Cumulative effects of wild ungulate and livestock herbivory on riparian willows

Abstract

We examined the effects of wild ungulates (deer and elk) and domestic sheep browsing on the growth, structure, and reproductive effort of two common willow species, Salix boothii and S. geyeriana, in a montane northeast Oregon riparian zone. With the use of exclosures, large herbivore effects on willows were studied in an area browsed by native mammals only and an adjacent area in which domestic sheep also lightly grazed during summer months. Growth variables were repeatedly measured on individual plants over a 5-year period to understand physiognomic and flowering responses of native willows to different levels of browsing pressure. At the beginning of the study, all willows were intensely browsed but were significantly taller in the area browsed only by native mammals than in the area also grazed by sheep (69 versus 51 cm, respectively). Willows inside exclosures responded with pronounced increases in height, crown area, and basal stem diameters while the stature of browsed plants outside exclosures stayed constant or declined. In the area browsed by both sheep and wild herbivores, the size of browsed plants remained at pre-treatment levels (<60 cm in height) for the duration of the study. There was no significant difference in growth rates of enclosed willows, indicating that current herbivory was the primary cause of growth retardation in the study area. Foliar area was strongly correlated with basal stem numbers for enclosed plants but much less so for browsed plants. Willows inside exclosures had more than twice as much foliar area per stem. Stem diameters were a positive function of crown area: stem-number ratios, suggesting lower photosynthetic potential was correlated with diminished radial growth among browsed plants. No flowering was observed until 2 years after exclusion when plants inside all exclosures and browsed willows in the wild ungulate area responded with a large pulse in flowering. Browsed plants in the sheep + wild ungulate area did not flower. The number of catkins produced per plant was significantly associated with willow height and plants <70 cm in height did not flower, thus suggesting a size threshold for reproduction in these species. Our results suggest that even relatively light levels of domestic livestock grazing, when coupled with intense wild ungulate browsing, can strongly affect plant structure and limit reproduction of riparian willows.

Riparian vegetation communities change rapidly following passive restoration at a northern Utah stream

“…Grazing retirement effectively allows annual plants to spread by seed and perennial herbaceous plants to expand vegetatively, but woody species may be more difficult to restore using passive restoration approaches alone. While some studies show rapid willow recovery following livestock grazing retirement (Booth et al, 2012), historic grazing has been shown to reduce sexual reproduction in willows (Brookshire et al, 2002), and there may be a reproductive lag in willows at Spawn Creek preventing new individuals from establishing. Willow growth and establishment can be constrained by low water tables and soil moisture availability, as well as native ungulate grazing (Bilyeu et al, 2008;Chambers et al, 2004;Pezeshki et al, 2007;Wolf et al, 2007).…”

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

“…Deciduous species include aspen (Populus tremuloides), cottonwood (Populus trichocarpa), and willow (Salix spp. ), all of which are strongly selected by cattle, elk, and mule deer in both upland forests and riparian communities (Beschta, 2005;Brookshire et al, 2002;Kay, 1997a,b;Ripple and Beschta, 2003;White et al, 1996). Strong grazing pressure by cattle, elk, and mule deer on aspen, cottonwood, and willow species, often combined with longterm fire suppression, is considered a major contributor to the widespread decline of these deciduous plant species and associated plant communities across millions of hectares in western North America (Bartos, 2001;Hann et al, 1997;Hessl and Graumlich, 2002).…”

Encroachment of upland Mediterranean plant species in riparian ecosystems of southern Portugal

“…Riparian ecosystems contain a high and unique number of plant species (Sabo et al 2005), adapted to disturbance (e.g., floods, drought) (Lyon and Gross 2005;Malanson 1993), in a restricted area of land (Lyon and Gross 2005;Malanson 1993). Riparian ecosystems also provide aquatic, water-land interface and terrestrial habitats for animal species, as well as drinking water for upland animals (Brookshire et al 2002;Hilty and Merenlender 2004;Iverson et al 2001;Machtans et al 1996;Matos et al 2008;Spackman and Hughes 1994;Virgós 2001;Williams et al 2003). Despite their high biological value, riparian ecosystems have seldom been included in systematic conservation planning (Nel et al 2009), and are becoming increasingly threatened by human activities (Salinas et al 2000) and upland plant encroachment (Huxman et al 2005), especially in the semi-arid Mediterranean region (Nel et al 2009).…”