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:This pattern of shifting N and P availability has been observed in a limited number of established long-term substrate age gradients (e.g. Crews et al. 1995; Wardle et al. 2004; Selmants and Hart 2008).
Category: