EFFECTS OF WILLOW QUALITY ON MOOSE DISTRIBUTION IN A MONTANE ENVIRONMENT

Abstract

Plant communities in Rocky Mountain National Park are influenced by a steep elevation gradient. Therefore, we hypothesized that summer forage quality of willow (Salix sp.) communities varies with elevation. Moose (Alces alces shirasi) in Colorado rely heavily on willow for summer browse, and browse patches differ in size as well as productivity in mountainous environments. We also hypothesized that moose distribution within the park is influenced by willow forage quality and that moose would distribute themselves by sex and age class across the elevation gradient in accordance with their need for forage quality and quantity. We measured crude protein and fiber content for 3 of the most abundant willow species. Where present, willows were collected at 3 different elevations during 5 sampling periods throughout the summers of 2003 and 2004. Because elevation influenced both chemical and physical properties of willow, we found its relationship with browse quality to be more complex than that reported for herbaceous forage. Whereas the relationship is often linear with herbaceous forage, we hypothesize that an apex in browse quality may be reached between the extremes of an elevation gradient. Although higher elevation willow communities often offered chemically superior forage (e.g., more nutritious), it was also physically inferior (e.g., lower achievable intake rates). Mid and high elevation sites were used predominantly by females. Therefore, moose with smaller body sizes (and thus smaller absolute energy requirements) probably exploit the most nutririous willow communities more efficiently, while larger moose (adult males) likely encounter intake constraints at higher elevations because of their higher absolute energy requirements. However, elevation-specific intake rates are needed to determine at which elevation moose are assimilating the greatest amount of nutrients.