Fall Colors and Autumn Leaves

This post was chosen as an Editor's Selection for ResearchBlogging.orgEarlier this year, researchers in Finland and Israel sought an explanation for the 35 million year puzzle as to why fall colors in the US are mainly red and why autumn leaves turn mainly yellow in Europe.

The green of the leaves of deciduous trees in spring and summer is caused mainly by the presence of the pigment chlorophyll, which absorbs sunlight for photosynthesis.

The change in color to reds and yellows in autumn is not caused by the leaves dying, but by a series of controlled biochemical processes. When the green chlorophyll in leaves diminishes, the yellow pigments that already exist become dominant and give their color to the leaves.

English Lake District in Autumn (by David Bradley)
English Lake District by David Bradley

Red autumn leaves result from a different process: As the chlorophyll diminishes, a red pigment, anthocyanin, which was not previously present, is produced in the leaf. These facts were only recently discovered and led to a surge of research studies attempting to explain why trees expend resources on creating red pigments just as they are about to shed their leaves.

According to a theory by Simcha Lev-Yadun of the Department of Science Education- Biology at the University of Haifa-Oranim and Jarmo Holopainen of the University of Kuopio in Finland, until 35 million years ago, large areas of the globe were covered with evergreen jungles or forests composed of tropical trees. During this phase there was a series of ice ages and dry spells and tree species evolved into deciduous varieties. Many of these trees also began an evolutionary process of producing red deciduous leaves in order to ward off insects. In North America, as in East Asia, north-to-south mountain chains enabled plant and animal ‘migration’ to the south or north with the advance and retreat of the ice according to the climatic fluctuations. And, of course, along with them migrated their insect ‘pests’ too. Thus the war for survival continued there uninterrupted.

In Europe, on the other hand, the mountains – the Alps and their lateral branches – reach from east to west, and therefore no protected areas were created. Many tree species that did not survive the severe cold died, and with them the insects that depended on them for survival. At the end of the repeated ice ages, most tree species that had survived in Europe had no need to cope with many of the insects that had become extinct, and therefore no longer had to expend efforts on producing red pigments as a toxic warning in their leaves.

Fall colors and Autumn leaves
Autumnal photo by David Bradley

Eastern Canada and the New England region of the United States are famous for the brilliance of their “fall foliage,” and a seasonal tourist industry has grown up around the few weeks in autumn when the leaves are at their peak. When you’re absorbing this brilliance and snapping photos, just think what might not have been if it hadn’t been for hungry tree pests 35 million years ago.

Thick forest cover and distinct seasonal changes make that part of the world an ideal setting for the types of deciduous trees that produce wonderful fall foliage. Fall colors are typically at their peaks in Early to mid October for much of the northern and interior parts of the area, late October for areas further south, and early November for the warmer subtropical areas of the region.

More… and even more…

Research Blogging IconLev-Yadun, S., & Holopainen, J. (2009). Why red-dominated autumn leaves in America and yellow-dominated autumn leaves in Northern Europe? New Phytologist, 183 (3), 506-512 DOI: 10.1111/j.1469-8137.2009.02904.x

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