In 1998, an experimental
10-year-old sweetgum plantation in Oak Ridge National Laboratory's Environmental
Research Park showed a 35% increase in growth over a nearby control
stand of trees. More wood was produced in the test forest's tree trunks
and more fine roots grew in the soil. The 15-m-tall sweetgum trees in
the plantation's 25-m-diameter plots grew more because they were being
exposed to air containing 50% more carbon dioxide (CO2) than
is in the atmosphere, thanks to free-air CO2 enrichment (FACE)
technology.
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|
Standing
in a hydraulic lift, Rich Norby collects leaves in the high-carbon
sweetgum forest for measurements of leaf mass, nitrogen concentrations,
and rates of photosynthesis.
|
In 1999, the second
year of the FACE experiment funded by the Department of Energy, some
of the data surprised Richard Norby and his ORNL collaborators Stan
Wullschleger, Carla Gunderson, Gerry O'Neill, Paul Hanson, Nelson Edwards,
Tim Tschaplinski, Mac Post, Don Todd, and Tony King. The growth rate
increase of the experimental plantation was reduced to 15% over that
of the control stand. These results differ from those at a Duke University
plantation dominated by loblolly pine trees with sweetgum trees in the
understory. In the past three years, Duke researchers have observed
a sustained growth rate increase of 25% per year in the trunks of high-CO2
pine trees over that of control trees in a normal atmosphere.
"The dramatic growth response
we saw in the first year in Oak Ridge disappeared in the second," says
Norby, leader of the collaboration at the FACE facility and a researcher
in ORNL's Environmental Sciences Division (ESD). "It could be year-to-year
variability or a blip in the first year's data because of the sudden
increase in CO2 concentration. It could be a short-term response
that is not indicative of the long-term response. Some of our data indicate
that the growth increase was actually maintained but the extra carbon
was not stored in the tree trunks."
Many of the physiological
responses observed in the second year were similar to those in the first
year of the FACE experiment. "We observed that photosynthesis remained
enhanced," Norby says, referring to the process by which plants use
the energy from sunlight to convert CO2 and water into sugars
needed for growth. "The leaf area stayed the same, and the trees conserved
water just as well in the second year as the first."
In both years, ORNL scientists
observed that the tree leaf pores (stomata), which allow CO2
to enter and water vapor to escape, were not open as wide in plots receiving
the extra CO2. "Trees use more water on sunny days and less
on overcast or rainy days," says Norby. "Because of high CO2
in air, they can close their stomata a little on days of high water
use and get the CO2 they need while letting out less water.
Thus, they draw less water from the ground, allowing soil moisture levels
to be higher. Higher soil moisture could result in more activity by
microbes that may make more nitrogen available to plants, fertilizing
them and promoting their growth."
In addition to the tree
growth rate difference in 1998 and 1999 at the FACE facility, ORNL scientists
also observed that the leaves of the high-CO2 trees became
heavier in 1999, probably because more of the extra carbon was used
to produce leaves than increase trunk growth. They also found that the
nitrogen concentration of leaves and litter (fallen leaves on the forest
floor) was lower, which could retard the cycling of nitrogen and carbon
in the ecosystem.
ORNL researchers
also observed an increase in production and mortality of fine roots
in the second year, resulting in a change in the belowground allocation
of carbon. "We saw an increase in carbon cycling because of high root
turnover," Norby says. "The fine roots took in more carbon and then
rapidly died, releasing more carbon to the soil than usual."
Because ORNL and Duke
scientists are doing similar experiments on plantations of equivalent
size and age in the same climate zone, they propose to collaborate on
a study of nutrient cycling and carbon sequestration in these test forests
if DOE funding is available. They will also try to help each other understand
the variability in the results from the Oak Ridge and Duke experiments.
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