"In autumn, as you will recall from your school biology classes (or, failing that, from “Mr. Wizard”), trees prepare for their long winter’s slumber by ceasing to manufacture chlorophyll, the chemical that makes their leaves green. The absence of chlorophyll allows other pigments, called carotenoids, which have been present in the leaves all along, to show off a bit. The carotenoids are what account for the yellow and gold of birches, hickories, beeches, and some oaks, among others. Now here is where it gets interesting. To allow-these golden colors to thrive, the trees must continue to feed the leaves even though the leaves are not actually doing anything useful except hanging there looking pretty. Just at a time when a tree ought to be storing up all its energy for use the following spring, it is instead expending a great deal of effort feeding a pigment that brings joy to the hearts of simple folk like me but doesn’t do anything for the tree.
What is even more mysterious is that some species of trees go a step further and, at considerable cost to themselves, manufacture another type of chemical called anthocycanins, which result in the spectacular oranges and scarlets that are so characteristic of New England. It isn’t that the trees of New England manufacture more of these anthocyanins, but rather that the New England climate and soil provide exactly the right conditions for these colors to bloom in style. In climates that are wetter or warmer, the trees still go to all this trouble-have done for years-but it doesn’t come to anything. No one knows why the trees make this immense effort when they get nothing evident in return."
Bill Bryson, “Fall in New England”
