Changing Snow Conditions and the Effects of Elk Foraging on Aspen and Willow

Jedediah Brodie, Pennsylvania State University; Wildlife Conservation Society, North America Program
Eric Post, Pennsylvania State University
Joel Berger, Wildlife Conservation Society, North America Program

Climate change is altering the depth, density, and duration of snow in Yellowstone National Park, and this will alter many ecological processes. Our research investigates how changing snow conditions may affect the interaction between elk and two of their key winter food sources: aspen and willow. Elk distribution and foraging are limited by deep snow, and as winters become milder, the severity of browsing on aspen and willow may increase. Increased browsing might reduce the survival of willow and aspen, and it could have cascading effects on other parts of the ecosystem. As the only deciduous component of upland forests in most of the northern Rocky Mountains, aspen and willow provide important habitat for many birds, mammals, and insects.  

Our initial observations indicate that higher snow levels of the past may have allowed fewer elk to winter on Yellowstone's northern range, and consequently aspen and willow in the past would have experienced greater protection from browsing. Two lines of evidence support this idea. First, aspen recruitment (new growth) has declined from its peak in the late 1800s at the same time as snow depth has decreased. Snow depth is now an average of 61 cm lower than it was when aspen and willow were achieving maximum recruitment. Second, the average height of aspen suckers on the northern range today is positively correlated to snow depth. Our research will determine whether these two correlations are indeed related as cause and effect.  

Specifically, we are testing the following hypotheses:  

1. Elk forage less in areas with deep or dense snow, and as a result, aspen and willow in these areas suffer lower levels of browsing.  
2. Historically, recruitment of aspen has occurred in high snow years or in areas that receive more snow and are thus less accessible to elk. Recent aspen recruitment failures are due to recent decreases in winter severity.  

We are testing these hypotheses by measuring elk browse rates in aspen and willow stands that span a naturally-occurring gradient of snow depth. To control for any confounding effects of topography (e.g., steep slopes) that may influence elk movements independently of snow depth, we are taking the same measurements in sites that have snow depth artificially increased with fences. We will also determine the age of aspen and willow in different areas and test for a relationship between historical records of snow depth/density and aspen/willow recruitment rate.