DRI's one-of-a-kind EcoCELL facility allows unique experiment

Researchers examine little piece of big prairie for impact of climate changes on ecosystems

Global question, local prairie. Dr. Jay Arnone stands inside of one of the "control" EcoCELLs for the study of grassland ecosystem responses to global warming. Colleague Paul Verburg examines the prairie foliage in the background. (Photo by John Doherty)

Though greatly simplified, this is the recipe that DRI scientist Dr. John "Jay" Arnone has been following since mid-2001 in a novel experiment that takes advantage of DRI's unique, large-scale, advanced controlled environmental facility in Reno.

Arnone heads an international team investigating how grassland ecosystems respond to year-to-year variation in climate conditions. Many believe this variation is increasing and that extreme climatic conditions are becoming more frequent. Scientists from the University of Nevada, Reno, the University of Oklahoma and Germany's Max Planck Institute are also involved in the project.

"Grassland ecosystems cover about 20 percent of the Earth's land surface," Arnone says. "Because of their ecological significance for agriculture, wildlife and global carbon storage, they are among the most studied in the world, so we knew a lot about them going into this study.

"What we learn from studying how grassland ecosystems respond to climate variability, however, can be extrapolated directly to understanding other kinds of ecosystems."

The study's focus is on carbon, the basic building block of living organisms. Scientists analyze the movement of carbon through ecosystems as a way of evaluating critical processes, measuring the system's productivity and understanding how ecosystems respond to environmental changes. This carbon exchange, or flux, occurs primarily as carbon dioxide, CO₂ , which is absorbed by plants for conversion into sugars and other carbohydrates through photosynthesis to create plant material. CO₂ is then released again back into the atmosphere by plants and soil microbial decomposers via their respiratory processes.

The concentration of CO₂ in the global atmosphere is increasing steadily due to the burning of fossil fuels, reversing millions of years of photosynthesis in a couple of hundred years. CO₂ is the primary culprit leading to the "greenhouse effect," a heat-trapping blanket around the planet. Many scientists believe this also leads to increasing climate variability, and that is believed to affect the movement of CO₂ in and out of terrestrial ecosystems. This is the subject of the 5-year study Arnone and his colleagues are currently conducting.

Inside an EcoCELL, a workman nudges the third of three 11-ton soil monoliths carefully into place in November 2001, in anticipation of beginning the grassland ecosystem study. (Photo by John Doherty)

With a $3 million grant from the National Science Foundation, Arnone and his colleagues extracted the huge monoliths and trucked them from Oklahoma to DRI's Reno campus in the fall of 2001. Cranes lowered the monoliths through the roof of DRI's greenhouse into four environmentally controlled chambers called EcoCELLs, an acronym for Ecologically Controlled Enclosed Lysimeter Laboratories. For a little more than a year, the monoliths were kept at average Oklahoma temperatures and precipitation levels.

Sensors, monitors and scales track multiple data points of factors such as humidity, CO₂ and water vapor fluxes, air and soil temperature, soil moisture, soil nutrients and solar radiation, all recorded automatically every five minutes since the study began. Far more than 50 million data points had been recorded by this summer.

"People tend to focus on what they see plants doing aboveground, but in grasslands, two-thirds of the real action goes on underground," Arnone says. "With these chambers we can watch all of these processes basically in real time."

On February 11, 2003, Arnone directed his laboratory technicians to turn up the temperature controls in two of the four EcoCELL chambers by 4 degrees Celsius-about 7 degrees Fahrenheit-above the average day-to-day temperature in Oklahoma. The monoliths in the other two EcoCELLs continued at the normal Oklahoma temperature as experimental controls.

Back on the real prairie, collaborating University of Oklahoma scientists began conducting parallel studies using twenty 4-foot by 8-foot plots, keeping some at normal conditions, some warmed by suspended heating elements, some treated with extra water and some with extra heat and water.

The response in the treated EcoCELLs, Arnone says, was almost immediate. Soil microorganisms and plants began respiring-exhaling-a larger amount of carbon dioxide into the chambers' atmosphere. Spring growth also started several weeks earlier in the treatment EcoCELLs than in the control chambers. These responses were expected.

Also anticipated was a subsequent earlier drying out of the ecosystems in the treated chambers as both warmer temperatures and greater plant activity consumed available soil water in the spring, with the resulting drop in late summer growth.

What was not expected was the dramatic shift observed in the mix of species of the plant communities in the treated chambers. "The dominant prairie grass species, big bluestem, declined dramatically, and broadleaf forb species took off and did really well."

Last February 11, Arnone returned the temperatures in the two treatment chambers to average Oklahoma conditions. This began another crucial phase of the experiment: observing how carbon flux in grasslands ecosystems would recover after the simulation of an unusually warm year.

Although a lot of data is still to be collected and analyses are far from complete, some results from the beginning of the third year of the project are already apparent. For one thing, spring in the treatment chambers started later than in the control chambers even though temperatures were now at the same level. Arnone thinks this was a lingering effect of the depleted soil moisture occurring in the warm year, in itself a significant response.

The project's primary hypothesis was that the experimental ecosystems would lose CO₂ during anomalously warm years, and, indeed, the carbon losses in the two heat-treated EcoCELLs matched the levels projected in the experiment, Arnone says. But it didn't happen the way their projections, or prevailing scientific theory, would suggest.

"We thought the system's CO₂ loss would come from the stimulation of microorganisms in the soil by the warmer temperatures. Instead, we found that it was the result of a decline in CO₂ uptake by the plants.

"Results from the Oklahoma plots will tell us how much this response may be caused by water restrictions," Arnone says. The results from the EcoCELL experiment also raise some pointed questions about the ability of ecosystems to absorb more of the increase in atmospheric carbon as global climatic extremes become more frequent.

"We've got our work cut out to integrate all our results and figure out what they all mean," he says. "But this facility is the only place in the world where this experiment could have been conducted, where we could even ask these questions. The answers will help us better project ecosystem responses to global change into the future."

— John Doherty