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Residential Life Goes
West • Chewing
on Evolution •
Atmospheric Ambiguities
Lobsters Play Biological Violins •
Remembering Wannamaker
When is a Platypus Not a Kangaroo?
• Solution
for Smokers •
In Brief
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Atmospheric
Ambiguities
 esults
from continuing experiments near Duke—where forest plots grow
in the higher atmospheric levels of carbon dioxide expected by the
mid-twenty-first century—suggest that trees and soil may not
sop up much of the extra gas over the long term under real-world conditions.
Future air apparent:research towers
release tomorrow's atmospheres to pine grove |
| photo:Les Todd |
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One of two articles in a May issue of the research journal
Nature shows that while twenty-year-old loblolly pine trees began
growing up to about 25 percent more wood after becoming continually
exposed to 1.5 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 eleven 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.
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.
An initial ring of computer-controlled towers, later joined
by another three, was designed to immerse 30-meter-diameter circles
of pine forest in constantly regulated equivalents of a mid-twenty-first
century atmosphere. Another three plots received no extra CO2, but
instead served as experimental “controls” for comparison.
Oren’s group also compared the FACE experimental
results with a separate study of twelve trees growing in low-nutrient
soils at the U.S. Department of Agriculture’s Forest Service
Southeast Tree Research and Education Site (SETRES) in North Carolina’s
Sandhills region. Half of those SETRES trees received 1.5 times normal
CO2 concentrations through open-topped growth chambers.
In addition to Department of Energy and U.S. Forest Service
funding, support came from the National Science Foundation.
Many scientists suspect the enhanced carbon dioxide is
already beginning to warm Earth’s climate by trapping solar energy
through a so-called “greenhouse effect.” Policy makers are
thus searching for ways to reduce CO2 levels. One suggested way is
to lock up some of the extra gas in growing wood, or in the soil humus
created by microbes from fallen leaves or pine needles.
Some earlier experiments had suggested that the extra
atmospheric carbon dioxide might itself have a “fertilizing”
effect on forests, causing them to lock up extra amounts of CO2 and
thus mitigate global warming impacts.
In a second article in the same issue of Nature, William
Schlesinger, James B. Duke Professor of Biogeochemistry and the Nicholas
School’s new dean, joined John Lichter of the biology department
and environmental studies program at Bowdoin College in Brunswick,
Maine, to examine whether the FACE experiment would also store extra
carbon dioxide in soils in the CO2-treated pine forest plots.
Comparing soils archived before the onset of the FACE
experiment with those collected later, the pair found significant
increases in the production of dead plant debris within the forest
floor of the CO2-enriched plots. Because the CO2 used in the FACE
treatment has less-than-normal concentrations of the carbon-13 isotope,
the scientists could also employ isotope ratios as a marker to trace
the chemical origins of carbon in the developing soil.
Using this information, Schlesinger and Lichter deduced
that while the initial bursts in wood production also produced extra
leaf litter, very little of the carbon originating from FACE-emitted
CO2 entered the mineral soil over the three-year study period. “In
the absence of significant changes in the carbon content of the mineral
soil, these data suggest only a limited potential for long-term soil
carbon sequestration in this forest in response to rising CO2 in Earth’s
atmosphere,” their Nature article concluded. .
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