GRAZING IN A CALIFORNIA SILVOPASTORAL SYSTEM - EFFECTS OF DEFOLIATIONSEASON, INTENSITY, AND FREQUENCY ON DEERBRUSH, CEANOTHUS-INTEGERRIMUSHOOK AND ARN

When understory species that suppress tree growth are preferred by liv estock to tree species, selective herbivory has practical application for forest or woodland management as part of a silvopastoral agrofores try system. Results of two studies of the prescriptive application of selective grazing designed to suppress growth of a common understory s hrub, deerbrush (Ceanothus integerrimus Hook. & Am.), to favor growth of conifer species are presented. Grazing for vegetation manipulation requires the same information needed to apply any chemical or mechanic al method: knowledge of the effective timing, frequency, and intensity of application, and the selectivity of impact. The first study, a two -year series of grazing trials, examined the degree of cattle preferen ce for deerbrush as compared to conifers, and response of deerbrush to grazing on a forest site. The second, a three-year study based on gra zing trial results, used clipping to examine the specifics of deerbrus h response to patterns of herbivory. Deerbrush was highly preferred by cattle in the grazing trials. Even at 90% utilization of the shrubs, no conifers were browsed. High degrees of utilization did not suppress shrub growth in the grazing trials. The clipping study found deerbrus h significantly responsive to frequency and intensity of defoliation ( P < 0.01), but not to season of use (P > 0.1). High intensity, frequen t grazing is required to suppress the shrub. Intermediate prescription s can be used to manage for various combinations of wildlife, timber, or forage-related objectives. These results and methods are applicable to any silvopastoral system where prescriptive grazing is used in con junction with tree crops.