Even here in Tucson, autumn has arrived, and many plants are beginning to lose their leaves for the winter. Most leaves change color before they die – some turn bright red, others yellow, orange, and others just a sad shade of brown. Why bother?

The answer I remember from grade school is that leaves are pulling nutrients back into the roots for winter, so that the loss of leaf mass isn’t so great. Apparently most of the useful nutrients are what make the leaf look green, and everything that’s left over is the fall coloration. This explanation isn’t entirely satisfying though – the red pigments still in the leaf are also expensive to produce, so why not pull them back too? And why do the leaves turn color when this happens? The strong convergence in fall coloration between species suggests there’s more to the story than just this.

Another idea is that the coloration protects leaves from damage from the sun or high carbon loss while they’re undergoing major chemical changes at the end of the year. Plant physiology suggests this is plausible, but why wouldn’t it operate during the rest of the year? Is the issue linked to environmental conditions the leaf experiences?

Another famous idea is that the coloration reflects co-evolution between insects and plants: plants display bright colors to signal their unpalatability (or lack of tastiness) to animals that might eat them, and insects use these color signals to determine which plants are good hosts. Over evolutionary time, perhaps very bright colors have become associated with very nasty leaves that retain many nutrients for the plant, while dull leaves are nutrient-poor leaves that are relied upon by insects. This is a nifty idea but most insects have a very hard time distinguishing red colors from yellow or green!

These hypotheses are all adaptive ones, purporting to explain some benefit the plant receives from investing resources in coloration. Unfortunately none have been tested over broad scales, leaving the question as being mostly unanswered!

If you have access to scientific journals, this is a good summary of some of the issues:
Red leaves, insects and coevolution: a red herring?

I hope you will agree with me that this common and beautiful fall phenomenon is far from simple, and far from being understood. The next time you see a tree, try and think of your own explanation for its colors!

(The other lurking issue you may have wondered about is why leaves are green to start with – and that is the subject for another post!)