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:More recently, discrete-return lidar has been applied to several broad-leaved forests in North America (Popescu et al. 2003, 2004; Lim et al. 2003a; Lim and Treitz 2004).
Category: