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NEWS RELEASE
12/5/02
CONTACT: Mark Shwartz, News Service: (650) 723-9296, mshwartz@stanford.edu
The prevailing view among scientists is that global climate change
may prove beneficial to many farmers and foresters -- at least in the
short term. The logic is straightforward: Plants need atmospheric carbon
dioxide to produce food, and by emitting more CO2 into the air, our
cars and factories create new sources of plant nutrition that will cause
some crops and trees to grow bigger and faster.
But an unprecedented three-year experiment conducted at Stanford University is raising questions about that long-held assumption. Writing in the journal Science, researchers concluded that elevated atmospheric CO2 actually reduces plant growth when combined with other likely consequences of climate change -- namely, higher temperatures, increased precipitation or increased nitrogen deposits in the soil.
The results of the study may prompt researchers and policymakers to rethink one of the standard arguments against taking action to prevent global warming: that natural ecosystems will minimize the problem of fossil fuel emissions by transferring large amounts of carbon in the atmosphere to plants and soils.
"Perhaps we won't get as much help with the carbon problem as we thought we could, and we will need to put more emphasis on both managing vegetation and reducing emissions," said Harold A. Mooney, the Paul S. Achilles Professor of Environmental Biology at Stanford and co-author of the Dec. 6 Science study.
He noted that the Stanford study is the first ecosystem-scale experiment to apply four climate change factors across several generations of plants.
"To understand complex ecological systems, the traditional approach of isolating
one factor and looking at that response, then extrapolating to the whole
system, is often not correct," Mooney said. "On an ecosystem scale,
many interacting factors may be involved."
Jasper Ridge Global Change Project
The findings published in Science are among the first results of the
Jasper Ridge Global Change Project -- a multi-year experiment designed
to demonstrate how a typical California grassland ecosystem will respond
to future global environmental changes.
Located in a fenced off section of Stanford's 1,189-acre Jasper Ridge
Biological Preserve, the novel experiment was designed to simulate environmental
conditions that climate experts predict may exist 100 years from now:
a doubling of atmospheric CO2; a temperature rise of 2 degrees Fahrenheit;
a 50 percent increase in precipitation; and increased nitrogen deposition
-- largely a byproduct of fossil fuel burning.
Launched in 1997, the Jasper Ridge experiment was conceived by Mooney
and Christopher B. Field, a professor by courtesy in Stanford's Department
of Biological Sciences and director of the Carnegie Institution's Department
of Global Ecology, also located on the Stanford campus.
"Most studies have looked at the effects of CO2 on plants in pots
or on very simple ecosystems and concluded that plants are going to
grow faster in the future," said Field, co-author of the Science study.
"We got exactly the same results when we applied CO2 alone, but when
we factored in realistic treatments -- warming, changes in nitrogen
deposition, changes in precipitation -- growth was actually suppressed."
To mimic future climate conditions, Field, Mooney and their colleagues
mapped out 36 circular plots of land, each about 6 feet in diameter.
Four plots are virtually untouched, receiving no additional water, nitrogen,
carbon dioxide or heat. Each of the remaining 32 circles is divided
into four equal quadrants separated by underground partitions to prevent
roots in one section from invading neighboring tracts. In these smaller
quadrants, researchers study all 16 possible combinations of elevated
and normal CO2, heat, water and nitrogen.
The plots thicken
The biggest surprise from the study was the discovery that elevated
carbon dioxide only stimulated plant growth when nitrogen, water and
temperature were kept at normal levels.
"Based on earlier single-treatment studies with elevated CO2, we initially
hypothesized that, with the combination of all four treatments together,
the response would be additional growth," said W. Rebecca Shaw, a researcher
with the Nature Conservancy of California and lead author of the Science
study.
But results from the third year of the experiment revealed a more
complex scenario. While treatments involving increased temperature,
nitrogen deposition or precipitation -- alone or in combination
-- promoted plant growth, the addition of elevated CO2 consistently
dampened those increases.
"The three-factor combination of increased temperature, precipitation
and nitrogen deposition produced the largest stimulation [an 84 percent
increase], but adding CO2 reduced this to 40 percent," Shaw and her
colleagues wrote.
The mean net plant growth for all treatment combinations with elevated
CO2 was about 4.9 tons per acre -- compared to roughly 5.5 tons
per acre for all treatment combinations in which CO2 levels were kept
normal. However, when higher amounts of CO2 gas were added to plots
with normal temperature, moisture and nitrogen levels, aboveground plant
growth increased by nearly a third.
Why would elevated CO2 in combination with other factors have a suppressive
effect on plant growth? The researchers aren't sure, but one possibility
is that excess carbon in the soil is allowing microbes to outcompete
plants for one or more limiting nutrients.
"By applying all four treatments, we may be repositioning the ecosystem
so that another environmental factor becomes limiting to growth," Field
observed. "For example, by increasing plant growth as a result of adding
water or nitrogen, the ecosystem may become more sensitive to limitation
by another mineral nutrient such as phosphorous, potassium or something
else we hadn't been measuring."
A new five-year experiment is under way at the Jasper Ridge site to
analyze potential limiting nutrients in the soil along with microbial-plant
interactions and the molecular biology of the vegetation.
Policy implications
Field and his colleagues say that their ultimate goal is to use the
results of the Jasper Ridge study to forecast what will happen to other
ecosystems -- from alpine tundra to tropical rainforests.
"In the past, people have argued that perhaps we don't really need
to worry about fossil fuel emissions, because increased plant growth
will effectively pull elevated CO2 concentrations out of the atmosphere
and keep the world at the appropriate equilibrium," he added. "But our
experiment shows that we can't count on the natural world, the unmanaged
world, to save us by pulling down all the atmospheric CO2."
Added Mooney: "Our study demonstrates that there is still a lot to
learn about the factors that regulate global climate change. But we
also know a lot already, more than enough to engage in a serious discussion
about action to reduce CO2 emissions from burning fossil fuels and clearing
forests."
Other coauthors of the Science study are former Stanford doctoral
student Erika S. Zavaleta, now a Nature Conservancy postdoctoral fellow
at the University of California-Berkeley; Nona R. Chiariello, research
coordinator of Stanford's Jasper Ridge Biological Preserve; and Elsa
E. Cleland, a graduate student in the Stanford Department of Biological
Sciences.
The study was supported by the National Science Foundation, the Morgan
Family Foundation, the David and Lucile Packard Foundation, the Jasper
Ridge Biological Preserve, the Carnegie Institution of Washington, the
U.S. Department of Energy, the U.S. Environmental Protection Agency,
the Switzer Foundation and the A. W. Mellon Foundation.
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By Mark Shwartz |
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