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Ancestor of modern trees preserves record of ancient climate change

Image: This times-200 magnification of the Coastal redwood, Sequoia sempervirens, a temperate rain forest tree from the western United States, shows the 'temperate' type of growth ring. Down is the inner late wood (with small cells), up is the outer early wood (with large cells).

BLACKSBURG, VA., November 2, 2006 -- About 350 million years ago, at the boundary of the Devonian and Carboniferous ages, the climate changed. There was no one around to record it, but there are records nonetheless in the rocks deposited by glaciers and in tissues preserved in fossils of ancient life.
“Events at the transition had terrific biological impact, marked by extinctions and the beginnings of new life forms,” said Stephen Scheckler of Blacksburg, professor of biological sciences and geosciences at Virginia Tech. He reported on evidence of climate change that he found in the fossils of the ancestors of modern trees at the Geological Society of America national meeting in Philadelphia Oct. 22-25.
“This glaciation was not widely understood until recently,” Scheckler said. “It was a worldwide event. The Europeans recognize the extinctions as the Hangenburg event, documented in a black shale deposit that contains a series of fauna changes. But the eastern United States was at a tropical latitude at that time, so the flora and fauna show less impact – but it is there. It is believed to be a time of coldness, because there was less diversity, but it is a subtle signal.”
Scientists exploring parts of the world farther from the equator have found glacial deposits, where the earth was scoured and sediment was dropped as the ice moved across Africa and Brazil. “Then glacial deposits were discovered in the former tropics. There is a widespread belt of rocks in Pennsylvania that were glacially deposited,” said Scheckler, who studied fossils from New York, Pennsylvania, West Virginia, and Ohio, from an age when the equator ran through New York and south through Virginia and the region was uniformly at a low elevation.
In his search for evidence of climate change, Scheckler, an authority on the earliest modern tree (Nature, April 22, 1999), looked at plants that made wood in the same way modern plants make wood. In modern trees, cambium tissue produces layers of wood cells on the inside and bark cells on the outside. The cambium moves outward as the tree grows and the kinds of cells it produces reflect seasonal dormancy induced by wet and dry or warm and cold conditions. The layers, of course, are tree rings.

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