Free-Air Concentration Enrichment

Free-Air Concentration Enrichment

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Free-Air Carbon dioxide Enrichment (FACE) is a method used by ecologist
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...

s and plant biologists that raises the concentration of in a specified area and allows the response of plant growth to be measured. Experiments using FACE are required because most studies looking at the effect of elevated concentrations have been conducted in labs and where there are often factors other than concentration that affect growth, for example the pot effect. Measuring the effect of elevated using FACE is a better way of estimating how plant growth will change in the future as the concentration
Carbon dioxide in the Earth's atmosphere
The concentration of carbon dioxide in Earth's atmosphere is approximately 392 ppm by volume and rose by 2.0 ppm/yr during 2000–2009. 40 years earlier, the rise was only 0.9 ppm/yr, showing not only increasing concentrations, but also a rapid acceleration of concentrations...

 rises in the atmosphere due to man-made emissions. FACE also allows the effect of elevated on plants that cannot be grown in small spaces (tree
A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to...

s for example) to be measured.


Horizontal or vertical pipes are placed in a circle around the experimental plot, which can be between 1m and 30m in diameter
In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints are on the circle. The diameters are the longest chords of the circle...

, and these emit enriched air around the plants. The concentration of is maintained at the desired level through placing sensors in the plot which feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...

 to a computer which then adjusts the flow of from the pipes.


FACE circles have been used across in parts of the United States in temperate forest
Temperate forest
Temperate forests correspond to forest concentrations formed in the northern hemisphere. Main characteristics include: wide leaves, big and tall trees and non seasonal vegetation...

s and also in stands of aspen
Populus section Populus, of the Populus genus, includes the aspen trees and the white poplar Populus alba. The five typical aspens are all native to cold regions with cool summers, in the north of the Northern Hemisphere, extending south at high altitudes in the mountains. The White Poplar, by...

 in Italy
Italy , officially the Italian Republic languages]] under the European Charter for Regional or Minority Languages. In each of these, Italy's official name is as follows:;;;;;;;;), is a unitary parliamentary republic in South-Central Europe. To the north it borders France, Switzerland, Austria and...

. The method is also utilized for agricultural research. For example, FACE circles have been used to measure the response of soybean
The soybean or soya bean is a species of legume native to East Asia, widely grown for its edible bean which has numerous uses...

 plants to increased levels of ozone
Tropospheric ozone
Ozone is a constituent of the troposphere . Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and by night...

 and carbon dioxide at research facilities at the University of Illinois at Urbana-Champaign
University of Illinois at Urbana-Champaign
The University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...

. FACE technologies have yet to be implemented in old growth forest
Old growth forest
An old-growth forest is a forest that has attained great age , and thereby exhibits unique ecological features. An old growth forest has also usually reached a climax community...

s, tropical forests, or boreal forests but future research projects will probably include these areas. TasFACE is investigating the effects of elevated CO2 on a native grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...

 in Tasmania
Tasmania is an Australian island and state. It is south of the continent, separated by Bass Strait. The state includes the island of Tasmania—the 26th largest island in the world—and the surrounding islands. The state has a population of 507,626 , of whom almost half reside in the greater Hobart...

, Australia
Australia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...

. The National Wheat FACE array is presently being established in Horsham, Victoria, Australia as a joint project of the Victorian Department of Primary Industries and the University of Melbourne.

A FACE experiment began at Duke University
Duke University
Duke University is a private research university located in Durham, North Carolina, United States. Founded by Methodists and Quakers in the present day town of Trinity in 1838, the school moved to Durham in 1892. In 1924, tobacco industrialist James B...

 in June 1994. The Blackwood Division of the Duke Forest
Duke Forest
Duke Forest is a forest managed by Duke University for research, teaching, and recreation. It is located in the edge of the Piedmont in Durham County, Orange County, and Alamance County in North Carolina...

 contains the Forest-Atmosphere Carbon Transfer and Storage facility. This consists of four free-air enrichment plots which provide higher levels of atmospheric concentration and four plots that provide ambient control. There have been 253 publication
To publish is to make content available to the public. While specific use of the term may vary among countries, it is usually applied to text, images, or other audio-visual content on any medium, including paper or electronic publishing forms such as websites, e-books, Compact Discs and MP3s...

s reporting on the findings of the experiment.


In 2004, a meta-analysis
In statistics, a meta-analysis combines the results of several studies that address a set of related research hypotheses. In its simplest form, this is normally by identification of a common measure of effect size, for which a weighted average might be the output of a meta-analyses. Here the...

 of 15 years of FACE studies, found the response to elevated using FACE only slightly increases yield in crop plants (5-7% in rice
Rice is the seed of the monocot plants Oryza sativa or Oryza glaberrima . As a cereal grain, it is the most important staple food for a large part of the world's human population, especially in East Asia, Southeast Asia, South Asia, the Middle East, and the West Indies...

 and 8% in wheat
Wheat is a cereal grain, originally from the Levant region of the Near East, but now cultivated worldwide. In 2007 world production of wheat was 607 million tons, making it the third most-produced cereal after maize and rice...

). These responses were lower than was expected from previous studies that measured the effect in labs or enclosures. This has important consequences as previous projections of food production have assumed that decreases in yield as a result of climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

would be offset by increased yield due to elevated .

As of 2010, a more complete picture is emerging, with significant difference in response being observed by different plant species, also water availability and the concentration of ozone. For example, the 2007-2010 Horsham FACE project (using wheat crops) in Victoria, Australia, found "The effect of eCO2 was to increase crop biomass at maturity by 20% and anthesis root biomass increased by 49%". This study also concludes that "a wide gene pool needs to be investigated to see if particular cultivars are able to respond more to eCO2". Increased atmospheric carbon dioxide has been found to reduce plant water use, and consequently, the uptake of nitrogen, so particularly benefiting crop yields in arid regions. The carbohydrate content of crops is increased from photosynthesis, but protein content is reduced due to lower nitrogen uptake. Legumes and their symbiotic "nitrogen fixing" bacteria appear to benefit more from increased carbon dioxide levels than most other species.