Aphid mothers tend to seek out trees in the fall to lay eggs on, and if they laid them on a very red tree (i.e, well protected chemically), their offspring would not survive as well as those whose mother placed them on a less well defended tree. In essence, their hypothesis states that some insects, aphids mainly, avoid brightly colored (i.e., red!) trees, because the bright color was a signal that this tree was well defended chemically against predators, and therefore the aphid mothers should not lay their eggs on them. Ok, you say, I believe that the glaciations caused trees to go extinct in Europe more often than in North America, but why would this lead to the preferential elimination of those species with red fall color? To answer this, we go refer back to two influential papers published in 20 by Marco Archetti, William Hamilton and Samuel Brown, who together developed a theory that red fall color serves as a warning to insect pests. In contrast, Great Smoky Mountains National Park, which is only a fraction the size of England, has 125 species of trees! As but one example, England has only 12 native tree species, the rest having been wiped out by glaciations and others prevented from migrating over from mainland Europe after England became separated by the English Channel. Thus, many species would have been trapped up against the mountains and extirpated.
However, in Europe during periods of glaciation, the southerly march of species would have been prevented by the fact that the mountains (the Alps) run predominantly east-west, creating a barrier to migration. We know this because when scientists analyze soil cores extracted from lakes and bogs in the south, they find pollen grains of species that today grow only in the north, such as red spruce and jack pine. In North America (and East Asia) the mountains are aligned primarily north-south, so species could migrate on the west and east sides to more southerly climes.
Plants and animals had to migrate southwards to avoid the ice and cold temperatures.
During the cool times, glaciers covered much of the land in the northern hemisphere. Beginning nearly 35 million years ago, in the Tertiary, the northern hemisphere was subjected to repeated episodes of cooling and warming. How could fall color displays in trees be related to the glaciers? Yev-Ladun and Holopainen reason as follows. They suggest that this asymmetry in red fall color is the result of the glaciers that have intermittently covered the northern temperate zone. So, why should there be this difference in fall color between the New and Old World and could there be a scientific explanation? These were the questions that researchers Simcha Lev-Yadun from Israel, and Jarmo Holopainen, from Finland, addressed in a recent article published in the scientific journal New Phytologist (see Lev-Yadun and Holopainen 2009 at end of this essay). In East Asia, the number of trees with red fall color is above 150. Here on this side of the pond there are at least 89 species that have red leaves in the fall, such as sugar maple, mountain ash, sumac, scarlet oak, dogwood, sweetgum, and sourwood, just to name a few. In fact, only four northern European tree species have red fall color: Prunus padus (bird cherry), Prunus spinosa (blackthorn), Sorbus aucuparia (European mountain ash) and Acer platinoides (Norway maple) while in all the rest of Europe there are just 24 species that turn red. Trees in northern Europe are dominated by yellow and orange fall color, and relatively few turn red. But one thing would be missing that we take for granted in the United States and Canada, and that is the color red.
For anyone who has ever been in Scandinavia in the fall (Finland, Norway, Sweden, and nearby countries) you would have been impressed by the fall color display of the trees, just as we are here in North America.
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