Fall Leaves


Art by Reese Green


By Anna Rees


The changing leaves in autumn are one of the most beautiful parts of the year, and one of the many reasons I chose to live in Vermont. But, why do trees change color in autumn? Researchers at Imperial College London sought out the answers to this question.


One cause is pretty simple: chlorophyll. Chlorophyll, the pigment that usually makes leaves green, breaks down as the leaves prepare to fall. This unmasks the other pigments already in the leaves like yellow carotenoids. However, anthocyanins, the pigments responsible for red leaf coloration, are newly synthesized in autumn— right before the leaves drop. Why would the trees want to spend energy on this, just for the leaves to fall shortly after? Researchers Döring et. al hypothesized that red could serve as a signal of the tree’s status to insects that migrate in autumn, which would make the effort to create this pigment worthwhile; they thought that perhaps tree leaf color and color preference by herbivores may have “coevolved in an arms race: autumn colours as an adaptation of the trees to reduce their parasite load, and preference for green in insects as an adaptation to find the most suitable host trees”.

Yet another mystery unfolds from this thought: aphids, the primary migrating insect in the fall, do not have photoreceptors for red. So, how could this be?

To test this idea, the researchers set up a large color-choice experiment out in the field by using water traps painted with 70 different colors varying from blue, green, yellow, orange, and red. Based on the proportion of aphids that were trapped by each color, they were able to test to see if the aphids had a preference for leaf color or not. They determined that, indeed, aphids were able to decipher between red and green leaves. They found that red leaves were the least attractive and yellow leaves were the most attractive to aphids. These findings support the researcher’s theory of coevolution between the aphids and the leaf color.

The mystery is not over yet, as we still do not know whether red has truly evolved as a signal to herbivores, which would, in fact, make it a coevolution between trees and aphids. One way to determine this would be to investigate whether the aphids are able to survive better on trees with green leaves versus red leaves in autumn. If aphids can survive equally well between the different colored trees, or even better on the green trees, then red leaf color would be unlikely to serve as a signal for the aphids for whether they should choose to land on the tree or not.

An alternative idea emerged from the finding, which is that red leaf coloration may serve to conceal the attractive yellow color that is unmasked when the chlorophyll in the leaves breaks down. It was found that aphids preferred trees with yellow leaves over any other color, so the trees may have evolved to produce the red pigment to protect themselves from the aphids. This would be an alternative to the co-evolution theory, and the above experiment would help to shed some light on this conclusion as well.

Though the mystery is still unsolved on the evolutionary story behind autumn leaves, we now have a possible explanation for why fall in Vermont is a postcard-worthy annual event. The next time you see a falling leaf, take a moment to appreciate the ages of evolution that lead to such a beautiful sight.


References:

F, D. T., Thomas F Döring Thomas F Döring Department of Life Sciences, Döring, T. F., Thomas F Döring Department of Life Sciences, Archetti, M., Marco Archetti Department of Zoology, Hardie, J., & Jim Hardie Department of Life Sciences. (2008, September 9). Autumn leaves seen Through Herbivore eyes. Proceedings of the Royal Society B: Biological Sciences. Retrieved September 23, 2021, from https://royalsocietypublishing.org/doi/10.1098/rspb.2008.0858?url_ver=Z39.88-2003&rfr_id=ori%3Arid%3Acrossref.org&rfr_dat=cr_pub++0pubmed.

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