Elk, multiple factors, and persistence of willows in national parks

Managers are concerned over declines in willows (Salix spp.) and increasing elk (Cervus elaphus) populations in Rocky Mountain and Yellowstone nationa l parks. We compare and contrast elk population trends, elk herbivory rates on willows, willow growth rates, climates, and responses of willows to cli pping. Both parks had similar patterns of elk densities (11-16 elk/km(2)), elk herbivory rates on willows (26-28% of current annual growth [CAG]), elk increases, long-term climate trends toward aridity; and recent wetter and cooler trends. Growth conditions for willows were better in Rocky Mountain National Park. Willow CAG was 250% greater, willow shoots were 100% heavier , and shoots were 41% longer in Rocky Mountain National Park compared to Ye llowstone National Park (P < 0.05). When previously protected willows were clipped (100%) over 3 years in Yellowstone National Park there was 250% les s CAG, a 22% decline in height, and no increase in tannins or phenolics. Ta nnins and phenolics increased in Rocky Mountain National Park. Elk had a gr eater negative effect on willows in Yellowstone National Park (P = 0.02) du e to less suitable growth conditions for willow; i.e., precipitation was lo wer and there were fewer beaver ponds compared to Rocky Mountain National P ark. Abiotic and biotic factors combined with elk densities affect willow p hysiology, regeneration, and persistence.