Soil fertility limits forests capacity to absorb excess CO₂

provided by University of Michigan

field study on the effects of elevated carbon dioxide (CO₂) on forest ecosystems raises doubts about the ability of trees to absorb excess CO₂ accumulating in the earth's atmosphere.

    Results of the seven-year study, published in the May 24 issue of Nature, show that some forests will not increase the amount of carbon they sequester at least not enough to compensate for increasing atmospheric CO₂. Soil fertility is a key factor in determining the long-term growth response to elevated CO₂, according to co-principal investigator David S. Ellsworth, assistant professor of plant physiological ecology in the School of Natural Resources and Environment at the University of Michigan.

    “When we exposed trees in low-nutrient soil to elevated CO₂, they maintained growth increases only with added nutrients,” said Ellsworth. “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.”

    The open-air field study described in Nature is the first of its kind to examine the effects of elevated CO₂ on forests growing in nutrient-limited environments over many years. The study included the longest running forest-based Free Air CO₂ Enrichment (FACE) experiment.

    By exposing trees to elevated CO₂ in an otherwise natural setting, the researchers were able to simulate conditions predicted for 50 years from now.

    “Other recent studies have shown that elevated CO₂ increases growth, measured as the amount of carbon sequestered in the tree's biomass, and increases nutrient uptake as well,” Ellsworth said. “But what happens if the tree does not take up soil nutrients in proportion to that growth increase, or the nutrients are not available?”

    That is the case for many northern mid-latitude forests, which comprise much land in the United States and Europe, Ellsworth noted. Forest soils tends to be low in nutrients because most of the nutrient-rich soil has been used for agriculture.

    “The debate over how much CO₂ trees will absorb should consider the limitations of soil fertility or other key resources in low supply.”

    The FACE experiment was conducted on a moderately fertile site at the Duke Forest of Duke University. A second field experiment used CO₂ enrichment in chambers on an infertile site in the sandhills of North Carolina. Both experiments exposed maturing loblolly pine trees to levels of CO₂ predicted to accumulate in the earth's atmosphere 50 years from now.

    In the FACE experiment, the researchers compared growth of CO₂-treated trees with untreated trees in an adjacent plot. Averaged over the first three years of the experiment, the elevated CO₂ plot showed a 34 percent increase in growth relative to the ambient CO₂ (untreated) plot. However, that increase dropped to 6 percent over the following four years.

    To test whether nutrient limitations reduce the tree response to elevated CO₂, the researchers added a balanced fertilizer to half the FACE area. Averaged over 1999 and 2000, trees grown under elevated CO₂ without nutrient addition increased growth at an annual rate of only 7 percent while the fertilized trees grown in ambient CO₂ increased annual growth by 15 percent.

    The combination of improved nutrition and elevated CO₂ increased growth by 47 percent at the site. This clearly indicates a synergistic effect of CO₂ and nutrient supply, the researchers concluded.

    At the infertile site, trees without added nutrition showed virtually no growth response to elevated CO₂ in two years. Under optimal nutrition and ambient CO₂, growth increased 21 percent. In trees subjected to the combination of improved nutrition and elevated CO₂, growth was 74 percent more than three times the sum of separate responses.

    These findings suggest that growth responses of pine forests to elevated CO₂ will be highly variable and depend on site fertility, to the point that trees growing on nutritionally poor sites may not respond at all. Moreover, other factors, such as water deficits, also could limit forest response to atmospheric CO₂.

    “Trees can sustain increases in biomass only as long as they find enough water and nutrients in their ecosystems,” Ellsworth said. “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 in high-nutrient soils.”

    Prof. Ram Oren of the Nicholas School of the Environment and Earth Sciences, Duke University, also was a principal investigator on the project. The research team included collaborators from Brookhaven National Laboratory, Boston University, and the US Forest Service.

    Brookhaven National Laboratory pioneered the free-air CO₂ enrichment technology used in the study and operated the FACE system.