For example, the Aspen FACE facility in Wisconsin (see photo) consists of 12 rings (approximately 30 m (98 ft) in diameter) containing natural forest. Each ring contains pipes around its perimeter where CO? can be emitted. Adjacent to the rings are patches of unaltered forest. Scientists can thus assess how increased CO? affects ecosystem attributes such as tree growth, leaf development, and soil carbon in the experimental rings in comparison with the adjacent unaltered forest. In this sense, FACE experiments allow scientists to conductfield research by comparing natural forests that have ambient, or unchanged, CO? concentrations with nearby natural forests that haveincreased CO? concentrations.