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:Such relationships between tissue [N] and two morphological parameters, SRL and tissue density, were also found in some studies with multiple species; for example, the variability of root tissue [N] in herbs and woody plants was strongly associated with tissue density and weakly with SRL (Roumet et al. 2006; Comas and Eissenstat 2009; Kong et al. 2014).
Category: