Fossils of Antarctic trees reveal surprising growth pattern
Although Antarctica is now much too cold to support
vegetation, it hasn’t always been: The fossil record indicates that plants
thrived on the now-icy continent from 400 million to 34 million years ago, says
Patricia E. Ryberg, a paleobotanist at the
But those ancient Antarctic trees grew nothing like today’s
trees do, Ryberg reported October 5 in
At each of these times, large amounts of planet-warming
greenhouse gases boosted global temperatures, giving even
Also, the structure of the Antarctic wood is quite different from wood in today’s trees, says Ryberg. In modern temperate-climate species, the dark wood that forms at the end of each growing season — the so-called latewood characterized by small, thick-walled cells — makes up about 40 percent of each annual growth ring. In the Antarctic fossils, however, the latewood makes up only between 5 and 12 percent of each growth ring. The number of cells in each growth ring suggests that the trees may have grown for no more than two months each year.
However, that estimate may be low, Ryberg notes, because it is based on today’s temperate-climate trees, which grow only during daytime hours. The Antarctic trees, Ryberg says, were exposed to 24-hour sunshine during the height of summer and might have grown continuously then.
Typically, three factors — cold temperature, lack of water and lack of sunlight — can strongly limit the growth rates of modern trees. The Antarctic fossils don’t show signs that the trees ever suffered frost damage. Also, the fossils were found in sediments usually deposited on the floodplains of rivers, so the trees probably didn’t experience drought often.
So, Ryberg says, the end of the ancient trees’ growing seasons probably came only when the sun dropped below the horizon and the perpetually dark winter began — a sight no Antarctic tree has seen for millions of years.