Duke Study Shows Carbon Dioxide Boosts Pine Tree Reproduction

While the long term-effects of such changes are still uncertain, "the implications for regeneration are substantial," said Shannon LaDeau, a Duke University doctoral student in ecology who prepared to describe the study Tuesday at the Ecological Society of America's annual meeting in Snowbird, Utah.

Economically, loblollies are important, LaDeau said in an interview. "They are the most important forest product in the Southeast. They tend to be one of the first tree species that will recolonize a disturbed site or an abandoned agricultural site."

Studies are under way in three 90-foot-diameter circular plots where entire forest ecosystems, including the 19-year-old loblollies, are being bathed in 1 1/2 times today's levels of atmospheric carbon dioxide. The extra gas is being delivered to each plot by rings of computer controlled towers at the U.S. Department of Energy-funded Forest-Atmosphere Carbon Transfer and Storage experiment (FACTS-1) in Duke Forest.

The FACTS-1 site is intended to evaluate how forests will respond to a higher CO2 atmosphere expected by mid-21st century from fossil fuel combustion and other human activities. Scientists are comparing responses in the carbon dioxide-enriched plots to results in three other matching plots where tower rings are delivering no gas.

About 100 pines, all planted at the same time, are growing inside each CO2-treated and untreated plot, said LaDeau, whose report covers loblolly reproductive changes within the FACTS-1 rings from 1997 to 1999. Her research group also includes Duke botany doctoral student Ines Ibanez and Duke botany professor James Clark.

The scientists used central towers in each FACTS-1 ring, as well as hydraulic lifts, to view and count cones growing in the pine branches. They also counted seeds collected in basin-shaped traps arranged on the forest floor.

None of these immature pines was reproductively active prior to 1998. But, in 1999, the researchers found that pines receiving extra CO2 were twice as likely as untreated pines to produce cones. Those receiving extra gas also produced three times as many cones per tree and more than twice as many seeds.

The scientists' data, as well as supporting modeling, also showed that the CO2-enriched trees are allocating a larger proportion of carbon into cone production. In addition, the investigators discovered that trees growing in elevated CO2 became reproductively mature at smaller sized trunk diameters than untreated trees. Ecologists have found that larger trunk diameters tend to correlate with greater reproductive output, LaDeau said.

There are also preliminary suggestions that pollen production will follow similar trends, LaDeau added.

Whether these trends will continue as more of the trees mature, and whether it would ultimately mean that loblollies may have more of a competitive advantage over other species in a future higher carbon dioxide world, remains to be seen, the scientists said.

This study deals only with early effects on loblolly pines stands alone, which under current natural conditions would be replaced by hardwoods after predominating for 50 to 100 years, LaDeau noted.

"If the seed production of everything was boosted, it's a tough question how that would play out," said Clark, who studies seed production and species competition in forests. "If just a few species really scored big because of high CO2, the answer seems more straightforward. Those are going to win more often and the others are going to lose more often."