Tall willow thickets return to northern Yellowstone

Abstract

AbstractNorthern Yellowstone National Park provides an example of passive restoration, as wetlands and riparian areas there lost most tall willows in the 20th century, due to intensive herbivory by elk (Cervus canadensis). Following large carnivore restoration in the late 1990s, elk numbers decreased, and some researchers reported willows growing taller with reductions in browsing, evidence of a shift toward willow recovery. Others questioned the extent and significance of these changes. To investigate how willow heights have changed in northern Yellowstone since the 1990s, and to assess the importance of browsing as a driver of willow height and canopy cover, we compared data from three time periods: 1988 to 1993 when elk densities were high and most willows very short, 2001 to 2004 when willows may have begun to recover, and 2016 to 2018. We found a strong contrast between sites along streams, compared to wet meadows (meadow sites). Willows in meadow sites did not increase in height, but willows in stream sites increased significantly, exceeding 200 cm mean height in summer by 2001–2004, and in spring by 2016, a height indicative of recovery. Where height did not increase, this was due to loss of annual growth to herbivory. Overall willow height distribution changed from mostly short to become clearly bimodal, with a new peak around 300–400 cm. Bison increased, and in some sites where bison congregate willows remained suppressed at heights below 120 cm, a condition strongly correlated with summer browsing. We also located and measured willow thickets (groups of 5 of more willows >200 cm spring height and <2 m apart) along streams in the study area. We found willow thickets in all stream reaches surveyed, a significant change from past conditions. Thickets occupied >80% of willow patches in some sites, but as little as 22% in others. Tall willow thickets are an important habitat feature and an indicator of willow recovery. Our results demonstrate that the reduction of elk herbivory over the last two decades in northern Yellowstone has allowed willows to grow taller in many places, despite a warming and drying climate, while increased herbivory by bison continues to suppress willows in some locations.