Influence of Cattle Stocking Rate on the Structural Profile of Deer Hiding Cover

Abstract

Hiding cover available for California (Odocoileus hemionus californicus) and Rocky Mountain (0. h. hemionus) mule deer was monitored during summer under no, moderate, and heavy cattle stocking rates in quaking aspen (Populus tremuloides) and meadow-riparian habitats in the central Sierra Nevada, California. Use of willow (Salix spp.) and herbaceous vegetation in meadow-riparian habitat was also measured using exclosure plots. Hiding cover in aspen and corn lily (Veratrum californicum) vegetation types was not reduced through mid-season in ungrazed treatments but was significantly (P < 0.05) reduced under moderate and heavy grazing. Increases in cover of aspen understory were detected after 2 years of cattle exclusion. Willow vegetation was resilient to the impacts of cattle under moderate grazing, but hiding cover was significantly (P < 0.05) reduced with heavy stocking rates. Browsing of willows by deer was light in ungrazed treatments but increased as the season progressed in cattle-grazed areas and as stocking rate increased. Natural weathering was partly responsible for overall hiding cover lost during the summer but reductions prior to mid-summer were attributed to cattle. The high proportion of hiding cover lost early in the season coincided with the 1st 2 months of life for fawns. J. WILDL. MANAGE. 51(3):655-664 The structure of vegetation provides important thermal and hiding cover for California and Rocky Mountain mule deer in the Sierra Nevada Mountains, California. An overstory canopy assists deer in minimizing energy expenditure for thermoregulation by creating a microclimate that buffers extreme weather conditions (Leckenby 1977, Peek et al. 1982). Understory vegetation provides relief from the weather for ungulate species that inhabit dense shrub or forest environments but is more important in providing deer with valuable hiding cover to escape danger (Taber 1961). Mule deer are regarded as classic hiders (Geist 1981). Most susceptible to predation are neonatal animals, which have not fully developed the coordination or speed for flight and are therefore dependent upon cover for escape. High rates of predation on deer fawns in the 1st 2 months of life have been reported in several studies (Cook et al. 1971, Salwasser 1974, Trainer 1975). Bowyer and Bleich (1984) suggested that lack of suitable cover due to cattle grazing may contribute to reduced fawn survival because of increased susceptibility to predation. Similarly, Smith and Lecount (1979) found that survival of fawns was influenced by vegetation because increased cover reduced predation. Pronghorn (Antilocapra americana) fawn survival has also been found to be higher on sites with more cover (Barrett 1981). Hamlin et al. (1984) concluded that poor hiding cover did not appear to be a factor in fawn mortality because fawns shifted habitat use to maintain adequate cover. Riley and Dood (1984) also reported that mule deer fawns selected habitats with dense vegetative cover. Hiding cover contributed significantly to habitat use by black-tailed deer (0. h. columbianus) in northern California (Loft and Menke 1984), and may be required even in the absence of predation risk if full use of a habitat is to occur (Black et al. 1976). The long history of livestock grazing in the western states has altered vegetation structure and species composition considerably from pristine conditions (Wagner 1978). Cattle are credited as having both detrimental (Mackie 1978) and beneficial (Urness 1976, Longhurst et al. 1982) effects on deer through their impact on plant communities. The overstory canopy is relatively free from the effects of cattle on a shortterm basis. Over several years though, regeneration and survival of young trees and vigor of mature trees can be adversely affected by browsing, trampling, soil compaction, trunk damage, and nutrient accumulation. However, understory vegetation, which provides the majority of wildlife hiding cover, can be affected by livestock grazing and trampling on a seasonal basis every year. On summer ranges, cattle and