by Zahra Hirji Thursday, January 5, 2012
As leaves change color every fall, the North American landscape transforms from a rolling verdure to a collage of vibrant yellows, oranges and reds. The autumn foliage in Europe, however, is rather bland, composed of mostly yellow leaves with red-leafed trees few and far between. Why this is the case has remained a mystery for years. But discovering why Europe’s leaves don’t turn red is only half of the battle; determining why the trees' leaves turn yellow in the first place is the other.
In the past decade, scientists have determined that green leaves produce yellow and red pigments differently. Yellow pigments are already present in green leaves, but only become the dominant hue when a leaf’s chlorophyll breaks down at the beginning of fall. But red pigments are newly produced in the leaf right before the leaf is shed from a tree — and this, say Simcha Lev-Yadun, a biologist at the University of Haifa in Israel, and Jarmo K. Holopainen, an environmental scientist at the University of Kuopio in Finland, is due to adaptations the trees made before glaciations set in millions of years ago to evade predation from insects.
To test this idea, Lev-Yadun and Holopainen started by looking at the geographic overlap between the distribution of 290 tree species with red autumn leaves and paleoclimate patterns across Asia, Europe and North America over the last 65 million years. In all of Europe today, there are only 24 species of trees with red autumn leaves. Eastern North America and East Asia currently have a greater abundance of red-leafed tree species, with at least 89 and 152 known species, respectively. This is probably related to three things, the researchers noted: insects, climate and geography.
Insects prey on the amino acids in leaves. Though amino acids are present in leaves year round, they build up in the leaves as they prepare to fall off the trees, Lev-Yadun says, and the different colors of leaves, red or yellow, are indicative of this transition. Many modern insects that eat leaves, such as aphids, are more attracted to yellow leaves than to red leaves, Lev-Yadun and Holopainen reported in the journal New Phytologist. The same was probably true for insects in the past, they say. This is because historically, the red pigment in leaves is often associated with greater defense mechanisms, such as toxins, than yellow-colored leaves.
Deciduous trees everywhere — Asia, Europe and North America alike — probably would have developed this autumn red pigmentation change to avoid predation, Lev-Yadun and Holopainen add. But things began to change about 35 million years ago in the Northern Hemisphere, as the climate began to cool and glaciations set in. Trees and insect populations migrated and changed in response to the glaciations. And since then, the scientists say, Europe’s leaves have been predominantly yellow, whereas trees in East Asia and eastern North America have had the range of red hues.
Eastern North America and East Asia also have similar geographies, Lev-Yadun and Holopainen say. In both regions, mountain ranges, such as the Appalachians in the United States and the Great Hig’an Range in northeast China, are oriented north-to-south, leaving clear pathways through which flora and fauna could migrate between the northern and southern parts of the continents. In Europe, on the other hand, mountain ranges such as the Alps trend east-to-west and serve as a geological barrier that divides the northern and southern parts of the continent, the researchers report.
Based on these patterns, Lev-Yadun and Holopainen suggest that the different continental geographies played a role in shaping red-leafed trees' modern distribution through the effects of alternating periods of glaciation and deglaciation, starting in the middle of the Cenozoic about 35 million years ago and ending in the Pleistocene epoch approximately 11,000 years ago.
During periods of glaciation, animals and plants, including red-leafed trees and the insects that preyed on them, would be pushed south to warmer regions. When the glaciers retreated during warmer periods, the plants and animals could return north. This is what happened in both North America and Asia during the Cenozoic, the researchers note.
But in Europe, trees and animals got trapped during glacial periods between the Alps to the south and glaciers to the north. As a result, most of the red-leafed trees and their insect predators went extinct, as did many yellow-leafed trees. When the glaciers retreated, trees once again repopulated Europe — but because the red-leaf-eating insects never fully recovered, there was no need for new tree species to evolve the red-leaf defense mechanism, according to Lev-Yadun and Holopainen.
Paleobiologists have observed the coupling between the Pleistocene glaciations and Northern Hemisphere geography for other plants and trees, such as the Tulip Tree and Sour and Sugar Gum Trees, for many years, says David Dilcher, a paleobiologist at the University of Florida in Gainesville, who is not affiliated with the study. But this is the first kind of study that bridges molecular biology and physiology with earth sciences, including paleobotany, Dilcher says. The argument that insects played a major role in the yellow/red autumn leaf distribution across the Northern Hemisphere in the past, however, needs a little more ground-truthing, he adds.
Lev-Yadun and Holopainen agree that they still need to find more tangible proof to support their hypothesis. The fossil record contains plenty of examples of plants adapting to large animals. For millions of years, plants have adopted defense mechanisms — from the chemical response of releasing toxins to physical mechanisms such as the development of spikes and thorns — in response to herbivory or predation by large animals, they note. But so far, researchers have found few to no examples of plants adapting to small animals like insects. Thus, they say, further study of fossil insects and insect-plant co-evolution is needed.
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