New Theory on Why Leaves Change Color in Fall
Different Fall Leaf Colors Evolved Due to Insect Variations
Aug 13, 2009
Scientists have long wondered why the predominant fall leaf color in North America is red and in Europe, yellow. And while types of trees can partially justify the difference, it doesn't fully explain the visual magic. Two scientists at the University of Haifa and the University of Kuopio in Finland recently proposed a theory based on geography, insects, and climate change.
Why Leaves Turn Colors in the Fall
In temperate climates filled with deciduous trees and shrubs, the annual fall color show is often eagerly anticipated. News reports and weather channels provide "leaf updates" with maps pointing avid leaf peepers to the brightest fall colors.
Why do leaves change color in the fall? Every leaf contains chlorophyll, the green pigment that helps plants transforms the sun's energy into simple carbohydrates necessary for energy and life. But within each leaf, other pigments co-exist. Each leaf has varying potential to show off glittering shades of deep purple, burgundy, scarlet, crimson, gold and pure yellow.
Cooler temperatures, shorter days, and the changing angle of the sun's rays upon the plant leaves signal the plant to stop producing chlorophyll. The internal factories in the leaves shut down, reducing the amount of green pigment. As the green pigment diminishes, the other pigments shine forth, and leaves change color in the fall.
Red Fall Leaves in North America, Yellow in Europe
Some trees are noted for specific colors. Certain types of maples, for example, produce striking shades of red and yellow, while oaks typically show brown, purple or dark burgundy colors. Observers have long noted, however, that the predominant color found in fall foliage in North America contains shades of red, while in Europe the main color is yellow.
Professors Simcha Lev-Yadun of the Department of Science Education-Biology at the University of Haifa-Oranim and Professor Jarmo Holopainen of the University of Kuopio in Finland recently published a paper entitled, "Why Red Dominant Autumn Leaves in America and Yellow in Europe?" (pages 506-512)in the August 2009 issue of New Phytologist that proposes an alternative understanding of why leaves change color, and why red predominates North America and yellow in Europe.
Evolution During the Ice Ages Influenced Leaf Color
According to Lev-Yadun and Holopainen's theory, 35 million years ago, large portions of the world were covered by evergreen jungles. As the climate changed from tropical to the cold and the Ice Ages began, dry spells were common and deciduous tree species evolved.
North America and East Asia contain mountain chains that run north-south, and as the glaciers advanced, animals and insects migrated southwards. In Europe, the mountain chains run east to west, and insects and animals could not migrate to escape the creeping ice age.
Over time, trees adapted to the insects and to wards them off, those with a red color in the leaves survived better. During the fall, many species of insects feed upon leaves or lay eggs on trees to shelter over the winter. Trees that could repel insect invaders lived to reproduce, while those who could not died out.
In Europe, however, no such migration occurred simply because the east-west path of the mountains offered no protection. Trees that couldn't survive the ice ages died, along with the insects dependent on them for survival. Trees maintained their common yellow color in the fall.
The result is that North American species that could repel insects thrived, and with them, red leaf colors. European trees kept their predominant yellow color because there was no need to keep the red to survive.
This theory is brand new and speculative. But it yields an interesting idea of what might have transpired far in the past to create the glorious fall colors seen today.
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