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Why are Fall Leaves Red in America, Yellow in Europe?

Autumn leaves of trees in North America often turn red. But in Europe the leaves mostly go yellow. Scientists think that the regional difference can be explained by the geographic orientation of each continent’s mountains.


NGS photo by Robert Sisson

A new theory provided by Simcha Lev-Yadun of the Department of Science Education-Biology at the University of Haifa-Oranim in Israel and Jarmo Holopainen of the University of Kuopio in Finland proposes taking a step 35 million years back to solve the color mystery, says a news statement by the University of Haifa.

“According to the theory provided by Prof. Lev-Yadun and Prof. Holopainen, until 35 million years ago, large areas of the globe were covered with evergreen jungles or forests composed of tropical trees,” the university said.

“Trees also began an evolutionary process of producing red deciduous leaves in order to ward off insects.”

“During this phase, a series of ice ages and dry spells transpired and many tree species evolved to become deciduous. Many of these trees also began an evolutionary process of producing red deciduous leaves in order to ward off insects.”

Scientists have determined that leaves turn yellow when the green pigment, chlorophyll, recedes prior to the onset of winter, as trees prepare to shed their leaves for the cold weather. Leaves that turn red are the result of trees producing anthocyanin, a red pigment, which some scientists think is an evolutionary response that deters insects from laying their eggs in the trees.

Whatever the reason for leaves turning red, it occurs a lot more commonly in North America than in Europe. Could it have something to do with the physical geography of the continents?

North-South Mountain Chains

“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,” said the University of Haifa. “And, of course, along with them migrated their insect ‘enemies’ too.

“Thus the war for survival continued there uninterrupted.

red-leaves-of-oak-tree picture.jpg

“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 warning leaves.”

According to the scientists, evidence supporting this theory can be found in the dwarf shrubs that grow in Scandinavia, which still color their leaves red in autumn.

NGS illustration by Walter A. Weber

“Unlike trees, dwarf shrubs have managed to survive the ice ages under a layer of snow that covered them and protected them from the extreme condition above. Under the blanket of snow, the insects that fed off the shrubs were also protected–so the battle with insects continued in these plants, making it necessary for them to color their leaves red.”

The research was published in the journal New Phytologist.