DURHAM, N.C. -- Duke University forest plots bathed in the higher carbon-dioxide atmosphere expected by the year 2050 experienced a 25 percent growth increase over the first two years of a continuing experiment, scientists from the University of Illinois at Urbana-Champaign and Duke reported Thursday.
"This study puts forests on the CO2 map," said Evan DeLucia, a U. of Ill. at Urbana-Champaign plant biologist who is the lead author of the latest report, published in the May 14 issue of the journal Science. While the potential for forests to sop up increasing human-caused carbon dioxide emissions had been only speculative, "now we have some real data that allows for global extrapolation," he added in an interview.
If the world's forests were all to be growing 25 percent faster 50 years from now, these results would suggest that woodland plant life could serve as a "sink" for about half the expected carbon dioxide emissions from fossil fuel combustion, according to estimates of DeLucia and Duke botany professor William Schlesinger, the article's other main author.
But both scientists cautioned that such a high sustained uptake is actually unlikely, for several reasons. First, the Duke Forest plots where the U.S. Department of Energy-funded study is being conducted are dominated by 13-year-old loblolly pines, among the world's fastest growing tree species, at their peak growing age. Second, open-air studies at Italian hot springs and another Duke Forest plot suggest that the carbon-dioxide inspired growth spurt will level off in a few more years or less.
"The crux of the matter is that vegetation can respond to higher CO2 and act as a carbon sink," Schlesinger said in an interview. "The 25 percent growth increase is probably an upper limit for what the world's vegetation can do. Nevertheless, it's interestingly high."
With technology pioneered by the Brookhaven National Laboratory on Long Island, N.Y., three 100-foot diameter forest parcels, each ringed by 16 towers, are receiving 1 1/2 times the current atmospheric concentrations of carbon dioxide.
The extra CO2 is being delivered round-the-clock by pipes and valves on the towers. Computer controls ensure that the right valves open on the right towers to keep the distribution constant within all parts of each plot, regardless of wind direction and speed.
The experiment is "fully replicated," meaning that it is being repeated at three locations. Three other identical tower ringed forest plots that receive no extra CO2 are serving as controls whose response can be compared to the three active sites.
Schlesinger is co-director of this Forest-Atmosphere Carbon Transfer and Storage (FACTS-1) experiment within part of the 7,700-acre university-owned Duke Forest research reserve, located several miles from the westernmost Duke campus. The other co-director is George Hendry, the Brookhaven researcher who devised the Free Air Carbon Dioxide Enrichment (FACE) delivery method.
In 1997, the first complete year of the replicated study, the overall growth rate of the dominant pine trees and underlying hardwoods, shrubs and vines increased 16 percent in the extra-CO2 plots when compared to the control plots, the authors reported in Science. In 1998, that increase swelled to 25 percent, an addition that to some degree reflected the inclusion of fine root growth that was not measured in 1997.
Both DeLucia and Schlesinger noted that those results occurred during two growing seasons that were affected by droughts.
"The growth rates declined in the control plots as a result of the drought," Schlesinger said, while the high-CO2 plots "were able to make up for the drought," he added. The Duke botanist speculated that plants growing at higher carbon dioxide levels may be able to better conserve water. Separate studies have shown that plant stomata, leaf pores that regulate water release, do not open as widely in elevated CO2.
DeLucia, one of a number of scientists at work at the FACTS-1 site, is studying the "carbon budget" of the experimental plots. Much like managing a checkbook, he is measuring how much of plants' carbon income is invested in new leaves, roots and stems, and how much is lost back into the atmosphere through respiration.
Shawna Naidu, a post doctoral researcher in DeLucia's laboratory who made one of eight presentations on the first year's FACTS-1 results at the August 1998 Ecological Society of America meeting in Baltimore, reported then that the 1997 growth rate in high-CO2 plots was 12 percent, a number that DeLucia has since recalculated as 16 percent.
DeLucia and Schlesinger are both skeptical that the current high growth rates will be sustained in part because of Swiss and Italian researchers tree ring studies around hot springs near Pisa and Siena, Italy that naturally emit carbon dioxide and other gases. Those studies, which also compared high-CO2 to control sites, found carbon dioxide's stimulatory effects decreased as trees aged.
An older tower ring at the FACTs-1 site built to test out the FACE concept has also logged reduced growth response to elevated CO2 beginning after the fourth of its six years of operation, Schlesinger added. But that plot only receives extra carbon dioxide during the daytime and is marred by "a lot of disturbance around the edge," he noted.
"In the few studies that have looked at how trees, such as those growing next to natural springs of CO2, respond over long periods of time, growth response is strong at first, then decays," said DeLucia. "Other models also predict this response.
"The mechanisms for this slowing of the growth response are not entirely clear; one reason is that trees may acclimate by slowing their photosynthesis rates."
Whether they end up being short term or longer term, the striking growth increases recorded during the first two years of the FACTS-1 study serve to highlight the environmental downside of suburban sprawl that is invading woodlands in the South and elsewhere, Schlesinger said.
"To the extent that we convert any of these fast growing forested lands to parking lots and shopping centers, that carbon sink is totally gone," he added. "Not only is the sink gone, but the carbon that used to be in the trees has probably been returned to the atmosphere by burning the trees or using the wood products in things like paper towels and cardboard boxes that later decompose or are burned."