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:…S. patens or absence S. virginica of arenchymatous tissue Anderson 1974 , and the impact of edaphic variables seems to be greater, resulting in very similar types of culturable diazotrophs associated with plant species that are themselves very different Bagwell et al. 2001; Bergholz et al. 2001 .
Category: