Warm spell spurred tropical biodiversity

Number of plant species rose along with atmospheric carbon dioxide

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

As global average temperatures spiked 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 biologist 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 study raises new questions about how tropical rain forests might respond as atmospheric carbon dioxide levels rise because of fossil fuel burning and other industrial activities. The researchers say that today’s forests may not respond to warming in the same way that ancient forests did, but 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 involved in the study. “We’re beginning to map out what happened in different places during this huge perturbation of the carbon cycle and climate system. It’s our best bet at seeing the results of something that’s already happened.”

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. Changes in ocean chemistry, such as “burps” of methane gas released from the seafloor, are thought to have rapidly built up greenhouse gases in the atmosphere and caused the warming. 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 increase over 10,000 years, researchers expect a 2-degree increase over just the next century, with more in store after that. 

HEAT LOVERS The diversity of pollen found in deposits from northern South America suggest that plants there thrived during a warm spell 56 million years ago. Francy Carvajal/Smithsonian Tropical Research Institute

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

To probe this question, Jaramillo and his colleagues spent seven years scouring South America for sedimentary rock outcrops whose ages span the Paleocene-Eocene boundary. Eventually the researchers narrowed their list to three sites in eastern Colombia and western Venezuela. By taking samples of pollen and other plant fossils from rock layers below and above the boundary, the team could gauge the number of plant types living 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. “It’s not just a matter of applying what we learned at that time, because today the forest is very fragmented,” Jaramillo says. “For the forests, I don’t think global warming is going to be good.”

Today’s tropical forests contain around 60,000 tree species, most of which are very rare, says ecologist Joseph Wright of the Smithsonian’s Panama research institute. Researchers expect that as temperatures rise over the next century, some of those species will vanish while others become much more common.

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.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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