HOUSTON — Trees that grew in Antarctica millions of years ago had a growth pattern much different than modern trees, according to a new fossil study reported during the Geological Society of America meeting.
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 University of Kansas in Lawrence. Indeed, she notes, lush forests covered the landscape during some parts of that interval, even though Antarctica sat at or near the South Pole then, as it does today.
But those ancient Antarctic trees grew nothing like today’s trees do, Ryberg reported October 5 in Houston during the annual meeting of the Geological Society of America. Modern trees living in temperate climates have annual growth rings no wider than 2 millimeters. But some samples of the well-preserved fossils of trees that covered Antarctica about 255 million years ago during the Permian have growth rings close to 10 millimeters wide. Trees that lived about 237 million years ago, during the Triassic, have growth rings as wide as 6.8 millimeters, Ryberg and University of Kansas colleague Edith L. Taylor found.
At each of these times, large amounts of planet-warming greenhouse gases boosted global temperatures, giving even Antarctica a temperate climate.
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.