FACE-ing the Future
It's the largest experiment ever to measure CO2
Photosynthesis taking place in wheat plants can be measured in
field chambers like this one being adjusted by plant physiologist Richard
At the U.S. Water Conservation Laboratory in Phoenix, Arizona, the future is
To see how crops will grow when carbon dioxide becomes more prevalent in our
atmosphere, ARS scientists there are pumping large quantities of CO2
across large, open-air test plots.
And while the gas bill for thisthe world's largest such
experimentreaches close to $100,000 annually, the money is well spent.
For like a factory, the experiment's large size produces an economy of scale.
Per dollar, the Free Air Carbon Dioxide Enrichment
projectFACEproduces 4 to 10 times as many high-CO2-grown
plants needed for research as other methods.
'Today's air contains about 360 parts per million of CO2. That's
up from levels of 265 in pro-industrial times and 315 as recently as
1958," says soil scientist Bruce A. Kimball. "Because there's every
reason to believe this trend will continue, we are conducting scientific
studies that will prepare us for the change."
Head of the ARS Environmental and Plant Dynamics Research Unit, Kimball
leads a team that is subjecting crops to environmental conditions like those
expected in the next century, especially air with a doubled CO2
"There's no reason to panic, because humans will not be directly
affected," Kimball says. "The air we breathe contains 78 percent
nitrogen and 21 percent oxygen; the remaining 1 percent is a mixture of
CO2, argon, neon, and helium. However, growth patterns of all major
food crops will change, and we need to find out exactly how."
Carbon dioxide affects both the process of photosynthesis and plant leaf
resistance to water vapor loss.
This large ARS study simulates field conditions representing those
anticipated in the next 50 to 75 years. Large amounts of CO2 are
released through upright pipes that maintain a constant 550 parts per million
in the air around plants in open fields. Previous experiments were in open-top
chambers whose walls affected plant growth and distorted findings. Other
experiments in greenhouses or growth chambers also produced data that were not
representative of natural, outdoor conditions.
Last year, there were 50 scientists from 25 different locations in eight
countriesCanada, Germany, Italy, Japan, Spain, The Netherlands, United
Kingdom, and the United Statescollecting or analyzing data from the large
plots located at the University of Arizona's Maricopa Agricultural Center.
And the ARS scientists based at Phoenix will make the data available to all
researchers who can tap into the Internet.
Two crops have been extensively studied: cotton and wheat. Smaller subplots
include corn, sorghum, sudangrass, and barley.
Facing Up to Future Change
Technician Lynette Eastman (left) and University of Idaho
graduate student Li Aiguo measure the spectral reflectance and transmittance of
wheat in studies to determine the effect of higher CO2 on photosynthetic
"FACE has already contributed a lot to understanding how elevated
CO2 will increase plant growthespecially, of plants we rely on
for food and fiber," says Kimball. "We are also getting basic
information on how individual factors such as carbon cycling and water vapor
pressure interact to produce that increase. This will help us predict our
future climate and water resources under a higher CO2
The data show that relative growth responses to CO2 are similar
in both FACE plots and open-top chambers. But only the FACE approach produces
data that can be relied on to represent future field conditions.
"One of the more interesting findings to come from our study is that
soil in our FACE experiments stored more carbon, compared to regular fields.
The cotton study indicated that soil carbon increased more than 10 percent in
only 3 years.
This observation may help clear up a mystery that puzzles modelers of global
carbon volume. Only about half of the carbon emissions from burning fossil
fuels can be accounted for in the atmosphere, and their models do not explain
where all the carbon is relocating.
"We hope our data will help them solve the missing carbon
question," says Kimball.
Information on the effects of increased CO2 will be important to
other researchers who are looking at the larger picture of global change.
Increased CO2 is only one component. Other changes include increased
temperatures and altered moisture distribution patterns.
The Bottom Line
Scientists summarize their results by saying elevated CO2 levels
are generally good for most plants. They produce more yield and use less water.
But increased amounts of the gas affect different plant species in different
Cotton, a woody summer crop, was highly responsive to the elevated
CO2. It yielded up to 50 percent more at 360 ppm, under
well-irrigated field conditions, and about 43 percent more under dry
In contrast, wheat, a cool-season grass, increased its growth about 20
percent at midseason under both wet and dry conditions. But the elevated
CO2 caused the wheat to mature sooner, thereby shortening the
growing season and giving a final yield only 8 percent higher in the wet plots.
Root growth increased about 50 percent in wheat, with most occurring in
midsize roots. Cotton taproots were from 40 to 70 percent larger.
Microbial populations varied, but total activity as measured by respiration
was up. So, not only was more carbon stored in the soil, more carbon was also
respired in the soil and returned to the atmosphere.
Cotton leaves grown under elevated CO2 levels experienced large
increases in leaf starch, while wheat leaves had large increases in both sugars
Varying CO2 concentrations in FACE field plots are tracked on
computer by University of Arizona associate engineer Michael Gerle.
Consistent with the increases in leaf starches and sugars, there was a
slight decrease in nitrogen concentration in FACE-grown wheat and significantly
less in cotton.
This implies that the leaves of plants in the future will provide less
protein per pound for cattle and leaf-eating insects.
Digestibility of sudangrass and wheat was not changed. There had been some
speculation that digestibility would decrease because cell walls are usually
thicker under FACE.
Canopy temperatures of cotton were an average 0.8 degrees warmer under FACE.
Wheat plants were 0.56 degrees warmer. In the case of the wheat, such an
increase in plant temperature probably contributed to the earlier maturity of
the FACE-grown plants.
Leaf photosynthesis increased between 20 and 40 percent in FACE cotton
plants. Photosynthesis increased about 20 percent in irrigated wheat and about
75 percent in dryland wheat. Although not the full story, these increases in
photosynthesis are undoubtedly responsible for the eventual increases in plant
growth, says Kimball.
Plants used water more efficiently. Cotton and wheat yielded 50 percent
product per unit of water used. While CO2 had little effect on
cotton's water, wheat used 11 percent less per acre, according to energy flow
The cotton data arc now being used to validate a computer program called
COTCO2, a cotton growth simulation model. Researchers plan to use it
to predict how increased atmospheric CO2 and any associated climate
change will affect cotton growth and water use in the future.
FACE team leader Bruce Kimball adjusts wind sensors used to
control the release of CO2 over wheat plots.
Other computer modelers should find the program valuable, too. For example,
the wheat data are being used to validate wheat growth models developed by
In addition to the U.S. Water Conservation Laboratory, other ARS
laboratories working on FACE include the Western Cotton Research Laboratory in
Phoenix, as well as labs in Temple, Texas; Athens, Georgia; Pullman,
Washington; Gainesville, Florida; Riverside, California; Auburn, Alabama; and
Fort Collins, Colorado.
The FACE project was started cooperatively by ARS, along with the U.S.
Department of Energy and its Brookhaven National Laboratory in Upton, New York,
and by Tuskegee University in Tuskegee, Alabama.
For the next 2 years, the FACE project will examine the interactive effects
of CO2 and soil nitrogen supply on wheat growth and carbon exchange,
especially on soil carbon sequestering and exchange. The project will he funded
by a grant from the U.S. Department of Energy to the University of Arizona, but
ARS scientists will continue to be major collaborators and will assist in the
management of the project. -- By Dennis Senft, ARS.
Kimball is in the USDA-ARS U.S.
Conservation Laboratory, 4331 E. Broadway Rd., Phoenix, AZ 85040; phone
(602) 437-1702, fax (602) 437-5291.
Less ThirstMore Growth!
It's probably no surprise that the Western range's famed "wide open
spaces" aren't as wide open as they once were. But one factor behind the
change might surprise you: Range plants may not be as thirsty as they used to
The reason is carbon dioxideCO2which is used by
plants for growth.
Plants take in CO2 from the atmosphere through tiny openings in
their leaves. When these openings, called stomates, are gaping to gulp in
CO2, precious water inside the plant escapes through the same
openings. So the plant must take in greater amounts of water from the soil to
maintain enough to survive.
Technician Ric Rokey takes the temperature of a wheat leaf with
an infrared thermometer.
Current atmospheric CO2 levels are about 360 parts per million
(ppm), compared with 280 ppm a century agoa gain attributed in part to
burning of fossil fuels and changes in land use. The greater abundance of
atmospheric CO2 means plant stomates don't have to open as wide to
take in sufficient carbon dioxide. That, in turn, means less water is used by
the plant, says ARS ecologist Hyrum B. Johnson.
"We know from photographs and other records that there is more brush on
the range today than a century ago," Johnson points out. "Rising
CO2 levels may have enabled plants to proliferate on parts of the
range that were formerly too dry, because the plants now use water more
At the ARS Grassland, Soil, and Water Research Laboratory at Temple, Texas,
Johnson and fellow ARS scientists Herman S. Mayeux and H. Wayne Polley have
experimented since 1988 with growing various plants in different CO2
levels. They range from today's 360 ppm back to the about 180 ppm of the last
Ice Age 15,000 years ago and forward to the anticipated 700 to 1,000 ppm of the
"We planted seeds of a rangeland brush species called acacia in large
soil boxes in August 1992," explains Mayeux. "We kept the boxes in
greenhouse bays maintained at different CO2 levels350 ppm, 700
ppm, or 1,000 ppm. We found that the plants in the higher CO2 grew
much faster, although they used about the same amount of water as those growing
in 350 ppm."
Between August and December 1992, stems on the acacia in 1,000 ppm of
CO2 grew an average of 23 feet, compared with an average of about 15
feet for the plants in 700 ppm and about 5 feet for plants in 350 ppm.
In a related project, Johnson, Mayeux, and Policy grew two varieties of
wheat in a specially constructed growth chamber in which flowing air's
CO2 content gradually declined from 350 ppm to 200 ppm at various
locations within the chamber. The wheat varieties were Yaqui 54, a variety
typical of the wheats grown in the 1950's, and Seri M82, representative of
varieties grown now.
"In 200 ppm CO2, the plants did very poorly and required
twice as much water to grow the same amount of forage or grain," reports
Polley. "As CO2 increased to modern levels, seed yield tripled
for both varieties. The size of the individual grains didn't change, but there
were more individual grains per spike."
"In the past 150 years, CO2 levels have risen 30 percent,
and the rate of increase has been most rapid in recent decades," Mayeux
adds. "In today's agricultural production, we may already be experiencing
an impact from higher CO2 levels." -- By Sandy Miller
S. Mayeux, and
Wayne Polley are in the USDA- ARS
Soil, and Water Research Laboratory, 808 East Blackland Road, Temple, TX
76502; phone (254) 770-6501, fax (254) 770-6561.
"FACE-ing the Future" was published in the
April 1995 issue of
Agricultural Research magazine.