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Warm spell spurred biodiversity in South American tropical rain forest: at least some plants could survive hotter climes than today's.

Some like it hot, including the plants living in South America's tropical rain forests 56 million years ago.


As average global temperatures spiked by 5 degrees Celsius over a period of 10,000 years--a geologic blink of an eye--plant diversity in northern South America also soared, researchers report in the Nov. 12 Science.

"We were expecting to find rapid extinction, a total change in the forest," says study leader Carlos Jaramillo, a paleobotanist at the Smithsonian Tropical Research Institute in Balboa, Panama. "What we found was just the opposite--a very fast addition of many new species, and a huge spike in the diversity of tropical plants."

The fossil study raises questions about how tropical forests might respond as atmospheric carbon dioxide levels rise because of fossil fuel burning and other industrial activities. Though today's forests may not respond to warming the same way ancient forests did, researchers say the findings do suggest that at least some plants are surprisingly adaptable.

"This kind of work is critically important," says Scott Wing, a paleobotanist at the Smithsonian Institution in Washington, D.C., who was not part of the study. "We're beginning to map out what happened in different places during this huge perturbation of the carbon cycle and climate system."

Researchers call the warming the Paleocene-Eocene Thermal Maximum, because it took place at the boundary between the Paleocene and Eocene epochs of geologic time. It's the closest analog scientists have to the global warming they expect in the future, though on a much slower scale; today, instead of a 5 degree Celsius increase over 10,000 years, researchers expect a 2 degree C increase over just the next century, with more warming after that.

Only a few places on land preserve evidence of how plants and animals responded to the Paleocene-Eocene heat, most in temperate or northern latitudes. In Wyoming, Wing and other researchers have found fossils suggesting that as things heated up, species from more southern regions moved into the area temporarily. But some tropical forests are already in the hottest places on the globe, so there is no still-warmer place from which other species might have moved to populate these spots. Many think these forests are already close to the maximum temperature at which they can survive.

To probe this question, Jaramillo and colleagues spent seven years scouring South America for sedimentary rock outcrops with ages spanning the Paleocene-Eocene boundary. Eventually the researchers narrowed their list to three sites in Colombia and Venezuela. By taking samples of pollen and other plant fossils from rock layers below and above the boundary, the team could gauge the diversity of plant types in those places before, during and after the hot spell.

Before the warming, the landscape was covered by a tall, damp rain forest with even more species than the Amazon has today, Jaramillo says. As temperatures rose into the Eocene, more plant groups appeared in the rock record--mainly angiosperms, the flowering plants that are the largest and most diverse plant group on Earth. Once the warming abated about 200,000 years later, those new plants stuck around for good.

Unlike in Wyoming, where native plants moved off the scene during hot spells and then returned, the South American plants apparently dealt with the heat by diversifying in a great evolutionary burst. "This shows that plants have the genetic variability already built in to cope with high temperatures and high CO2," says Jaramillo.

But that doesn't mean tropical forests will necessarily thrive under future climate change. Oliver Phillips, a tropical ecologist at the University of Leeds in England, says that it's a stretch to suggest that plants at the end of the Paleocene would have responded to warming the same way modern plants would. "Very few modern genera, let alone species, were extant 56 million years ago," he says, "and our modern plants therefore evolved with different climate tolerances." Tropical plants are far more likely to go extinct in the near future than to diversify into new species, he says.

So even though global warming is expected to raise temperatures the most at polar latitudes, it may have the greatest biodiversity impact in the tropics.
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Title Annotation:Earth
Author:Witze, Alexandra
Publication:Science News
Geographic Code:30SOU
Date:Dec 4, 2010
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