Elimination of large predators and reduced hunter harvest have led to concerns that an increasing elk (Cervus elaphus) population may be adversely affecting vegetation on the low-elevation elk winter range of Rocky Mountain National Park, Colorado, USA. Beaver (Castor canadensis) and their impoundments also have declined dramatically (94%) in the same area over the past 50 years coincident with a 20% decline in willow (Salix spp.) cover. From 1994 to 1998, we studied vegetation production responses of willow communities to elk herbivory and water availability. We estimated willow production by measuring current annual growth of shrubs in 9.3-m2 circular plots, and we measured herbaceous production by clipping vegetation within 0.25-m2 circular plots. Elk herbivory suppressed willow heights, leader lengths, annual production, and herbaceous productivity of willow communities. Water impoundment had a positive effect on herbaceous plant production, but little effect on shrubs, possibly because water tables were naturally high on the study sites even without beaver dams. Nevertheless, the winter range environment previously included more riparian willow habitat because of more stream area (47-69%) due to larger beaver populations. Elk herbivory appears to be the dominant force determining vegetation productivity in willow sites, but the effects may be exacerbated by lowered water tables. Fewer elk or protection from browsing, and water enhancement for <10 years along with management to encourage elk movement away from willow communities, could possibly work as strategies to reestablish sustainable willow communities.
