Pie-shaped plots of grass
may help solve riddle of global climate change
BY MARK SHWARTZ
At first glance, the
small, grassy field above Stanford's main campus seems an
unlikely place to make predictions about the global
environment.
But this unassuming meadow
in the Jasper Ridge Biological Preserve is now the site
of a dynamic experiment designed to forecast the effects
of rising temperatures and other environmental changes on
grassland ecosystems throughout the planet.
The Jasper Ridge Global
Change Project is a multi-year study focusing on what
scientists believe will be four of the most significant
changes affecting Earth in the next century: higher
levels of carbon dioxide (CO2) in the atmosphere, warmer
temperatures, increased use of nitrogen fertilizers and
changes in precipitation.
Researcher
Maria Macedo carefully inserts an underground video
camera specially designed to photograph the root systems
in each of the pie-shaped experimental plots that make up
the Jasper Ridge Global Change Project. The video images
will help scientists measure root growth during the
three-year experiment. Each plot is equipped with a
watering system, carbon dioxide gas emitters and an
infrared heat lamp positioned above the circle. (Photo: P. Cohen)
The goal of the project is
to document the effects of all four factors -- alone and
in combination -- on the plants, microbes, animals and
soil that make up the grassland ecosystem.
"Global change is
going to be caused by more than just increased carbon
dioxide and higher temperatures," says Harold A.
Mooney, the Paul S. Achilles Professor of Environmental
Biology, who helped design the project.
Mooney, a leading
authority on global change, points out that farmers
around the world are using ever-increasing amounts of
water and nitrogen-based fertilizers to enhance crop
production. These factors, he says, should be included in
any long-range projection of changes in the biosphere --
the fragile envelope of air, water and soil that allows
life to exist on Earth.
Mooney and biologist
Christopher B. Field of the Carnegie Institution of
Washington are leading the study, which is located in a
fenced-off section of grassland inside Stanford's
1,189-acre Jasper Ridge Biological Preserve.
So far, the results have
been dramatic.
Some plots of experimental
soil have produced an explosion of wildflowers, while
others are filled with tall, thick grass. And still
others are virtually bare. It all depends on how much
carbon dioxide, heat, nitrogen and water are added to the
environment -- and in what combination.
"This is the only
experiment of its kind in the world," says Philippe
S. Cohen, administrative director of the Jasper Ridge
Biological Preserve.
"The project promises
to make important contributions to our understanding of
how ecosystems respond to human-induced global
changes," Cohen adds.
Experimental circles
In 1997, project
scientists plotted 36 circles in the ground, each about 6
feet in diameter. A large rectangular fence was placed
around the perimeter of the circles to prevent deer from
prematurely consuming the results of the experiment.
All the circles were
equipped with watering systems, carbon dioxide gas
emitters and infrared heaters located about 3 feet off
the ground.
"The idea is to
simulate the kinds of changes predicted over the next 100
years," says Cohen -- a doubling of current levels
of atmospheric carbon dioxide, accompanied by an average
temperature increase of 2 to 6 degrees Fahrenheit.
A computer makes sure that
carbon dioxide levels remain consistent by adjusting the
release of CO2 gas to changing wind speeds and
direction.
To measure the effect of
the experimental equipment itself, four of the circular
plots receive no additional water, nitrogen, carbon
dioxide or heat.
Each of the remaining 32
circles is divided into four equal quadrants separated by
underground fiberglass partitions to prevent roots in one
section from invading a neighboring tract.
The result is 32 plots,
each containing four different experiments using every
possible combination of carbon dioxide, heat, water and
nitrogen.
Plot 13, for example,
receives twice the normal amount of atmospheric carbon
dioxide gas 24 hours a day, but no additional heat.
Experimental Plot 13
receives twice the normal amount of atmospheric carbon
dioxide gas 24 hours a day, but no additional heat. It is
divided into four quadrants, each receiving different
treatments that produced dramatically different results.
The upper-left quadrant (1) put forth a showy display
of blue wildflowers after being given extra nitrogen,
while the upper-right quadrant (2), which is exposed to
extra CO2 and water, produced a mix of flowers
and grass. The lower-right quadrant (3) -- treated with
both nitrogen and 50 percent more water –
grew tall grass but no wildflowers. Meanwhile, the
lower-left section (4) – fed only elevated CO2–
ended up with a sparse collection of short plants. (Photo: P. Cohen)
By mid-spring, Plot 13's
upper-left quadrant, which receives extra nitrogen, put
forth a showy display of blue wildflowers.
But just inches away, in
the lower-right quadrant of the same circle, the
vegetation looks totally different. This section is
treated with both nitrogen and 50 percent more
water. The result: a thick cover of tall grass, but no
wildflowers.
Meanwhile, the upper-right
quadrant, which is exposed to extra carbon dioxide and
water, contains a mix of flowers and grass, while the
lower-left section -- fed only elevated CO2
-- contains a sparse collection of short plants.
What could cause such
extreme differences in plant growth in such a tiny patch
of earth?
"The researchers
can't answer that question yet," says Cohen,
pointing out that this is only the second year of a
three-year experiment.
"We do know that,
originally, all four quadrants looked alike," he
adds.
Miniature ecosystems
According to Field, each
test plot can be viewed as a true ecosystem with several
thousand individual plants, mostly grasses and
wildflowers.
"Overall," Field
says, "these ecosystems appear much more sensitive
to elevated levels of water and nitrogen than to excess
carbon dioxide and warming alone. Therefore, we expect
the biggest effects of global warming to be
indirect."
For example, he points out
that warmth indirectly affects the availability of water
and nitrogen, which in turn affects a plant's growth
rate.
Graduate student Erika
Zavaleta also found that shrub seedlings actually did
better in some experimental plots with elevated carbon
dioxide levels, suggesting that CO2 could play a role in transforming
grassland to shrubland.
Plants use carbon dioxide
to produce carbohydrates, but Field says preliminary
results show that increasing CO2 in the atmosphere does not always
mean a big increase in the amount of carbon in the plant
itself.
"The big unknown in
the global carbon cycle is below ground," notes
Rebecca Shaw, a postdoctoral researcher whose work on the
project is supported by the Department of Energy.
"How are the roots
responding to environmental changes?" asks Shaw.
Specifically, what is
happening to the rich mix of roots, soil, microbes and
animals that we rarely see?
To find out, Brazilian
scientists Aristotelino Ferreira and Maria Macedo have
been taking thousands of photographs of the root systems
in every experimental plot using a video camera lowered
into tubes buried in the soil.
The pictures will help
scientists measure root growth and determine if extra
carbon from the air is in fact being stored underground.
Researchers are also using
chemical analysis to trace the movement of carbon and
nitrogen through the plants, microbes and soil.
Unlike similar studies
involving forests and other large ecosystems, the Jasper
Ridge Global Change Project has the advantage of
monitoring several generations of plants in a short time
period.
"The annual grassland
goes from seed to seed every six to nine months,"
says Mooney, "so you can observe an entire cycle of
vegetation."
The project is
underwritten by the National Science Foundation, and
researchers have asked the NSF to continue funding the
experiment an additional four years. SR
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