Global Warming Could Mean Wetter Grasslands

STANFORD, California, August 4, 2003 (ENS) - A two year long experiment conducted in the grassy foothills above Stanford University's main campus has shown that wetter grassland ecosystems could result from global warming. The study, by researchers from Stanford and the Carnegie Institution of Washington, contradicts many climate models that forecast drier natural landscapes, including grasslands, due to climate warming.

Instead of causing the soil to dry up, higher temperatures increased soil moisture by as much as 10 percent, researchers found.

"Warming accelerates evaporation, so we expected warmer to mean drier," said lead author Erika Zavaleta, a former Stanford doctoral student now on the faculty of the University of California-Santa Cruz.

"We were surprised to find that warming actually increased moisture in our grassland plots during those critical weeks in late spring at the end of the growing season, when moisture shapes which plant species prevail," Zavaleta said. "We traced this unexpected moisture increase to the plants themselves."

"This doesn't mean climate change is good for California grasslands, but it reinforces the importance of paying attention to how plants and animals could modify its effects," said Zavaleta.

The study adds to a growing body of knowledge about the major role that plants can play in global warming, said coauthor Christopher Field, a professor by courtesy of biological sciences at Stanford and director of the Carnegie Institution's Stanford based Department of Global Ecology.

Christopher Field and field technician Julia Silvis analyze plant growth at an experimental plot in the Jasper Ridge Global Change Project. (Photo by L.A. Cicero courtesy Stanford University)

To observe the long term effects of global warming, the research team fenced off 32 circular plots of grassland, each six feet in diameter, inside Stanford's 1,200 acre Jasper Ridge Biological Preserve.

Each plot was equipped with an infrared heat lamp capable of raising ambient air temperatures an additional two degrees Fahrenheit, and a carbon dioxide (CO2) gas emitter that can double the concentration of atmospheric CO2, a greenhouse gas.

Over a two year period, heat and carbon dioxide were continuously applied to specific plots. The soil moisture in each plot was measured weekly duringmost of the experiment.

Researchers were surprised to discover that moisture increased in every plot where the heat lamp was kept on, while soil moisture increased in plots that had been exposed to elevated levels of carbon dioxide, as they expected.

"Simulated warming increased spring soil moisture by five to 10 percent under both ambient and elevated CO2," the authors wrote. "While elevated CO2 has been shown to increase soil moisture in other field experiments, stimulation of soil moisture by warming has not been previously reported."

The discovery that higher temperatures can dampen soil is at odds with several climate models that predict global warming will make grassland ecosystems drier, not wetter. Those models are based on the assumption that higher temperatures will increase the amount of water that evaporates from the soil and the surface of living plants, a process called "evapotranspiration."

At the Jasper Ridge site, most soil moisture evaporates through plants. But during the course of the experiment, researchers discovered that warming caused the early death of numerous grasses and wildflowers.

Some experimental plots that were exposed to higher temperatures lost 17 percent of their green vegetation prematurely. Since evapotranspiration only occurs through living plants, the fact that so many died early could explain the unexpected rise in soil moisture, the authors wrote.

"In California grasslands, plants control most of the water exiting the system by transpiring water through their leaves until they die," Zavaleta said. "Simulated global warming accelerated the death of the dominant grasses in our plots, leaving slightly more water in the soil for other species like oaks and summer wildflowers to use."

Researcher inserts an underground video camera designed to photograph the root systems in each of the experimental plots that make up the Jasper Ridge Global Change Project. (Photo by P. Cohen courtesy Stanford University)

Funded by a grant from the National Science Foundation to Professors Field and Harold Mooney, this is the first experimental study to address this particular set of interacting global changes in the field.

The Jasper Ridge Global Change Projec is designed to demonstrate how grassland ecosystems will respond to global changes that could occur in the next 100 years if increased fossil fuel consumption doubles the amount of carbon dioxide in the atmosphere.

According to climate experts, the additional CO2 could produce a global greenhouse effect that would raise worldwide temperatures by more than two degrees Fahrenheit.

The Jasper Ridge Global Change Project has also published studies in the last eight months showing that doubling the amount of carbon dioxide in the air significantly reduces the number of plant species that grow in the wild, and that elevated atmospheric carbon dioxide reduces plant growth when combined with other likely consequences of climate change such as higher temperatures, increased precipitation or increased nitrogen deposits in the soil.

Published in the Proceedings of the National Academy of Sciences (PNAS), the Stanford grasslands study will be posted this week on PNAS Online at: http://www.pnas.org

The study was supported by the National Science Foundation, the David and Lucile Packard Foundation, the Morgan Family Foundation, the Jasper Ridge Biological Preserve, the Carnegie Institution of Washington, the U.S. Department of Energy, the U.S. Environmental Protection Agency, the Switzer Foundation, the A.W. Mellon Foundation and the Nature Conservancy.