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

Abstract

Herbivory 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.

Bison alter the northern Yellowstone ecosystem by breaking aspen saplings

“…on the northern range have focused on the elk that wintered there (Houston, 1982 ; NRC, 2002 ; Painter et al, 2018 ; Wagner, 2006 ). Only recently, as elk have decreased and bison increased, have bison emerged as an important factor affecting the dynamics of these woody plants (Beschta et al, 2020 ; Painter & Ripple, 2012 ; Ripple et al, 2010 ; Rose & Cooper, 2016 ). Their impact on aspen saplings adds a new complication to the assessment of potential aspen recruitment.…”

“…The amount of browsing varies with season and availability of shrubs and other forage (Bork et al, 2013 ; Larter & Gates, 1991 ; Peden, 1976 ; Waggoner & Hinkes, 1986 ; Zeigenfuss & Schoenecker, 2021 ). These consumptive effects can be significant where bison are present in high densities year after year, as in northern Yellowstone (Beschta et al, 2020 ; Painter & Ripple, 2012 ; Rose & Cooper, 2016 ). Browsing often removes new shoots, and repeated browsing can suppress growth and prevent the recruitment of new trees.…”

Does restoring apex predators to food webs restore ecosystems? Large carnivores in Yellowstone as a model system

“…However, it should be noted that the mid‐1990s, when wolves were reintroduced, had the highest snowpack and stream flows on record. This led to the most significant cottonwood recruitment events of the 20th century on the Lamar River and other streams, and probably stimulated higher willow growth due to increased water availability (Rose & Cooper, 2017). Moreover, half of the sites studied by Beyer et al were on streams too large to be dammed by beaver, so they would not be expected to be influenced by effects of changes in the hydrologic regime experienced on the smaller streams.…”

“…We estimated the total biomass of elk and bison inhabiting the northern range using a weighted average body mass (Rose & Cooper, 2017) multiplied by the count of individuals in the population, assuming that the age and sex composition of each population was constant. Total forage consumed per day by elk and bison is roughly proportional to total biomass because dry matter intake of ruminants is well approximated as a constant fraction of body mass (Alldredge et al, 1974; Cordova et al, 1978; Holechek & Vavra, 1982).…”

“…Our data suggested otherwise; 20% of all our annual observations of bison in willow communities showed them browsing on willows, suggesting that bison feed on willows at least 20% of the time that they are in willow communities. Waggoner and Hinkes (1986) and Rose and Cooper (2017) showed that willows and other woody deciduous plants can be a substantial component of bison diets. We are the first to analyze the consequences of a reorganized guild of large herbivores for browsing pressure on willows (Figure 19).…”