Effects of Ungulate Herbivory on Aspen, Cottonwood, and Willow Development Under Forest Fuels Treatment Regimes

Herbivory by domestic and wild ungulates can dramatically affect vegetation structure, composition and dynamics in nearly every terrestrial ecosystem of the world. These effects are of particular concern in forests of western North America, where intensive herbivory by native and domestic ungulates has the potential to substantially reduce or eliminate deciduous, highly palatable species of aspen (Populus tremuloides), cottonwood (Populus trichocarpa), and willow (Salix spp.). In turn, differential herbivory pressure may favor greater establishment of unpalatable conifers that serve as ladder fuels for stand-replacing fires. The resulting high fuel loads often require silvicultural fuels reductions to mitigate fire risk, which in turn may facilitate additional recruitment of deciduous species but also additional herbivory pressure. Potential interactions of ungulate herbivory with episodic disturbances of silviculture, fire, and other land uses are not well documented, but are thought to operate synergistically to affect forest dynamics. We evaluated individual and joint effects of ungulate herbivory and fuels reduction treatments in grand fir (Abies grandis) and Douglas-fir (Psuedotsuga menziezii) forests that dominate large areas of interior western North America. We applied fuels reduction treatments of mechanical thinning and prescribed fire and then evaluated the responses of aspen, cottonwood, and willow species to these treatments (N = 3) versus areas of no treatment (N = 3), and to exclusion from ungulate herbivory versus areas subjected to extant herbivory by free-ranging cattle (Bos taurus), elk (Cervus elaphus), and mule deer (Odocoileus hemionus). Densities of deciduous species were >4 times higher in response to fuels reduction treatments (84.4 individuals/ha) compared to areas of no treatment (19.7 individuals/ha). Additionally, when ungulates were excluded from fuels treated sites, the density of cottonwood was >5 times higher (122.5 individuals/ha) than fuels treated sites subjected to extant herbivory (24.3 individuals/ha). Similarly, densities of Populus spp. and Salix spp. were >3 times higher (211.6 individuals/ha) on fuels treated sites excluded from ungulate herbivory versus fuels treated sites subjected to extant herbivory (66.1 stems/ha). Deciduous species subjected to extant ungulate herbivory also were significantly lower in height, canopy surface area, and canopy volume than the same species inside the ungulate exclosures. Recruitment and long-term survival of aspen, cottonwood, and willow species in coniferous forests of interior western North America require a combination of episodic disturbances such as silviculture and fire to facilitate deciduous plant recruitment, followed by reductions in grazing pressure by domestic and wild ungulates during the time intervals between episodic disturbances to facilitate plant establishment, growth and survival.