New Worlds: Where have all the red leaves gone?

August 29, 2009 21:08
4 minute read.
for judy's column

new worlds 88. (photo credit: )

Immigrants from North America will notice that the leaves of deciduous trees fated to fall to the ground in autumn are not red but yellow. Now researchers at the University of Haifa and Finland's University of Kuopio have a new theory that reaches 35 million years back in time to explain why. Americans and tourists to the US travel long distances to see ruddy autumn leaves, especially in the northeast. The color is the same in East Asia, but not in Europe or Israel. Prof. Simcha Lev-Yadun of the university's science education department and Prof. Jarmo Holopainen of the Finnish university have just published their theory in the journal New Phytologist . The green of a tree's leaves is from the larger proportion of chlorophyll pigment. The change in color to red or yellow as autumn approaches is not the result of the leaves' dying but of a series of processes - which differ between the red and yellow autumn leaves. When the chlorophyll in leaves diminishes, the yellow pigments that already exist become dominant. Red autumn leaves result from a different process: As the chlorophyll diminishes, a red pigment called anthocyanin, not previously present, is produced. These facts were only recently discovered, and led to a surge of research studies attempting to explain why trees expend resources on creating red pigments just as they are about to shed their leaves. Explanations vary, and there is no agreement as yet, says Lev-Yadun. One discipline suggests that the red pigment is produced as a result of physiological functions that make the retranslocation of amino acids to the woody parts of the tree more efficient in setting up protection against the potential damage of light and cold. Other explanations suggest that the red pigment is produced as part of the tree's strategy for protecting itself against insects that thrive on amino acids. But whatever the answer, these explanations do not help us understand why the process of creating anthocyanin, the red pigment, does not occur in Europe or Israel. An evolutionary approach infers that the strong autumn colors result from the long evolutionary war between trees and the insects that use them as hosts. During the fall, which is when the insects suck amino acids from the leaves and later lay their eggs, the tree colors its leaves red because aphids are attracted to yellow ones, as if to lower the tendency of the insects to occupy the leaves for nutrition and the bark for breeding. In this case too, the protective logic of red pigmentation may be sound, but the yellow leaves cannot be reconciled with this approach. The new theory can be applied to settle this point, says Lev-Yadun. According to the new theory, until 35 million years ago large areas of the globe were covered with evergreen jungles. During this phase, a series of ice ages and dry spells caused many tree species to become deciduous. Many of these trees also began an evolutionary process of producing red leaves to ward off insects. In North America, as in East Asia, north-to-south mountain chains enabled plant and animal "migration" with the advance and retreat of the ice. And of course, along with them migrated their insect "enemies" too. Thus the war for survival continued 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 did not survive the severe cold, and with them perished the insects that depended on them for survival. At the end of the ice ages, most tree species that had survived in Europe had no need to cope with the insects that had become extinct, and therefore no longer had to expend effort on producing red 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. Unlike trees, dwarf shrubs managed to survive the ice ages under a layer of snow that 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. JELLYFISH MIX IT UP Jellyfish and other small swimming marine creatures can have a huge impact on ocean mixing, according to marine researchers in California. Scientists at the California Institute of Technology believe that such ocean animals may play a role in the process by which layers of water interact to distribute heat, nutrients and gases, according to a UPI report on an article in the journal Nature. "The perspective we usually take is how the ocean - by its currents, temperature, and chemistry - is affecting animals," says John Dabiri, a Caltech bioengineer who headed the team that discovered the new mechanism. "But there have been increasing suggestions that the inverse is also important - how the animals themselves, via swimming, might impact the ocean." Dabiri said oceanographers had dismissed the idea that animals have a significant effect on ocean mixing, believing that the viscosity of water would balance out any turbulence created by small drifting animals. "Results from this study will change some of our long-held conceptions," comments David Garrison, director of the National Science Foundation's biological oceanography program, which funded the research.

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