THE SUMMER FORAGE QUALITY OF WILLOW COMMUNITIES AND ITS INFLUENCE ON MOOSE FORAGING ECOLOGY IN ROCKY MOUNTAIN NATIONAL PARK, COLORADO

Stumph, Bradley

Riparian willow communities in Rocky Mountain National Park are influenced by a steep elevation gradient and heavy utilization by ungulates. I hypothesized that species composition and summer forage quality of willow (Salix sp.) communities varies with elevation and that moose (Alces alces shirasi) would distribute themselves by sex and age class across the elevation gradient in accordance with their needs for forage quality and quantity. I measured species composition, annual biomass production, crude protein, fiber, tannin and macronutrient contents of selected willow communities. Where present, willows were collected at 3 different elevations during 5 sampling periods throughout the summers of 2003 and 2004. Forage quality of willow varied between years, species, elevation and sampling period. The highest quality forage was rarely found at low elevations. However, the superior chemical quality of higher elevation communities may be counterbalanced by their less desirable physical properties (reduced biomass yield and leader length) which could reduce ungulate intake rates on both daily and seasonal scales. Therefore, larger moose (adult males) are more likely to encounter intake constraints at higher elevations because of their higher absolute energy requirements. In this study, mid and high elevation sites were used predominantly by females. Therefore, moose with smaller body sizes (and thus smaller absolute requirements) probably exploit higher elevation willow communities more efficiently. However, elevation-specific intake rates are needed to determine at which elevation moose are assimilating the greatest amount of nutrients. Indirect evidence suggests that ungulates may be altering willow community composition both across the elevation gradient and within individual communities.