C[O.sub.2]-enhanced trees weather ice storms

The increased levels of carbon dioxide in the atmosphere predicted for later this century may reduce the damage that future ice storms will cause to commercially important loblolly pine trees, according to a study by Duke University, Durham, N.C.

Researchers working at an outdoor test facility found that loblolly pines growing under carbon-dioxide levels mimicking those predicted for the year 2050--roughly one and a half times today's levels--fared better during and after a major ice storm than did loblollies growing under current concentrations of the gas.

"Before the storm, I was absolutely certain the pines would be more susceptible to ice damage under elevated concentrations of carbon dioxide," admits Ram Oren, a professor in the School of the Environment and Earth Sciences. "My impressions were absolutely wrong. Instead of increasing the sensitivity to ice-storm damage, carbon dioxide decreased [it]."

At the Free-Air Carbon Dioxide Enrichment (FACE) experiment, located in the Duke Forest Research Reserve, ecologists have been studying how the predicted extra carbon dioxide would affect a typical Southeastern forest ecosystem. At the site, there is a computer-controlled network of pipes and valves that feeds carbon dioxide from elevated towers to trees growing in three different open-air plots. Three other identical "control" plots receive no additional carbon dioxide.

Following a severe ice storm, researchers collected and measured ice-felled tree parts. They found "fewer damaged trees" and "less damage per tree" in plots with elevated carbon-dioxide concentrations. Overall, the researchers discovered that about 20% of the trees in plots treated with carbon dioxide were damaged, compared with 29% of those in control plots. Tree tops broken off by the ice in the treated plots also tended to be shorter in length. Moreover, in the growing season following the storm, trees in the treated plots recovered better since they had suffered less damage.

Oren suggests that higher carbon-dioxide levels may have induced the trees to reshape their tops, making them thicker and thus better able to withstand the ice load without breaking.

The test results may hold meaning for commercial pine forests in the Southeast, Oren notes. If future studies confirm the loblolly pines growing under higher carbon-dioxide concentrations are better able to resist ice damage, then this might "generate scenarios in which loblollies could actually migrate farther north." Ice storms now are major barriers to the northward migration of loblollies, which are susceptible to cold temperatures and ice damage, but climate changes under global warming would tend to favor the spread of such fast-growing trees.

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