The Oak Ridge Experiment on CO₂ Enrichment of Sweetgum was a Free-Air CO₂ Enrichment (FACE) experiment at the Oak Ridge National Laboratory in Tennessee, USA. From 1997 to 2010, we tested critical hypotheses about the responses of a closed-canopy deciduous forest to the atmospheric CO₂ concentrations of future decades. The research was sponsored by the U.S. Department of Energy.

The response of natural ecosystems to an increasing concentration of carbon dioxide in the atmosphere is a key component of analyses of the current and potential impact of global change. Efforts to understand how eastern deciduous forests will be affected by carbon dioxide enrichment of the atmosphere had previously been addressed by studying components of the forest system (individual small trees, specific processes), but this experiment took the critical leap to measuring the integrated response of an intact forest with a focus on stand-level mechanisms.

Goal
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.

Features
A free-air CO₂ enrichment (FACE) facility, comprising five 25-m plots was constructed in a deciduous forest on the Oak Ridge National Environmental Research Park. The study site was a sweetgum (Liquidambar styraciflua) monoculture planted in 1988. This closed-canopy, 18-m tall stand offered the opportunity for rigorous tests of hypotheses that address the essential features of a forest stand and how they could influence the responses to CO₂. These features include:

• the closed canopy, which constrains growth responses
• full occupancy of the soil by the root system, which constrains the nutrient cycle
• the larger scale of the trees compared to saplings in open-top chambers, which changes the functional relationships of carbon cycling
• and the longer time scale that can be addressed, permitting studies of soil carbon changes.

Support
The Oak Ridge FACE facility was operated by the Environmental Sciences Division of the Oak Ridge National Laboratory (ORNL). The FACE facility was established with support from the ORNL Director’s R&D Fund and operated with continued support from the Biological and Environmental Research program of the U. S. Department of Energy (DOE) Office of Science. Additional support was provided by the Terrestrial Ecology and Global Change (TECO) program through the National Science Foundation.

Information about the DOE FACE research program is here: