U. CHICAGO (US)—The world is a cooler, wetter place because of flowering plants, according to new climate simulations.

The effect is especially pronounced in the Amazon basin, where replacing flowering plants with non-flowering varieties would result in an 80 percent decrease in the area covered by ever-wet rainforest.

The simulations demonstrate the importance of flowering-plant physiology to climate regulation in ever-wet rainforest, regions where the dry season is short or non-existent, and where biodiversity is greatest. Details are published in the journal Proceedings of the Royal Society B.

“The vein density of leaves within the flowering plants is much, much higher than all other plants,” says the study’s lead author, C. Kevin Boyce, an associate professor in geophysical sciences at the University of Chicago. “That actually matters physiologically for both taking in carbon dioxide from the atmosphere for photosynthesis and also the loss of water, which is transpiration. The two necessarily go together. You can’t take in CO2 without losing water.”

Why vein density matters
This higher vein density in the leaves means that flowering plants are highly efficient at transpiring water from the soil back into the sky, where it can return to Earth as rain.

“That whole recycling process is dependent upon transpiration, and transpiration would have been much, much lower in the absence of flowering plants,” Boyce says.

“We can know that because no leaves throughout the fossil record approach the vein densities seen in flowering plant leaves.”

For most of biological history there were no flowering plants—known scientifically as angiosperms. They evolved about 120 million years ago, during the Cretaceous Period, and took another 20 million years to become prevalent. Flowering species were latecomers to the world of vascular plants, a group that includes ferns, club mosses and confers. But angiosperms now enjoy a position of world domination among plants.

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Researchers at the University of Chicago conducted a climate study, which found that the tropics are cooler and wetter because of flowering plants. (Credit: Dan Dry)

“They’re basically everywhere and everything, unless you’re talking about high altitudes and very high latitudes,” Boyce says.

Plants and dinos
Dinosaurs walked the Earth when flowering plants evolved, and various studies have attempted to link the dinosaurs’ extinction or at least their evolutionary paths to flowering plant evolution. “Those efforts are always very fuzzy, and none have gained much traction,” Boyce notes.

Boyce and Lee are, nevertheless, working toward simulating the climatic impact of flowering plant evolution in the prehistoric world. But simulating the Cretaceous Earth would be a complex undertaking because the planet was warmer, the continents sat in different alignments and carbon- dioxide concentrations were different.

“The world now is really very different from the world 120 million years ago,” Boyce says.

Supercomputer simulation
So as a first step, Boyce and coauthor Jung-Eun Lee, a postdoctoral scholar at Chicago, examined the role of flowering plants in the modern world. Lee adapted the National Center for Atmospheric Research Community Climate Model for the study.

Driven by more than one million lines of code, the simulations computed air motion over the entire globe at a resolution of 300 square kilometers (approximately 116 square miles). Lee ran the simulations on a supercomputer at the National Energy Research Scientific Computing Center in Berkeley, Calif.

“The motion of air is dependent on temperature distribution, and the temperature distribution is dependent on how heat is distributed,” Lee says. “Evapo-transpiration is very important to solve this equation. That’s why we have plants in the model.”

The simulations showed the importance of flowering plants to water recycling. Rain falls, plants drink it up and pass most of it out of their leaves and back into the sky.
In the simulations, replacing flowering plants with non-flowering plants in eastern North America reduced rainfall by up to 40 percent. The same replacement in the Amazon basin delayed onset of the monsoon from Oct. 26 to Jan. 10.

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