Open-Air Experiment Shows High CO2 Boosts Tree Growth

However, the scientists who conducted the study said such high growth rates probably will not be sustained as the experiment continues. They emphasized that the results do not indicate that more lush plant growth would soak up much of the extra CO2 entering the atmosphere from fossil fuel burning.

The researchers reported in a presentation prepared for Thursday, Aug. 6 at the annual meeting of the Ecological Society of America. The research team included scientists from the University of Illinois at Champagne-Urbana, Duke, Brookhaven National Laboratory on Long Island and West Virginia University in Morgantown.

"In colloquial terms, if you've got plants that are starving for CO2, and all of a sudden they find themselves bathed in air richer in CO2 than they were getting before, they should have the greatest response right away," said William Schlesinger, co-director of the Forest-Atmosphere Carbon Transfer and Storage experiment (FACTS-1) now underway at Duke Forest, a research reserve outside Durham, N.C.

"We found a 12 percent growth increase in the whole forest in response to addition of CO2 during 1997," added Schlesinger, a professor at Duke's Nicholas School of the Environment and botany department, in an interview before the meeting. "I would not be at all surprised if the growth response is somewhat lower in '98. I certainly expect it's going to decline after a few years as the forest adjusts,"

At FACTS-1, three patches of loblolly pine-dominated woodlands are being enveloped around-the-clock by the CO2 equivalent of 21st century air delivered by computer controlled rings of towers. Another three identical tower rings, each encircling similar patches of pine forest, are not providing extra carbon dioxide. The gas-less rings thus serve as "control" sites that scientists can compare to the high-CO2 plots.

The scientists' goal is to get an early look at how ever-increasing CO2 levels emanating from industrial smokestacks, vehicle exhausts and forest-clearing -- the same human causes being blamed for forecasted global warming -- could change future ecosystems.

Similar past experiments in greenhouses and open-top growth chambers have hinted that higher carbon dioxide levels could increase growth levels in some plant species. But until experiments like FACTS-1, scientists have had no way to evaluate the effects of high-CO2 in a "real-world" setting.

In 1995, researchers from Brookhaven and Duke had reported elevated photosynthetic activity at an initial Duke Forest tower ring erected to test the reliability of maintaining 1 1/2 times higher than current levels of the gas in the open air. But, standing alone, with no other test sites or matched controls to compare their results to, that initial Free-Air Carbon Dioxide Enrichment (FACE) experiment was not considered "fully replicated," a gold standard for research.

The fully replicated FACTS-1 experiment began in 1997 after additional funding from the U.S. Department of Energy allowed construction of six more rings. Project organizers at Duke and Brookhaven hope it can continue operation for at least a decade, allowing scientists time to gauge longer range impacts on a complex ecosystem.

A total of eight different scientific papers on FACTS-1's first year results were scheduled for the current ESA meeting.

Delivering a multi-authored analysis of overall tree growth, Shawna Naidu, a post doctoral researcher at Illinois, reported that the 12 percent increases at the active FACTS-1 sites occurred in the face of low soil fertility and periods of drought. The scientists documented the increased growth by making tree-girth measurements about three feet above the ground.

Another study by Duke graduate student Jacqueline Mohan showed that high CO2 at FACTS-1 also boosted the growth of tree seedlings and saplings that will form the future forest. And in a poster presentation, Elke Naumburg of Duke and David Ellsworth of Brookhaven reported a 120 percent increase in photosynthesis in red maple and sweetgum saplings.

But, Naidu's report also noted that computer models suggest continued CO2 stimulation will have a lessening impact in future years, a conclusion Schlesinger expanded on in his interview.

"One might guess that plants growing at high CO2 would drop leaves having lower concentrations of nitrogen and phosphorus in them, not because they are taking up less nitrogen and phosphorus from the soil, but because they're growing so fast that the same nitrogen and phosphorus concentrations are diluted by this higher growth rate," Schlesinger said.

When microbes then decompose those nitrogen and phosphorus diluted leaves in the soil, the net result might be less nutrients -- especially nitrogen -- that trees would need to fuel the higher demand.

"So that higher demand would not be met," he added. "Then the growth rate will slow. And the upshot of this would be that the pine forest would get to its ultimate size faster, but it won't end up being any larger then it would have been had it just grown without the extra CO2."

Another report bolsters that scenario. Duke graduate student Andrew Allen used a multivariant statistical analysis method to evaluate nitrogen levels in the decaying plant matter in FACTS-1 soils in the fall of 1997. Allen's results showed a "significant decrease" in nitrogen within the soil at treated sites.

But, undercutting the nitrogen deficit predictions, Duke postdoctoral researcher Adrien Finzi reported that he found no difference in the chemistry of the leaf litter at the treated versus the untreated sites, at least during the experiment's first year.

Interpreting those conflicting results, Schlesinger suggested that the nitrogen deficiencies contributing to slower future growth might be showing themselves in the underground root systems of trees rather than in their leaves. Tree growth is considered a potentially important reducer of atmospheric CO2 -- and thus an antidote to global warming -- because plants take in carbon dioxide through the photosynthetic process, break down the gas, and store its carbon in their tissues, meanwhile releasing oxygen.

Schlesinger noted that current forecasts for carbon dioxide emissions from fossil fuel combustion around 2050 is about 15 billion metric tons a year. "The enhanced growth seen in this forest, if applied globally, would take up about 20 percent of that fossil fuel release," he estimated.

"There are a lot of reasons why that 20 percent number is pushing the envelope," he cautioned. "It assumes that there will be a lot of forests around then, that humans haven't cut them all down. And it assumes that they will all grow as fast as loblolly pines, which are one of the fastest growing species on Earth.

"That should be very discouraging to those who would expect plants to solve the carbon dioxide problem for us," he said.