Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Although the declining CO2 stimulation of leaf production observed here is supported by theory (Körner 2006) and observations from similar experiments (for example, Gielen and others 2003), less is known about the long-term response of forests to elevated O3.
Category: