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.
To back up this theory, the researchers offer an example of the exception that proves the rule: Dwarf shrubs, which grow in Scandinavia, still color their leaves red in autumn.
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 conditions 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 thinking goes.
http://www.livescience.com/5749-fall-col…