Another Global Warming Surprise: Grasslands May Become Wetter As Temperatures Rise

The study, to be posted the week of Aug. 4 on PNAS Online, is based on a two-year field experiment conducted in the grassy foothills above Stanford's main campus. Instead of causing the soil to dry up, the experiment revealed that higher temperatures actually increased soil moisture by as much as 10 percent.

"Warming accelerates evaporation, so we expected warmer to mean drier," said lead author Erika S. 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. We traced this unexpected moisture increase to the plants themselves."

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

"We found that, once the plants shut down, the moisture is effectively trapped in the soil," he noted.

Global Change Experiment

The PNAS study is the third in a trilogy of papers published in the last eight months by the Jasper Ridge Global Change Project. This multiyear experiment 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 2 degrees Fahrenheit or more.

To observe the long-term effects of global warming, the research team fenced off 32 circular plots of grassland, each 6 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 2 F, and a carbon dioxide gas emitter that can double the concentration of atmospheric CO2.

In the PNAS study, heat and/or CO2 gas were continuously applied to specific plots for about two years. Soil moisture in each plot was measured on a weekly basis throughout most of the experiment.

As expected, soil moisture increased in plots that had been exposed to elevated CO2. But researchers were surprised to discover that moisture also increased in every plot where the heat lamp was kept on.

"Simulated warming increased spring soil moisture by 5-10 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."

Conflicting results

The discovery that higher temperatures can significantly dampen soil is at odds with several climate models that predict that 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 demise of numerous grasses and wildflowers. In fact, some experimental plots that were exposed to higher temperatures suffered the premature loss of 17 percent of their green vegetation. Since evapotranspiration only occurs through living plants, the fact that so many died early could explain the unexpected rise in soil moisture, the authors noted.

"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. 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."

Other discoveries

"Mediterranean-type ecosystems throughout the world experience similar growing season dynamics to our site and even share some common species as a result of exotic introductions," the authors concluded. "It is reasonably likely that they will respond in qualitatively similar ways to warming."

In the past year, Jasper Ridge scientists have reported other discoveries that run counter to widely held assumptions about global climate change. In a Dec. 2002 study in the journal Science, Field, Zavaleta and their colleagues discovered that excess atmospheric CO2 can retard plant growth instead of increasing it. And in a PNAS study published in June, the researchers found that excess CO2, nitrogen and water caused a significant reduction in the diversity of plant species.

Other coauthors of the August PNAS study are Brian D. Thomas, a former doctoral student at Stanford now with Environmental Defense; Nona R. Chiariello, research coordinator of the Jasper Ridge Biological Preserve; Gregory P. Asner, assistant professor (by courtesy) in Stanford's Department of Geological and Environmental Sciences and assistant professor in the Carnegie Institution's Department of Global Ecology; and M. Rebecca Shaw of the Nature Conservancy.

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.