DURHAM, N.C. Results from continuing experiments near Duke University where forest plots grow in the higher atmospheric levels of carbon dioxide expected by the mid 21st century suggest that trees and soil may not sop up much of the extra gas over the long term under real-world conditions.
One of two articles in the May 24 issue of the research journal Nature shows that while 20-year-old loblolly pine trees began growing up to about 25 percent more wood after becoming continually exposed to 1 1/2 times current levels of carbon dioxide (CO2), that initial growth spurt dropped back to only marginal gains after the first three years.
Researchers found they were able to enhance wood production as much as 74 percent at a nearby experimental site by providing extra nitrogen fertilizer as well as CO2 to trees growing in nutrient-poor soils. But growth did not increase at all without the supplemental nitrogen.
"That suggests that CO2 effects on tree growth in pine forests will be highly variable and depend greatly on site fertility, perhaps to the point of not responding at all on the nutritionally poorest sites," concluded the article's 11 authors, led by Ram Oren, an associate professor at Duke's Nicholas School of the Environment and Earth Sciences.
Another variable was moisture. An early growing-season drought in 1999 delayed growth in the elevated CO2 plots at the Duke Free Air Carbon Enrichment (FACE) study site, the scientists found. Even with extra nutrients and CO2 provided, drought in 1999 reduced sequestration of carbon by more than 25 percent relative to the moist growing season in the year 2000.
Funded by the U.S. Department of Energy, the FACE experimental research site at Duke Forest is designed to mimic effects that extra CO2 expected from continued industrial and vehicular emissions and other human activities will have on typical forest ecosystems.