Finding answers in the soil

October 10, 2000

Mary Ann Travis

While many scientists who study global warming look up to investigate the Earth's atmosphere, Jay Gulledge looks down, exploring the planet's soil. Most scientists agree that average temperatures around the world are getting hotter, and they correlate this increased heating with more carbon dioxide concentrated in the atmosphere.

But Gulledge, assistant professor of ecology and evolutionary biology, is letting others track atmospheric carbon dioxide emissions from internal combustion engines, burning forests and other human activities commonly linked to global warming. Instead, he's unraveling the global warming conundrum by investigating the soil in a bottomland coastal forest just across the river from New Orleans.

Scientists forecast that warmer global temperatures will melt a portion of polar ice caps. The increased water volume resulting from the melting ice will cause sea levels to rise everywhere. While not primarily associated with melting ice masses, much of southeastern Louisiana is already sinking and losing land at an accelerated rate because of the leveeing of the Mississippi River. And that makes it a particularly attractive place to model the effects of actual sea-level rise on coastal ecosystems.

Gulledge says, "Southern Louisiana at this time happens to be a particularly good place to get an early jump on the question, 'what if we do get a true global sea-level rise?'"

Gulledge and Julie Whitbeck, a University of New Orleans researcher, have received funding from the National Institute for Global Environmental Change to study the effects of sea-level rise on the ecosystem of the Barataria Preserve in Marrero, La. This study has relevance worldwide, Gulledge says. "Global sea-level rise is going to inundate a lot of coastal forests."

The Barataria Preserve is an old-growth, wetland forest dominated at its highest elevation by oak, sweet gum and maple trees. The forest hasn't been cut recently, and, as a national park, is protected from development. It slopes from high ground down to a cypress swamp, which typically is under water nine months of the year.

The incline of the Barataria site allows Gulledge an opportunity to "substitute space for time," he says. The higher, drier ground simulates a later time before inundation by rising water. As water in the Barataria forest creeps up from lower elevations, it represents what can happen worldwide as sea levels rise.

Although this year's drought has affected the site's water coverage by drying out the mucky swamp, Gulledge isn't concerned that the atypical dry conditions in this first year of the three-year study will skew his findings.

"It's actually going to give us an interesting contrast between two different climate conditions," he says. Without rain boots, in late September, Gulledge walked the site from top to bottom, installing sampling receptacles at three different spots.

In these, he'll collect samples of carbon dioxide released into the soil. He'll return with the samples to his Israel Building lab where, with a brand-new gas chromatograph, he'll measure the concentration of carbon dioxide at each point in the forest. That's the first step.

Gulledge says, "We can't do anything until we basically know how much carbon dioxide is coming out." All of this has to do with the carbon cycle. We breathe in oxygen released by plants and, in turn, breathe out carbon dioxide. All living things are involved in the exchange of gases in the carbon cycle, which includes the combined processes of photosynthesis, decomposition and respiration by which carbon circulates between the atmosphere, oceans and living organisms.

Gulledge has a hypothesis that the carbon cycle will slow down in the Barataria forest in places where it is covered with water. The forest is a complex ecosystem, says Gulledge, with plants, bacteria, fungi, worms and "creepy crawlies" munching on dead roots and leaves.

"It's important to understand which ones are controlling the release of carbon from the soil to the air. That's what we're trying to get at down there. That's our fundamental question."

Gulledge says his experiments "will tell us what happens to the below-ground carbon cycle in that ecosystem when sea level rises. "Then that has to be evaluated within a larger body of information to decide what it means globally."

But that's somebody else's job. For now, Gulledge will leave the big global warming picture to other scientists. He'd rather focus on a slice of a Louisiana wetland forest.

"We don't know if the plants are going to change the rate at which they put carbon into the soil when they get covered with water. It's complicated. We can't say exactly what's going to happen. And that's why we need to do the research."

Tulane University, New Orleans, LA 70118 504-865-5000