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:These changes have been associated with reductions in tree basal area, increases in tissue N-to-P (N: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.
Category: