Free-Air Carbon Dioxide Enrichment (FACE) Experiment

Pumping CO₂ into live forests to measure the effect of future climates on vegetation.

The Free-Air CO₂ Enrichment (FACE) technology was first developed by Brookhaven National Laboratory (BNL) for use in an agricultural setting. Elevated CO₂ experiments, combined with manipulations of water and nitrogen supply, were conducted from 1989 to 1999 in Maricopa, Ariz., with cotton, wheat, and sorghum. Cotton was shown to be highly responsive to CO₂ enrichment, but the C4 sorghum was not. Inter–actions with water and nitrogen varied across species.The BNL FACE system was scaled up for use with tall vegetation, and a prototype forest FACE system was tested in the Duke University Forest in 1994 and 1995, leading to a fully replicated FACE experiment in the loblolly pine forest in 1996. Other FACE experiments with forest stands and native vegetation began in 1997 at Oak Ridge National Laboratory (ORNL) in Tennessee (ORNL FACE in a sweetgum plantation), at the Nevada National Security Site (Nevada Desert FACE), and on U.S. Forest Service land in Wisconsin (Rhinelander FACE with trembling aspen mixed with sugar maple and paper birch). Over 10 to 12 years, these experiments exposed the vegetation in replicated, 25-m to 30-m diameter plots to ambient or elevated CO₂ (about 550 to 565 parts per million). In the Rhinelander FACE experiment, the CO₂ treatments were combined with ambient or elevated ozone, and the Duke FACE experiment added a nitrogen fertilization treatment toward the end.

Key Results from FACE Experiments

• Increased net primary production (NPP) in elevated CO₂ was sustained throughout the Duke FACE experiment and resulted in greater woody biomass accumulation. The increased production was supported by an increased flux of carbon below ground, which stimulated tree uptake of nitrogen bound in soil organic matter and created a positive feedback through increased canopy nitrogen and greater photosynthesis. Slow adjustments in canopy structure meant that short-term leaf-scale responses of stomatal conductance to elevated CO₂ were not translated directly to canopy water-use responses, emphasizing the importance of long-term experiments whose durations are sufficient for slow responses to manifest.
• An initial stimulation of aboveground growth by elevated CO₂ in the ORNL FACE experiment was quickly dissipated, but NPP was enhanced with the additional productivity accounted for by enhanced production of fine roots, especially deeper in the soil, which sustained increased nitrogen uptake needed to support increased NPP. However, nitrogen availability steadily declined, leading to a loss of photosynthetic enhancement and the NPP response. This decline occurred faster in elevated CO₂, supporting the premise of progressive nitrogen limitation.
• The Nevada Desert FACE Facility was the only DOE FACE experiment that examined the impact of elevated atmospheric CO₂ on an arid ecosystem and took place in an undisturbed, natural ecosystem. Responses of the desert vegetation to elevated CO₂ were highly dependent on precipitation patterns. Photosynthesis was stimulated by elevated CO₂, increasing leaf-level carbon gain, with larger effects in wet years than during extended drought years. This response supported increased aboveground growth only in the wet years. After 10 years, there was no detectable effect of elevated CO₂ on aboveground biomass or community structure of the perennial plant community.
• The Rhinelander FACE experiment was initiated in a seedling stand and, there-fore, was an expanding system through most or all of the experimental duration. Many of the responses reported early in the experiment were not sustained, emphasizing the importance of projecting the responses of seedlings and young trees to mature forests, as well as the value in maintaining experiments for as long as is feasible. Ozone was found to counteract some of the effects of elevated CO₂, suggesting that projections of CO₂ responses will be overstated if the co-occurrence of ozone is not considered. However, the negative effects of ozone on growth dissipated by the end of the experiment. [1]

These projects were part of the CO2 research network fostered by the Global Change and Terrestrial Ecosystems core project of the International Geosphere-Biosphere Programme. Results from the experiment contribute to the Terrestrial Ecosystem Response to Atmospheric and Climatic Change (TERACC) project, a 5-year initiative integrating experimental data and global change modeling. [2]