Sheep grazing as a silvicultural tool to suppress brush

Abstract

The possibility of using livestock as a biological agent to control unwanted ground vegetation in Pacific Northwest coniferous forests has been discussed for over 50 years. However, little quantitative information has yet been published documenting the efficacy of livestock in suppressing brush and other ground vegetation in commercial Douglas-fir (Pseudotsuga menziesii) plantations. Therefore, a study was conducted in 1981 and 1982 to evaluate the potential for using herded sheep to control competing vegetation in Douglas-fir plantations in Pacific Northwest coniferous forests. Three 4- to 6-year-old plantations were grazed once each year during the May to September grazing season. Estimates of current year's growth present in October, both inside and outside a livestock exclosure on each study plantation, were used to evaluate the effects of grazing. In general, utilization of brush by sheep was moderate to heavy, except in the spring of 1982, when brush was lightly utilized. Sheep grazing effectively reduced (p<0.01) both total understory plant growth and brush net current year's growth on all plantations. Reduced brush biomass on grazed areas was associated with greater Douglas-fir diameter growth in 1981-82 and 1982-83. By 1985, trees in grazed areas were 5% taller (p<0.05) and 7% greater in diameter (p<0.01) compared to ungrazed controls. Our data and observations suggest that sheep may be effectively used as a biological control agent for brush control in coastal Douglas-fir forests.