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:Recent studies have clarified that such mycoheterotrophic gametophytes are associated with a particular AM fungal group (Winther and Friedman 2007, 2008, 2009), but knowledge about mycorrhizal associations of aboveground photosynthetic gametophytes is still unclear for most pteridophyte families.
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