certain environment. That's where the red pigment comes in.
Anthocyanin is a chemical generated by a plant in late summer when it has started to pull nutrients, like phosphates, out of the leaves. The withdrawal of nutrients changes the way that the plant breaks down the sugar it produces, and the byproduct is that red color.
The more sugar that has to get broken down, the more red color will get produced, which means that the most brilliant reds come when the days of early fall are bright and cool. As the red starts to predominate and the chlorophyll starts to die out, the time comes for the tree to drop the leaf.
It doesn't take any great knowledge of chemistry or biology to stand in the midst of these trees and get a feeling for the grandeur of the process. Still, the fall affords us a moment to think about what goes on in the secret world of plants.
The forlorn beauty of autumn has inspired a lot of mythmaking in temperate climates. In New England, the changing of the leaves becomes a brilliant backdrop for celebrations of the harvest and of the plenty that nature has helped deliver, a last gasp before it is hidden under a blanket of snow for the winter.
Information in this article was taken from the Massachusetts Department of Conservation and Recreation, the US Department of Agriculture and the Massachusetts Office of Tourism and Travel. To learn more about fall foliage in Massachusetts visit www.mass.gov/dcr/fallfoliage.htm.