Effects of ungulate herbivory on aspen, cottonwood, and willow development under forest fuels treatment regimes

Abstract

a b s t r a c tHerbivory 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.

Effects of Browsing by Captive Elk (Cervus canadensis) on a Midwestern Woody Plant Community

“…Our results are consistent with numerous other studies that have shown differences in composition but not diversity of woody plant communities when experiencing elk and other ungulate browse pressure (Ammer, 1996;Hobbs, 1996;Moser and Witmer, 2000;Peinetti et al, 2001;Rooney and Waller, 2003;Sankaran et al, 2008; but see Johnson and Cushman, 2007;Beschta and Ripple, 2008). Elk foraging is also known to reduce tree sapling and seedling density, biomass, and regeneration (McPherson, 1993;Scheiner et al, 1995;Baker et al, 1997;Zeigenfuss et al, 2002;Endress et al, 2012) and shrub biomass and regeneration (Scheiner et al, 1995;Singer and Renkin, 1995;Huffman and Moore, 2003;Johnson and Cushman, 2007). White-tailed deer appear to affect woody plant communities similarly (Rooney and Waller, 2003).…”

“…Western elk populations can significantly alter plant community composition and reduce biodiversity, including the loss of shrub species richness (Johnson and Cushman, 2007), reduction of tree and shrub mass and frequency (Schreiner et al, 1995;Riggs et al, 2000;Endress et al, 2012), and reduction in tree seedling numbers and sapling growth (Kuiters and Slim, 2002;Schoenecker et al, 2004). Woody browse is a major component of elk diet and is often negatively affected by elk foraging (reviewed in Toweill et al, 2002;Huffman and Moore, 2003;Johnson and Cushman, 2007).…”

Roles of maternal condition and predation in survival of juvenile Elk in Oregon

“…The positive influences of disturbance such as wildfire or logging on developing diverse and productive plant communities may be countered by chronic herbivory because these community types are often highly attractive foraging areas (Wisdom et al , Endress et al ). A classical perception is that reducing densities of herbivores may help bolster per capita nutrition, nutritional condition, and productivity (Fowler , McCullough ).…”

Trophic Cascades in a Multicausal World: Isle Royale and Yellowstone

“…2002, Beschta 2005. The conspicuous alleviation of one factor, elk herbivory, during the past few years may not be a strong basis for concluding that it is the primary cause of a sea change in aspen demography, as studies have further suggested that aspen do not always respond to experimental protection from herbivory (Endress et al 2012). To date, however, there is an absence of research capable of isolating the effect of elk herbivory from other factors that may influence aspen, including temperature (Brown et al 2006), moisture (Romme et al 1995), snow pack (Brodie et al 2012), bovid herbivory (Kimble et al 2011, and competition with conifers (Bartos 2001).…”