Free-Air CO2 Enrichment (FACE)
When plants are grown in an atmosphere with a CO2 concentration greater than the current concentration, increases in leaf photosynthesis and the size of pores on the leaf (known as stomata) are observed. This results in increased carbon availability to a plant and the water use efficiency of a plant. Although these physiological responses to CO2 are well understood, their ecological effects as they cascade through many plant and ecosystem processes are subject to multiple interactions and feedbacks operating at various timescales. Therefore, the response of the land carbon sink to increasing atmospheric CO2 remains the largest uncertainty in global carbon cycle modeling to date, and this is a huge contributor to uncertainty in climate change projections.
Building on past research at ORNL with tree seedlings in growth chambers and saplings in open-top chambers, the goal of the Oak Ridge Free Air CO2 Enrichment (FACE) experiment (http://face.ornl.gov) was “To understand how the eastern deciduous forest will be affected by CO2 enrichment of the atmosphere, and what are the feedbacks from the forest to the atmosphere. This goal is being approached by measuring the integrated response of an intact forest ecosystem, with a focus on stand-level mechanisms.” The experiment was sited in a sweetgum (Liquidambar styraciflua) plantation located on the Oak Ridge National Environmental Research Park (35º54' N; 84º20' W). Oak Ridge is in East Tennessee in the Ridge and Valley province between the Cumberland and Blue Ridge Mountains. Tree, soil and ecosystem measurements were made on five 25-m diameter plots in the forest, two elevated CO2 plots (528-560 ppm) and three unchanged CO2 plots (384-405 ppmv).
CO2 treatment started in April 1998 prior to leaf out, and ran through September, 2009. Key findings from the experiment were:
- Fine-root production was stimulated by elevated CO2, resulting in more carbon added to the soil system.
- Initial enhancement of net primary productivity was not sustained because of feedbacks through the nitrogen cycle.
- Stable isotope analysis indicated that N availability declined faster in plots exposed to elevated CO2
- Carbon storage in the soil increased in CO2-enriched plots, including in protected forms.
The FACE Model-Data Synthesis (FACE-MDS) was established in 2008 to evaluate multiple carbon cycle model predictions using data from the Duke and Oak Ridge FACE experiments (two of the longest and most comprehensive experimental datasets on ecosystem responses to elevated CO2). The project delved deep into model results and pulled apart the underlying reasons for model behavior, a method we refer to as assumption centered modeling. The FACE-MDS working group has advanced understanding of terrestrial ecosystem responses to elevated atmospheric CO2, identified areas for model improvements and questions for further experimental analysis, and advances a new standard for model inter-comparison which we hope will be adopted by many modeling groups. The FACE-MDS is ongoing and is now analyzing results from 12 models against data from 6 CO2 experiments.
Following on insights from the ORNL FACE experiment and the FACE-MDS project, ORNL researchers are involved in establishing a new FACE experiment in the Amazon forest of Brazil (https://amazonface.org). Funded by the Inter-American Development Bank and Brazilian science institutions, AmazonFACE will help to answer the question: “How will rising atmospheric CO2 affect the resilience of the Amazon forest, the biodiversity it harbors, and the ecosystem services it provides?” AmazonFACE is an international effort led by Brazilian scientists and with participants from the US, UK, Germany, and the Netherlands.
The Oak Ridge FACE experiment and FACE-MDS are national and international collaborations of scientists at many institutions across at least six nations led by principal investigators at Oak Ridge National Laboratory. In addition to the core group of six ORNL researchers who led the ORNL FACE experiment, numerous ORNL researchers and students participated in the project, as well as collaborators from more than 20 other institutions (http://face.ornl.gov/people.html) Key partners in the FACE-MDS project include researchers at Hawkesbury Institute for Environment at Western Sydney University in Australia, Max Planck Institute for Biogeochemistry in Germany, and Macquarie University in Australia.
We thank DOE Office of Science, Biological and Environmental Research for their support, additional sponsors, and all of our collaborators.