October 27, 2007 | David F. Coppedge

Fall Colors Have a Function

Deciduous trees have an investment decision to make when fall chill sets in: do they send their sunlight-produced nutrients to the roots early, and so risk damage to the leaves from autumn sunlight, or should they spend more energy creating a sunscreen that allows them to produce nutrients longer, and thereby increase food storage in the roots for the upcoming winter?
    The determining factor may be the amount of nitrogen in the soil, claims a press release from the Geological Society of America.  According to research done by a graduate student at University of North Carolina, in nitrogen-poor soils the balance is tipped toward investing in sunscreen.  It takes energy to produce the reddish anthocyanins, but these allow the leaves to work overtime producing nutrients from the impoverished soil.  Trees in more nitrogen-rich soils can simply fade to yellow before falling.
    The student’s advisor remarked, “The rainbow of color we see in the fall is not just for our personal human enjoyment — rather, it is the trees going on about their lives and trying to survive.”

There are a couple of problems with the advisor’s remark.  First, it ascribes personality and intent to a plant.  The plant has no brain to figure out what to do; rather, it has been pre-programmed with elaborate mechanisms that ensure its survival.  Did the tree figure out how to manufacture anthocyanins?  Did it know in advance that these molecules will allow it to extract more nutrients from the nitrogen-poor soil?  Was it “trying to survive”?  Of course not.  The mechanisms discovered are marks of engineering for robustness.
    The second problem is an either-or fallacy between plant survival and human enjoyment.  These functions are not mutually exclusive.  They also raise interesting philosophical questions.  Why should humans feel enjoyment at the rainbow of color in the fall?  What is the evolutionary advantage to pleasure at a scene that does not produce food or offspring?  Why shouldn’t a human sense alarm at the apparent death of a source of food?
    Actually, the advisor left open the possibility that joy might be a purpose, by saying it is not just for our personal human enjoyment.  But to push the point, she would have had to explain how the pleasure response in humans to apparently useless beauty had adaptive value.  That would not be very romantic, would it?  It would be just another example of evolutionism taking the soul out of life.  If you are an evolutionist feeling good walking through the rainbow of colors in a fall display, maybe you should be surprised by joy.

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Categories: Amazing Facts

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