Experiment suggests limitations to carbon dioxide 'tre...( from plot to plot using averages coul...)

"In some areas, the growth is maybe 5 or 10 percent more, and in other areas it's 40 percent more," Oren said. "So in sites that are poor in nutrients and water we see very little response. In sites that are rich in both we see a large response."

The researchers found that extra carbon dioxide had no effect on what foresters call "self thinning" -- the tendency of less-successful trees to die off as the most-successful grow bigger.

"We didn't find that elevated CO2 caused any deviation from this standard relationship," said McCarthy, now a postdoctoral fellow at the University of California, Irvine.

Also unchanged by the CO2 enrichment were the proportions of carbon atoms that found their way to various components of plant systems -- wood, leaves, roots and underlying soil. Only a few of those components will store carbon over time, noted Oren and McCarthy.

"Carbon that's in foliage is going to last a lot shorter time than carbon in the wood, because leaves quickly decay," McCarthy said. "So elevated CO2 could significantly increase the production of foliage but this would lead to only a very small increase in ecosystem carbon storage."