A world without flowering plants wouldn't just be drab, it would be hotter and drier, particularly in parts of the tropics, a new study concludes. Flowering plants' rainmaking ability might also have stoked tropical evolution, the work suggests.
Plants are pipelines to the atmosphere, imbibing water with their roots and releasing it through evaporation from their leaves, a process called transpiration. The impact on climate is huge: About 10% of the moisture in the atmosphere emanates from plants, meaning that they can essentially make their own rain. Flowering plants, or angiosperms—the group that includes elms, oaks, tulips, and roses—transpire more than other kinds of plants, thanks to their superior plumbing .
Because flowering plants account for almost all vegetation on Earth, paleontologist C. Kevin Boyce and climate modeler Jung-Eun Lee of the University of Chicago in Illinois wondered what impact they've had on global weather patterns ever since they arose more than 100 million years ago in the Cretaceous period.
To simulate a world without angiosperms, the researchers rejiggered climate models, cutting transpiration by 75%—the approximate amount contributed by these plants. The effects were complex, with some areas drying and some even becoming wetter. Eastern North America, for example, received 30% to 50% less precipitation. But the biggest impact occurred in tropical areas of South America, the pair will report  online tomorrow in the Proceedings of the Royal Society B. Without angiosperms, average annual rainfall in the area declined by 300 millimeters. In the eastern Amazon basin, the length of the wet season decreased by nearly 3 months. The extent of the wettest rainforests, which receive more than 100 millimeters of rain per month, shrank by 80%. The effects weren't as severe in other tropical areas, such as in Africa, which already has lots of dry tropical forests.
A drier world would have been bad for other species, too. As a rule, less precipitation translates into fewer species of animals and plants—that's why deserts are biologically deprived. So angiosperms' transpiration capacity might have been important not just ecologically but also evolutionarily, spurring the origin of more tropical species, including other angiosperms. "They modify their environment in ways that bolster their own diversity," Boyce says.
"It's a good paper," says paleobotanist Carlos Jaramillo of the Smithsonian Tropical Research Institute in Panama. "This work shows that angiosperms play a big role in the climate of the tropics." A further question is how the appearance of angiosperms affected climate in the Cretaceous period.