Effects of Jackrabbit Grazing, Clipping, and Drought on Crested Wheatgrass Seedlings

Abstract

The effects of black-tailed jackrabbits on crested wheatgrass (Agropyron desertorum) seedling establishment and utilization were monitored on a mesic and 2 xeric Wyoming big sagebrush (Artemisia trdentata spp. wyomingensis) sites in Nevada. Although jackrabbit densities (1.4 to 2.9/ha) and utilization were high, seedling survival was not significantly different (p?0.05) inside or outside rabbitproof exclosures even on the xeric sites during drierthan-normal springs. Seedling density was greater on the mesic than xeric sites, but good grass stands were produced on all sites. The smaller seeded areas (less than 60 ha) and the edge of the large seeded area (400 ha) had the greatest forage utilization by rabbits. To determine the effects of defoliation and drought on seedling survival, seedlings in small tubes (3.8 cm diameter and 20 cm deep) and in large tubes (10 cm diameter and 1 m deep) were watered and clipped at 7 different intervals. Seedlings grown in the smaller volume were more sensitive to clipping and drought than those grown in the larger soil volume. Seedlings watered less than weekly showed a trend toward higher survival when clipped every 3 or 4 weeks than when clipped more or less frequently. Clipping weekly and watering less than weekly greatly reduced root growth and seedling survival. Although infrequent grazing by rabbits could slightly increase seedling survival in a dry year, the additive effects of frequent grazing and drought could result in stand failure, especially on shallow or rocky soils with a small soil volume that limits water retention and root growth. Rangelands of western North America are inhabited by several species of hares of the genus Lepus, commonly known as jackrabbits. The black-tailed jackrabbit (Lepus californicus) is probably the most abundant and widely distributed of the jackrabbits and therefore, most generally responsible for damage to crops and forage plants. Jackrabbit populations are very cyclic and virtually every western state has gone through severe crop depredation during population peaks (McAdoo and Young 1980). Large populations of jackrabbits have been considered a result and symptom of overgrazed rangelands (Fichter 1952) and a threat to crested wheatgrass seeded for range improvement (Goodwin 1960). High consumption of range forage by jackrabbits has been well documented (Taylor et al. 1935, Currie and Goodwin 1966, Westoby and Wagner 1973). Although Westoby and Wagner (1973) suggested and Goodwin (1960) observed that jackrabbits affected crested wheatgrass seedling establishment, these effects have not been quantified. McGinnies (1973) showed that clipping crested wheatgrass seedlings to ground level once during the 5month period following germination can reduce seedling survival by 50%o. However, for many years and clipping dates, clipping caused little or no seedling mortality, while for other years and clipping dates, high mortality was observed. Cook and Stoddart (1953) suggested that crested wheatgrass clipped early in the season would not produce regrowth under arid conditions. Mohammad et al. (1982) found 40% defoliation increased production of crested wheatgrass grown at -0.5MPa water potential while 80% defoliation resulted Authors are range scientist, formerly with USDA/ ARS, Reno, Nev., now at School of Renewable Natural Resources, University of Arizona, Tucson 85721; range agronomist, formerly with University of Nevada, Reno, now at Department of Crop and Soil Science, University of New Mexico, Las Cruces; wildlife scientist, Renewable Resources Center, University of Nevada, Reno; and range scientist, USDA/ ARS, Reno, Nev. Manuscript accepted March 19, 1985. in 100% mortality of plants at -l.5MPa water potential. The purpose of this study was to determine the effects of black-tailed jackrabbits and clipping on crested wheatgrass seedling survival in relation to precipitation.