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:It appears that the initial stimulation of growth by elevated CO2 allows forests to more quickly reach the point in stand development where environmental factors (light, water, nutrients, and so on) limit canopy production, but eventually, trees growing under ambient conditions reach a similar set of limitations to growth (Körner 2006).
Category: