I went leaf-peeping with some friends over the weekend. While I was avidly relishing the innumerable shades of autumn foliage, I wondered why trees put on this spectacular “make-up” every year before the winter season begins. Some googling revealed a very interesting theory that might explain the reasons behind this vibrant display by various deciduous trees.
According to the coevolution theory of autumn colours, the bright colours of leaves in autumn are a warning signal to insects that lay their eggs on the trees in that season. If the colour is linked to the level of defensive commitment of the tree and the insects learn to avoid bright colours, this may lead to a coevolutionary process in which bright trees reduce their parasite load and choosy insects locate the most profitable hosts for the winter.
Granted, this is a new theory (proposed in 2001) and have not been verified to the degree of getting a scientific consensus, but it certainly promises a very intriguing explanation. Previous explanations focused only on the proximate reason for this phenomenon – describing it as a by-product of leaf senescence. The green pigment called chlorophyll degrades in the aging leaf, which unmasks the color of other pigments. Leaf senescence and abscission also have an adaptive explanation: the cost of keeping the leaves in winter is higher than the benefits it would return by photosynthesis during the cold wintry days with scant sunshine. But this pigment-explanation fails to address the ultimate reason: why this particular trait was developed and why it has survived the test of time? What evolutionary benefit(s) does this trait provide?
Now, the coevolution theory suggests a possible ultimate reason. The defensive signaling mechanism can very well be the ultimate cause for the bright autumn colors. And what’s particularly interesting about this trait is that not only the bright colors benefit the trees in keeping the parasites away, they also help the insects by directing them towards more profitable hosts for the winter (hence the term ‘coevolution’).
P.S. Here’s link (PDF) to a white-paper about the coevolution theory.