Most scientists agree that growing carbon-dioxide levels are trapping enough heat to induce global warming, but some have suggested that loblolly pines and other fast-growing trees could serve as "sinks" to lock excess human-produced carbon dioxide in long-term storage. A new Duke study shows that while these trees do appear to grow and conserve water somewhat better in the carbon-dioxide-enriched atmosphere expected by midcentury, such growth spurts appear to diminish over time, due at least in part to the kind of hot and dry weather that is likely to become more common in the future. The results provide little reassurance for proponents of loblolly sinks.
The findings of this growth-ring and wood-chemistry study were described by Duke graduate student Ashley Ballantyne at the Ecological Society of America's 2005 national meeting in Montreal. Ballantyne, a fourth-year doctoral student in paleoclimatology at the Nicholas School of the Environment and Earth Sciences, did his study with research associate Jeffrey Pippen at the Free-Air Carbon Dioxide Enrichment (FACE) experiment.
At the FACE site in Duke Forest, stands of loblolly pines and other tree species receive extra carbon dioxide through tower-borne valves under otherwise natural conditions. Results from the enriched trees are compared with those in matched controlled plots not treated with enhanced carbon dioxide. The FACE experiment is designed to emulate the atmospheric environment that plants will be subjected to if CO2 levels continue to increase as expected because of human activities such as the burning of fossil fuels. Ballantyne and Pippen's work was funded by the U.S. Department of Energy.
Ballantyne says he and Pippen evaluated the pine trees' response to higher than normal carbon-dioxide levels by measuring annual growth rings in cores extracted from treated and control trees. Their analysis, which began within the year 1997, revealed increasing growth percentages for the first three years, peaking with 25 percent extra growth in 1999, and then dropping unevenly. In 2000, it was 19 percent. By 2004, it was down to .01 percent. Those years of declining enhancements were marked by "an approximately 15 percent decline in soil moisture during the growth season," Ballantyne says, while the earlier years coincided with the peak moisture of El NiÒo.
In fact, Ballantyne says, the uneven year-to-year growth differences seemed more closely related to soil moisture than to other possible factors. Increased temperatures may cause a decline in soil moisture, thereby suppressing growth, he adds. Other possible factors in growth declines would include the tree stands' increasing ages and the relatively low amounts of nitrogen in Duke Forest soils drained of nutrients by previous farming.
The Duke researchers also studied how the gas-enriched trees processed carbon dioxide and water by analyzing cellulose in extracted tree-wood samples. During photosynthesis, carbon is drawn from the atmosphere as carbon dioxide to be incorporated into tree tissue, Ballantyne says. At the same time, water is drawn in from the soil through the roots, and some of that water eventually escapes into the atmosphere. Analyzing the chemistry of cellulose "can tell us the relative amounts of carbon gain versus water loss." When carbon-dioxide levels are higher than normal, "more carbon is being drawn in, and less water is being emitted into the atmosphere. In future climates, this might be a way for loblolly pines to deal better with water stress or drought."
"A world with double CO2 is undoubtedly going to be warmer," he says. "However, predictions for precipitation changes are not as clear, with some climate models predicting dryer conditions and others wetter. If we do see dryer conditions, we might expect less carbon to be stored."