Impacts of ungulate herbivores on a rare willow at the southern edge of its range

Abstract

In northeastern Arizona, USA, ungulate herbivory was identified as a potential threat to the continued existence of Arizona willow, Salix arizonica. In a series of multi-year experiments, I examined the impact of domestic and wild ungulates on growth and reproduction of the protected species in a natural and artificial habitat. Both wild and domestic ungulates significantly reduced above-ground biomass, height, survival, and sexual reproduction. The degree of impact of wild vs. domestic ungulates was related to the amount of time plants were exposed to herbivory, the amount of herbivore-free recovery time, and the numbers and kinds of herbivores present. Because experimental plants did not fully compensate or replace the amount of tissue lost or the reproductive capacity lost to herbivory within 1 year, I predict S. arizonica will require years to recover fully from herbivory. Herbivore impacts appear to be more pronounced at the southern edge of the species’ range than in more northern sites, and are probably exacerbated by the historically heavy domestic grazing and enhancement of wild ungulate populations. These studies support ongoing conservation actions by land managers to protect S. arizonica from ungulate herbivores: (1) fencing, (2) augmenting natural populations, and (3) reducing wild ungulate herds and domestic cattle grazing in the species’ habitat.