The FACE Program
The atmospheric carbon dioxide concentration ([CO2]) has risen by 35% since the start of the industrial revolution, it is higher now than at any time in the past 25 million years and is predicted to increase an additional 50% by 2050 (IPCC). Plants respond to rising [CO2] through increased photosynthesis and reduced transpiration. Photosynthesis removes CO2 from the atmosphere and respiration by plants and heterotrophs add it back.
Therefore, the terrestrial biosphere is not just a passive respondent to rising [CO2] but can play a fundamental role in determining the rate of global change. Before FACE, much of what we knew about plant and ecosystem responses to rising [CO2] came from studies conducted in enclosures where the response of plants is modified by their growth conditions. FACE was developed as a means to grow plants in the field at a controlled elevation of [CO2] under fully open-air conditions. Results from FACE experiments provide perhaps the best estimate of how plants and ecosystems will respond in a future high CO2 world.
What is FACE?
A typical FACE plot is approximately circular and surrounded by a ring of pipes that release CO2, or air enriched with CO2, at vertical intervals from just above the ground to just above the top of the plant canopy. Wind direction, wind velocity, and [CO2] are measured at the center of each plot and this information is used by a computer-controlled system to adjust CO2 flow rate to maintain the target elevated [CO2].
Only pipes on the upwind side of the plots release CO2, unless wind velocity is very low, at that time CO2 is released alternately from adjacent release points. For vegetation of low stature only one or two vertical release points are necessary, whereas for tall vegetation several vertical release points are needed to enrich the whole canopy.
Fast feedback algorithms avoid large overshoots in response to fluctuations in [CO2] and provide a stable elevation of [CO2]. This basic design has been utilized with some variations and technical developments in over ten experiments in plots that are as large as 30m diameter that can accommodate vegetation as tall as 25 meters.
What are the advantages of FACE?
FACE studies are fully open air and have many benefits over controlled environment and open-top chamber (OTC) experiments. FACE allows the investigation of an undisturbed ecosystem and does not modify the vegetationís interaction with light, temperature, wind, precipitation, pathogens and insects.
This, in combination with the large size of FACE plots, allows the integrated measurement of many plant and ecosystem processes simultaneously in the same plot, avoids many of the problems associated with edge effects prevalent in OTCs, enables significantly more plant material to be harvested without compromising the experiment, and allows plants to be studied throughout their life cycle, including trees that have enough space to develop to canopy closure.
Last Modified: February 1, 2008