The evolutionary meaning of autumn colors

As a kid growing up in Ontario, Canada, I have vivid memories of vast expanses of forests set ablaze by their autumn colors. Whole landscapes look like the canvas of a painter whose love of red, orange, gold and yellow are readily apparent. But, like most biologists, I had been taught that these colors are simply the by-product of leaf senescence, nothing more than a biochemical accident. I was amazed to read Marco Archetti's recent work showing that there may actually be adaptive benefits to changing leaf color in autumn and for particular colors. Generally the adaptive benefits involve either protection against abiotic factors or as a response to plant-animal interactions. One of his interesting results is that autumn coloration has evolved repeatedly and cannot be explained by being related to an ancestor who changed colors, rather that there must be some other evolutionary or adaptive explanation. While he suggests a large number of candidate hypotheses, some more plausible than others, I'll list five for example:

1) Sunscreen: Pigments provide photoprotection against photooxidation during the recovery of nutrients.

2) Leaf warming: Colors absorb light and warm the leaves during cooling temperatures.

3) Coevolution: Tells overwintering insects that the tree is not suitable (poisonous or low nutrition) for hibernation.

4) Camouflage: Many insects lack red photoreceptor, making leaves difficult to see -thus protecting trees from overwintering pests.

5) Unpalatability: Pigments (e.g., red -anthocyanins) are unpalatable.

So, we may quibble about particular hypotheses, but the point for me is that there may be deeper explanations as to why certain species produce the vivid colors they do. At a minimum, Archetti provides ammunition to experimental botanists and evolutionary biologists for testing new hypotheses. I'll never look at an autumn forest the same again.

Archetti, M. (2009). Classification of hypotheses on the evolution of autumn colours Oikos, 118 (3), 328-333 DOI: 10.1111/j.1600-0706.2008.17164.x

Archetti, M. (2008). Phylogenetic analysis reveals a scattered distribution of autumn colours Annals of Botany, 103 (5), 703-713 DOI: 10.1093/aob/mcn259

Archetti, M., Döring, T., Hagen, S., Hughes, N., Leather, S., Lee, D., Lev-Yadun, S., Manetas, Y., Ougham, H., & Schaberg, P. (2009). Unravelling the evolution of autumn colours: an interdisciplinary approach Trends in Ecology & Evolution, 24 (3), 166-173 DOI: 10.1016/j.tree.2008.10.006