Why Do Leaves Change Color in the Fall?

“Why” questions in science often find ready answers.  Why do we have night and day?  The Earth turns on its axis.  Why do we have seasons?  The tilt of the Earth in its path around the sun.  What makes the wind blow?  Solar warming of the atmosphere.  The physics and chemistry of a situation provides us with answers.

Sometimes “why” questions are more difficult.  Why are oranges orange and apples red?  Why do birds migrate? Why do leaves change color in the fall?  Those questions do not depend directly on physics at all.  Do they even have answers?

In the case of leaves changing color, there actually is an answer based on physics and chemistry.  As the days shorten, plant hormones cause a layer to form in the leaf stem (an abscission layer) that cuts off water supply to the leaf.  Leaf cells with chlorophyll die off, that green pigment rapidly degrading.  What is left are more resilient pigments, the yellow carotenes and the red anthocyanins.   Trees turn red and orange and yellow and, Presto!  We have explained why leaves change color.

But another “why” question remains: of what advantage is it to the tree that leaves change color?  Here evolutionary biologists wage pitched battles.  Is color change somehow “adaptive?”  That is, does it have something to do with the tree’s survival and reproduction?  Or is it just something that happens, unrelated to those things?

Though relatively ignorant about these matters, I tend to cling to the belief that some things “just happen.”  They have nothing to do with enhanced survival and reproduction of species.  The question “why” is only an expression of our human intelligence, ever demanding explanations for phenomena that have none.

I could be wrong about it—and sometimes I wonder how anyone could ever prove conclusively certain traits are adaptive.  Is that because my own nature causes me to lean one way or the other?  Is that very quality adaptive?  Understandably, those concerned with such questions are prone to headaches.  I hope you are not so afflicted.

Richard Fidler is co-editor of Grand Traverse Journal.

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