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::P) ratios, and progressive P limitation over time in forested systems (Wardle et al. 2004) and observed across nine chronosequences spanning 6–4100 ky in length in boreal, temperate, and subtropical zones (Peltzer et al. 2010).
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