Ungulate herbivory of willows on Yellowstone's northern winter range

摘要: Effects of unmanaged populations large mammalian herbivores, especially elk (Cervus elaphus on vegetation is a concern in Yellowstone National Park, since wolves (Canis Lupus) are extirpated, ungulate migrations altered by human activities and the disruption natural process possible. Stands low, hedged (height-suppressed) willows (Salix spp.) observed throughout greater Park area where high densities wintering or moose (Alces alces) exist. The height 47% willow stands surveyed Yellowstone's northern winter range has been suppressed. Mean leader use all heights was (P < 0.05 1987-88, increased to 60% 1988-89, following drought fires 1988, then declined 44% 1989-90 1990-96. Height-suppressed (43 +/- 2 cm, mean SE) were about one-half as tall (83 4 cm). Percent twig suppressed summer (25%) (59%) significantly more than for intermediate 0.05). Suppressed produced one-fourth aboveground annual biomass compared taller willows; even after 27 31 years protection, previously-suppressed only one-third willows, suggesting grow sites with lower growth potential. Growth conditions may have due warmer drier climate this century, locally reduced water tables--because decline beaver (Castor canadenis), fire suppression be responsible changes. Tall intermediate-height contained higher concentrations nitrogen they exhibited stress height-suppressed same species. More xeric century last during decades 1920's, 1930's, 1980's, explain low rates chemical defenses against herbivory willows. We propose locally-reduced tables likely contributed declines range, but that proximate factor native ungulates.