Mitigating Global Warming

Trees May Not Play as Big a Role as Scientists Hoped p.2

A report in the May 1999 issue of Science, co-authored by Evan DeLucia of the University of Illinois at Urbana-Champaign and several Nicholas School faculty members, showed tree growth rates were boosted about 25 percent during the first two years of exposure to extra carbon dioxide at three of the FACE rings.

In the April 6 Science, Duke biology professor James Clark and his graduate student Shannon LaDeau reported that pines at FACE receiving the enriched concentrations produced three times as many cones per tree and more than twice as many seeds.

Results like those from the United States Department of Energy-funded facility would suggest that the extra CO₂ will rev-up pine production to new plateaus. According to that line of reasoning, perhaps high carbon dioxide emitting nations like the United States could reap add-on benefits from policies that would sop up additional gas by planting more trees.

Another win-win scenario would pass CO₂ in a bucket brigade from the air to the trees to the soil, where the carbon might remain locked away from the atmosphere for many years.

These wishful notions were dashed by two articles in last May 24's Nature.

The first report took another look at trees. "The idea was that forests respond to elevated CO₂ with a very significant increase in growth," said Ram Oren, Nicholas School associate professor of ecology and ecophysiology and FACE co-principal investigator, who served as principal author of the report. "Our paper showed that in fact the response was transient."

Oren's team measured trees in the FACE project's initial "prototype" ring of 17 towers, from which computer-controlled valves started releasing extra carbon dioxide about two years before the three other rings were "turned on." There was thus that much more time to see how forests would adjust to the change.

Just as DeLucia's group had found in other FACE rings, Oren and his 10 co-authors reported an initial 25 percent surge in wood production among the prototype ring's trees. But "the forest settled after four years to a marginal gain if any gain at all," he said.

Despite this deflating evidence, industrial nations can still sop up more CO₂ from the atmosphere simply by planting more trees, Oren said. "If we have more forests those will do more, and less forest, of course, will do less."

The second article studied the fate of the carbon taken up by FACE trees as they dropped pine needle litter to the ground. That study was possible because carbon dioxide released by FACE rings has a distinctive isotopic signature.

There were two possibilities, said Schlesinger, who co-authored the second Nature report with John Lichter of Bowdoin College.

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