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:For example, although some research indicates that forests can sustain increased productivity under elevated CO2 by enhancing N uptake (Finzi and others 2007), few studies have followed forests through important stand development phases (for example, establishment, canopy closure, age-related declines in stand productivity) to examine whether the early stimulation of plant productivity by elevated CO2 persists as stands mature after reaching maximum productivity (Körner 2006).
Category: