This fall if you have been listening, you may have noticed I’ve been talking a lot about how it seems like everything in the world changes its behavior with the seasons, everything in the world except us, myself included. Yet I find that, as the end of October nears, my calendar is actually starting to clear, and for once I am in no rush to fill it. The signs of fall are all around us, flocks of sparrows and shore birds are arriving, resting a day or two as they migrate south for the winter. The golden rod and last of the asters have gone to seed, and the trees of course are turning color and gradually losing their leaves.
Leaves are green in spring and summer due to the photosynthetic pigment chlorophyll. Chlorophyll is the substance that allows plants to capture carbon dioxide and water and use the sun’s energy to recombine them into sugars and oxygen. Chlorophyll is what absorbs the sun’s energy and uses it to move electrons around in this process. Here’s the thing about the sun, its rays have a lot of energy, more than the plant can use, and some of it in wavelengths the plant can’t use. It is the same with us, many of us get a sunburn if we absorb too many of the sun’s rays. We stay in the shade or wear sunscreen to prevent over exposure to the sun, but plants can’t do that. They are, by evolutionary design, required to be in the sun as much as possible. Trees solve this problem by having other substances in their leaves that absorb some of the excess sun, and the harmful ultraviolet rays (the same ones that burn us). They have related substances that act as on board antioxidants, so that when sun damage does occur to the delicate chlorophyll molecule, it can be repaired as soon as possible. These substances are the carotenoids and xanthophylls, a group that includes over 60 different pigments. We are familiar with some of the better known carotenoids, beta carotene, the precursor molecule to Vitamin A is found in dark leafy greens and deep orange vegetables, and lycopene, the molecule found in tomatoes that is reported to offer cardiovascular benefits. We know these molecules to be beneficial to our health, and they play much the same role for the trees. Carotenoids also assist chlorophyll in the capture of light energy in photosynthesis.
When the season changes and the trees stop metabolically maintaining their leaves, as broad leaves are simply an energetic and hydration liability in the winter, the chlorophyll molecule breaks down rapidly. As the chlorophyll goes away, so does the green. The carotenoids don’t break down as fast and their yellow and orange color is revealed as the green chlorophyll decomposes. The yellow and orange were there all along, helping to protect the leaf as it did the important work of capturing and storing energy for the tree.
The red color of many a maple, as well as other trees, is a different group of substances called anthocyanins. They weren’t there all along, like the carotenoids. Anthocyanins appear to only form in the fall, and seem to play a role in helping the tree recapture the any remaining sugars or other nutrients that remain in the leaves before they fall off. It would be a tremendous waste to simply let all of that nutrition fall to the ground. They are powerful antioxidants, and this is at least in part an aspect of their role in the leaf in the fall.
So as the season changes productivity shifts from the external to the internal, resources are drawn in, and what is no longer needed is cast off. In the process we see a different side of the trees around us. The change in season provides an opportunity for us to show our other colors, and show appreciation for the hidden strengths and supports, be they friends, family, community that enable our fullest summer expressions. Enjoy this slow motion dive into fall, and take a moment to look at yourself, and see what hidden colors are now shining out of you.
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References:
Fun site about Autumn for kids, from the Wisconsin Department of Natural Resources:
http://dnr.wi.gov/eek/veg/trees/treestruecolor.htm
Really nice info on the chemistry and roles of different leaf pigments, from the University of Georgia School of Forest Resources:
http://warnell.forestry.uga.edu/warnell/service/library/index.php3?docID=144
An authoritative voice, from the University of Illinois Extension service: http://web.extension.illinois.edu/forestry/fall_colors.html
