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Why Leaves In U.S. Turn Various Colors In Fall, Europe Only Turn Yellow

September 22, 2009

This is fascinating.  (Not a bad topic to start with on the first day of fall.)

While the fall foliage in North America and East Asia takes on a fiery red hue, perplexingly, autumn leaves in Europe are mostly yellow in color.

A team of researchers has a new idea as to why the autumnal colors differ between the continents, one that involved taking a step back 35 million years in time.

Until 35 million years ago, the idea goes, large areas of the globe were covered with evergreen jungles or forests composed of tropical trees, say Simcha Lev-Yadun of the University of Haifa-Oranim in Israel and Jarmo Holopainen of the University of Kuopio in Finland. During this phase, a series of ice ages and dry spells transpired, and many tree species evolved to become deciduous, dropping their leaves for winter.

Many of these trees also began an evolutionary process of producing red deciduous leaves in order to ward off insects, the researchers say.

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 ‘enemies’. 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 warning leaves.

 

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