In Europe, the tree leaves appearing during the Autumn season are predominantly yellow, whereas in America they are mostly red. Why? An article recently published in New Phytologist attempts to solve this mystery. The fact that plants produce anthocyanin, a pigment that causes leaves to appear red once the chlorophyll in them starts to diminish during the Autumn season, has been known for some time now.

But Professor Simcha Lev-Yadun of the University of Haifa-Oranim and Professor Jarmo Holopainen of the University of Kuopio in Finland present a new theory that explains the difference in Autumn color as observed in America and Europe.

According to their theory, the reason that American trees have red leaves is that the red pigment is required to ward off insects that have historically sucked nutrients from the leaves of plants during Ice Ages and dry spells. During an Ice Age in America, the North-South mountain range created a respite to plant and animal species. Because the plants and the insects who fed on them survived, the “arm’s race” between plants and insects was maintained and the red pigment remained necessary as a defense mechanism.

In Europe, however, the mountain ranges run East to West. Lev-Yadun and Holopainen explain that, as a result, the European mountain ranges did not provide any protection for plants and insects during an Ice Age. Thus many plants did not survive the cold winters and the insects that depended on them died out. The surviving trees were quite strong and had no need to protect themselves from insects, so there was no need to produce the red pigment.

This theory is supported by the presence of red leaves in small shrubs in Europe. Unlike trees, which are exposed to the ravages of stormy winters, small shrubs often survive by hiding beneath the snow cover. However, when these plants are covered under the snow and protected, the insects that depend on the plants are also protected. Thus the competition between the two persists, and the red pigment remains.

This kind of evolutionary conjecture has always been an integral part of science. It serves as a reminder that the best way understand the present is to look to the past. With this knowledge of why the red pigment in leaves is created, botanists have a whole new way of ‘viewing’ colored leaves in plants.

Discussion question: Why do some places in the United States have a more diverse mix of deciduous trees? What connections can be made between the leaves of those trees and the geography of the regions in which they are found?

News Article: http://www.sciencedaily.com/releases/2009/08/090813142150.htm
Journal Article: http://www3.interscience.wiley.com/journal/122453432/abstract?CRETRY=1&SRETRY=0