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Leaf fall in ancient polar forests still a mystery

Explorers in the 1800s discovered through fossils that deciduous forests once covered the poles, but researchers still do not know why leaf-dropping trees were preferred over evergreens.

"The dominant idea since the 1940s was that because of the polar light regime of continuous darkness and warmth, leafless branches had an advantage over evergreen canopies in the polar forests," says Dana Royer, research associate in geosciences, Penn State.

This carbon loss hypothesis states that the amount of carbon lost when a canopy of leaves is shed annually is less than the total carbon lost by canopy respiration during the warm, dark winter months and the small amount of leaf loss in evergreens. This would give deciduous trees an advantage during long, very dark winters.

During much of the past 250 million years, the Earth's poles were devoid of ice, and nearly 40 percent of the area was covered by forests consisting mostly of deciduous trees.

"Today, we do not have these types of forests in the polar latitudes so we have no analog," says Royer. "In most places, the trees we see at the tree line today are evergreens, not deciduous trees."

Working with Dr. David J Beerling, professor and Dr. Colin P. Osborne in the department of animal sciences, University of Sheffield, Sheffield, UK, the Penn State researcher tested trees considered living fossils to see if the carbon loss hypothesis was correct. They looked at gingko, dawn redwood and bald cypress, all deciduous trees that are considered living fossils because they have existed much as they are since there were polar forests. Also tested were two living fossil evergreens, sequoia and southern beech.

In Sheffield, the trees grew under controlled temperatures, carbon dioxide and light. The researchers monitored the amount of leaf litter and the amounts of carbon lost to respiration for both groups of trees.

The researchers report in this week's issue of Nature
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Contact: A'ndrea Elyse Messer
aem1@psu.edu
814-865-9481
Penn State
2-Jul-2003


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