Understanding trophic cascades in terrestrial wildlife communities is a major challenge because these systems are difficult to sample properly. We show how a tradition of nonrandom sampling has confounded this understanding in a textbook system (Yellowstone National Park) where carnivore [Canis lupus (wolf)] recovery is associated with a trophic cascade involving changes in herbivore [Cervus canadensis (elk)] behavior and density that promote plant regeneration. Long-term data indicate a practice of sampling only the tallest young plants overestimated regeneration of overstory aspen (Populus tremuloides) by a factor of 3-8 compared to random sampling because it favored plants taller than the preferred browsing height of elk and overlooked non-regenerating aspen stands. Random sampling described a trophic cascade, but it was weaker than the one that nonrandom sampling described. Our findings highlight the critical importance of basic sampling principles (e.g., randomization) for achieving an accurate understanding of trophic cascades in terrestrial wildlife systems.

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).

This dataset has 18,792 records, including 18,623 records of individual young aspen (plants > 1 year-old & < 600 cm) and 169 records of plots with no young aspen ("zero plots"). Records of individual young aspen (N = 18,623) were used in the majority of analyses (Fig. 2-5a,b), including generalized linear mixed models (GLMMs) that tested how the effect of year on browsing, height, and recruitment of stems differed by sampling method. This dataset was also used to model the effect of stem height on browsing to estimate the preferred browse height (PBH) and browse escape height (BEH). The full dataset that includes plots with no young aspen was only used to calculate the percentage of plots and stands each year with median heights greater than 200 (Fig. 5c) or 300 cm (Fig. 5d).  

The dataframe has the following 6 columns:  

1.  Plot: individual identifier for each of 113 plots distributed randomly across the study area. Each plot was a 1 × 20 m belt transect located randomly within an aspen stand 
2. Year: year in which aspen was sampled
3. Tree: individual identifier for each stem within a plot
4. Browse: denotes the browsing status (browsed = 1,unbrowsed = 0) of the leader (tallest) stem. A leader was ‘browsed’ if its growth from the previous growing season had been eaten
5. Height: height (cm) of the leader stem of each individual aspen
6. Type: sampling method. Every young aspen within a plot is a "random" stem, and each of the five tallest young aspen within the stand is a "5T" stem.

Plots that had no young aspen in a given year have a value of "NA" for browse and height.