MONTREAL -- Southern pines appear to grow and conserve watersomewhat better in the carbon-dioxide-enriched atmosphere expected bymid-century, a Duke University study has found. However, any growthspurts appear to diminish over time, due at least in part to the kindof hot and dry weather that likely may become more common in thefuture. Thus, the researchers concluded, enhanced growth of pines maynot constitute a long-term sink for human-produced carbon dioxide whichmight ameliorate global warming.
These findings of a growth ringand wood chemistry study will be described by Duke graduate studentAshley Ballantyne at 9 a.m. Eastern Time on Tuesday, Aug. 9, 2005,during the Ecological Society of America's 2005 national meeting inMontreal.
Ballantyne, a fourth-year doctoral student inpaleoclimatology at Duke's Nicholas School of the Environment and EarthSciences, did his study with research associate Jeffrey Pippen at theFree-Air Carbon Dioxide Enrichment (FACE) experimenthttp://face.env.duke.edu/main.cfm.
At the FACE site in DukeForest, near the university campus, stands of loblolly pines and othertree species are receiving extra CO2 through tower-borne valves underotherwise natural conditions. Results from the enriched trees arecompared with those in matched controlled plots not treated withenhanced carbon dioxide.
The FACE experiment is designed toemulate the atmospheric environment that plants will be subjected to ifCO2 levels continue to increase as expected due to human activitiessuch as fossil fuel burning. Ballantyne's and Pippen's work was fundedby the United States Department of Energy.
Ballantyne said he andPippen evaluated the pine trees' response to higher-than-normal CO2levels by measuring annual growth rings in cores extracted from treatedand control trees.
Their analysis revealed that pines underelevated CO2 levels experienced 25 percent extra growth in 1999, and alower 17 percent and 21 percent boost, respectively, during the twoprevious years. The 1999 level was the maximum measured to date.
Thoseprevious years, 1997 and 1998, "coincided with one of the highestmagnitude el Niño events ever recorded," Ballantyne said in aninterview. "We suspect el Niño plays some role in regulating soilmoisture, probably due to extreme temperatures and changes in seasonalprecipitation patterns."
After those most elevated years,enhanced growth in the CO2 began dropping unevenly in a zigzag fashion.In 2000 it was 19 percent. By 2004 it was down to .01 percent. Thoseyears of declining enhancements were marked by "an approximately 15percent decline in soil moisture during the growth season," he said.
Infact, the uneven year-to-year growth differences seemed more closelyrelated to soil moisture -- the water available for use by the trees --than to other possible factors. Increased temperatures may cause adecline in soil moisture, thereby suppressing growth, he said.
Otherpossible factors in growth declines would include the tree stands'increasing ages, and the relatively low amounts of nitrogen in DukeForest soils drained of nutrients by previous farming, he said.
TheDuke researchers also studied how the gas-enriched trees processedcarbon dioxide and water by analyzing cellulose in extracted tree woodsamples.
During photosynthesis, carbon is drawn from theatmosphere as carbon dioxide to be incorporated into tree tissue,Ballantyne said. At the same time, water is drawn in from the soilthrough the roots, and some of that water eventually escapes into theatmosphere. Analyzing the chemistry of cellulose "can tell us therelative amounts of carbon gain versus water loss," he added.
TheDuke investigators found that under higher-than-normal carbon dioxidelevels "more carbon is being drawn in and less water is being emittedinto the atmosphere," Ballantyne said. "In future climates, this mightbe a way for loblolly pines to deal better with water stress ordrought."
Most scientists agree that the growing carbon dioxidelevels are trapping enough heat to induce global warming. Andloblollies and other fast-growing trees have been suggested as possible"sinks" to lock some of the excess carbon dioxide in long-term storage.
Overall,the Duke researchers' findings suggest extra carbon dioxide mightenhance loblolly water-use efficiency. But the results may provide lessreassurance for proponents of loblolly sink, Ballantyne acknowledged.
"Aworld with double CO2 is undoubtedly going to be warmer," he said."However predictions for precipitation changes are not as clear, withsome climate models predicting dryer conditions and others wetter.
"If we do see dryer conditions, we might expect less carbon to be stored."
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