Large carnivore extirpation linked to loss of overstory aspen in Yellowstone

Abstract

The striping of bark on the lower portion of aspen trees (Populus tremuloides) by Rocky Mountain elk (Cervus canadensis), or “barking”, increases entry points for disease organisms such as wood-decaying fungi, thereby increasing aspen tree mortality from heart rot. We hypothesized that this has occurred in Yellowstone's northern range aspen stands as part of a trophic cascade and has contributed to the premature and widespread loss of overstory trees. To evaluate these potential effects, we randomly selected aspen stands along a 60-km traverse across the park's northern range. For overstory trees ≥15 cm in diameter at breast height (DBH) within these stands, which were accessible to elk, we measured the height of barking (as indicated by deeply furrowed/blackened bark) and the proportion of increment core lengths with heart rot. Sampled trees had an average barking height of 2.2 m and 93.8% of them had heart rot. In contrast, only 13.3% of aspen trees that had grown in an environment protected from elk had heart rot. Heart rot comprised 45.2% and 2.5% of increment core lengths for the elk-accessible and protected stands, respectively. Results support a multi-level trophic cascade, from predator-to-prey-to-plants-to-fungi, whereby an incomplete large carnivore guild, over a period of seven decades, allowed the widespread barking of aspen by elk to occur. This, in turn, may have increased the prevalence of heart rot and has contributed to an accelerated loss of overstory aspen across the northern range.