Sampling bias exaggerates a textbook example of a trophic cascade

We measured browsing and height of young aspen (≥ 1 year-old) in 113 plots distributed randomly across the study area (Fig. 1). Each plot was a 1 × 20 m belt transect located randomly within an aspen stand that was itself randomly selected from an inventory of stands with respect to high and low wolf-use areas (Ripple et al. 2001). The inventory was a list of 992 grid cells (240 × 360 m) that contained at least one stand (Appendix S1). A “stand” was a group of tree-size aspen (>10 cm diameter at breast height) in which each tree was ≤ 30 m from every other tree. One hundred and thirteen grid cells were randomly selected from the inventory (~11% of 992 cells), one stand was randomly selected from each cell, and one plot was randomly established in each stand. Each plot likely represented a genetically-independent sample (Appendix S1). We measured aspen at the end of the growing season (late July to September), focusing on plants ≤ 600 cm tall, which we termed “young aspen.” For each stand, we measured every young aspen within a plot (‘random stems’) and each of the five tallest young aspen within the stand (‘5T stems’). For all young aspen, we measured browsing status (browsed or unbrowsed) and height of the leader (tallest) stem. A leader was ‘browsed’ if its growth from the previous growing season had been eaten, which we identified by a sharp, pruned edge at the base of the current year’s growth. Most plots were measured nearly every year since 1999 (Ripple et al. 2001) and our analysis focused on data from 10 years (2007-2014, 2016-2017) in which sampled stands included measurements of random and 5T stems. Elk were likely the primary ungulate species browsing young aspen in our plots during the study (Fig. S1).