Researchers have found that although higher CO₂ concentrations in the atmosphere do increase forest growth, that growth is limited by soil fertility. As a consequence, more plant growth won't compensate¿as some had hoped¿for higher CO₂ emissions from burning fossil fuels. "When we exposed trees in low-nutrient soil to elevated CO₂, they maintained growth increases only with added nutrients," explains investigator David Ellworth of the University of Michigan. "While CO₂ initially acts as a stimulus to the tree's physiology, our experiments suggest that short-term increases in growth are not sustainable over the long term in low-nutrient environments."

Ellsworth and his colleagues, who published their findings in today's Nature, looked at two low-nutrient spots: the moderately fertile Duke Forest of Duke University and the infertile sand hills of North Carolina. In both places they exposed maturing loblolly pine trees to CO₂ levels equivalent to what the earth's climate may contain 50 years from now. At the first site, they compared the growth of the test plants with that of normal trees nearby. For the first three years, the test plants grew 34 percent more than the others. In the following four years, though, their added growth dropped to 6 percent. They also found that fertilized plants grew much faster.

At the infertile site, extra CO₂ led to virtually no additional growth, whereas extra fertilizer did. Plants given both fertilizer and more CO₂ grew by 74 percent. "Trees can sustain increases in biomass only as long as they find enough water and nutrients in their ecosystems," Ellsworth concludes. "I don't think we can assume existing forests, with their fertility limitations, will completely offset rising CO₂ without soil amendments. We will more likely find solutions in measures such as burning less fossil fuel and planting more trees with high-nutrient soils."

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