Department of Fisheries, Wildlife, and Conservation Sciences, Oregon State University Corvallis, Oregon, United States
Abstract: American bison (Bison bison) are sometimes called a keystone species, due to their strong influence on the ecosystems they inhabit. Among their known effects is the removal of shrubs and trees. We observed bison bulls browsing and breaking the branches and stems of young quaking aspen (Populus tremuloides) plants, and assessed the extent of these effects on aspen in the northern ungulate winter range (northern range) of Yellowstone National Park. For most of the 20th century, young aspen in northern Yellowstone were suppressed by herbivory by elk (Cervus canadensis) and rarely grew taller than 200 cm. Young aspen began growing taller in the early 2000s, following sustained reductions in elk density that followed large carnivore restoration in the late 1990s. We sampled young aspen during the years 2020-21, using random plots in 87 randomly selected aspen stands across the northern range, and noted the number of young aspen saplings that were live or broken. Saplings were defined as young aspen >200 cm tall and < 5 cm dbh (diameter-at-breast-height); broken saplings were identified by stumps >2 cm in diameter at 50 cm height. Overall, 18% of saplings were broken with a much higher incidence of breakage in areas of high bison density. In a focal stand frequented by bison, 58% percent of aspen saplings were broken, and data from previous surveys suggested this was an underestimate of the actual number that had been removed. Some portions of the stand had been cleared of saplings, and aspen sprout growth was prevented by browsing on the edges of the stand. Broken aspen may resprout at a low height, but the new shoots are then vulnerable to browsing. Saplings also were killed when the bark was scraped off by horns. Saplings grew in the partial protection of many fallen trees, but by 2020 these trees presented less of a barrier.
Saplings taller than 200 cm have been used as indicators of potential aspen recruitment in areas affected by intensive browsing, but the propensity of bison to break down these saplings reduces the usefulness of this metric. Northern Yellowstone bison have increased four-fold since the early 2000s, and current plans are for greater numbers. Thus, the suppressing effects of bison on aspen recruitment, both by browsing young sprouts and breaking saplings, are likely to increase, resulting in a partial reversal of the recent trend toward aspen recovery.
