The influence of floods and herbivory on cottonwood establishment and growth in Yellowstone National Park

Abstract

AbstractHerbivory by ungulates can shape the structure and function of riparian forests. However, our understanding of the interactions between herbivores and cottonwoods (Populus spp.) is largely derived from studies of domestic livestock grazing and may not reflect free ranging herds of wild ungulates. In this study, we quantified the influence of stream hydrologic regime and herbivory by wild ungulates on cottonwood establishment and growth along three rivers in Yellowstone National Park's northern range.Approximately 1.36 of the existing 1.37 million cottonwoods representing approximately 66% of the cottonwood stand area in Yellowstone's northern range was established between 1995 and 2008 by the largest flood sequence in recorded history. Coincidentally, the flood sequence began in the years immediately following wolf (Canis lupus) reintroduction to Yellowstone National Park. The flows caused large‐scale channel changes and provided suitable habitat for cottonwood seedling establishment and survival. Over 92% of the cottonwoods occurred along the Lamar River, and recruitment along this river appears to follow infrequent large peak flows. Soda Butte Creek and the Gardner River cottonwoods exhibited nearly annual recruitment. The resulting cottonwood biomass from the flooding events has exceeded herbivore demand. Even at relatively low consumption rates, bison are able to remove a significant proportion of total cottonwood production in the study areas limiting plant height and forage available to wintering elk. We conclude that the large cottonwood recruitment event that began in the late 1990s was due to a rare series of large snowmelt‐driven floods.