Herbivore effects on seeded alfalfa at four pinyon-juniper sites in central Utah

Abstract

Effects of rabbits (Lepus californicus, Sylvilagus spp.), mule deer (Odocoileus hemionus), and livestock on seeded alfalfa (Medicago sativa) were studied on 4 sites in central Utah. Sites were dominated by pinyon pine (Pinus edulis)-and Utah juniper (Juniperus osteosperma) and were doubled-chained and seeded with a mixture of grasses, forbs, and shrubs between 1959 and 1962. A 4-way exclosure was built on each site in 1962 which included the following treatments: (1) control (rabbits, deer, and livestock excluded), (2) rabbit access, (3) deer access, and (4) rabbit plus deer access. The fifth treatment (outside the exclosure) was accessible to rabbits, deer, and livestock. Alfalfa density and production were estimated at 1- to 5-year intervals between 1963 and 1986. Alfalfa growth form was measured in 1986. Stand densities declined from 0.5 to 8.5 plants/m2 to 0.5 to 2.5 plants/m2 during the 23-year sampling period. Reproduction by seed was not evident. Alfalfa production fluctuated greatly (4 kg/ha to 4,104 kg/ha) with precipitation and decreased with increased herbivore access. Treatment effects varied. Rabbits had a negative effect on alfalfa density at 2 sites, but no effect on alfalfa production. Deer use had inconsistent effects on alfalfa density, but reduced alfalfa production at 2 sites. The addition of livestock use reduced alfalfa density at 1 site, and alfalfa production at 3 sites. Grazing treatments had a marked effect on alfalfa growth form. Decreases in height and increases in basal cover were associated with increased herbivore access. Results of this study indicate that alfalfa can be an important and persistent component of seeding mixtures used on semiarid pinyon-juniper ranges.