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:Although fast growing woody plants have been widely used for cleaning up HM contaminated soils, limited information is available about the physiological and molecular mechanisms of HM tolerance and detoxification in woody plants (Konlechner et al., 2013; Luo et al., 2016).
Category: