Grazing in a California silvopastoral system: effects of defoliation season, intensity, and frequency on deerbrush, Ceanothus integerrimus Hook. & Arn.

Abstract

When understory species that suppress tree growth are preferred by livestock to tree species, selective herbivory has practical application for forest or woodland management as part of a silvopastoral agroforestry system. Results of two studies of the prescriptive application of selective grazing designed to suppress growth of a common understory shrub, 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 mechanical 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 preference for deerbrush as compared to conifers, and response of deerbrush to grazing on a forest site. The second, a three-year study based on grazing trial results, used clipping to examine the specifics of deerbrush 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 deerbrush significantly responsive to frequency and intensity of defoliation (P < 0.01), but not to season of use (P > 0.1). High intensity, frequent grazing is required to suppress the shrub. Intermediate prescriptions 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 conjunction with tree crops.

Influence of Cattle Stocking Rate on Browsing of Norway Spruce in Subalpine Wood Pastures

“…On average, 4% of the sample trees were browsed by cattle. This falls below the threshold level of 12% browsed apical shoots given by Eiberle and Nigg (1987), and goes in line with the results of Liss (1988) and Huntsinger (1996), who also found low browsing rates by cattle on wood pastures. In our study, only one sample tree was browsed on both, lateral and apical shoots.…”

“…Cattle, which stay on subalpine wood pastures only during part of the growing season, are known to prefer herbaceous species compared to tree shoots, and thus are classified as grazers rather than as browsers (Hofmann 1989). Liss (1988) and Huntsinger (1996) found that cattle avoided browsing shoots of Norway spruce (Picea abies (L.) Karst. ), though Haeggstro¨m (1990) observed that animals begin consuming less palatable plants, such as spruces, when grazing intensity is high.…”

“…Little is known about the influence of the present extensive cattle grazing on the regeneration of young trees in subalpine forests. Liss (1988) and Huntsinger (1996) emphasised that tree browsing by wild ungulates in winter might exceed tree browsing by cattle. Wild ungulates are known to cause severe browsing damage to young trees in winter (Motta 1996).…”

Experimental determination of the effects of cattle stocking density and grazing period on forest regeneration on a subalpine wood pasture

“…Since the heifers spent their first summer on a subalpine pasture and there were no adult cattle present to be imitated, the heifers had to find out by themselves which plant species were palatable. Experimenting as the assumed major reason for spruce browsing is confirmed by the known aversion of most cattle against using spruce shoots as forage [22]. Experimenting might also have been the reason for lethally damaging some T2 spruces by rooting out.…”

Sciences, knowledges, and the practice of forestry

“…This study used local harvesters' selection criteria (stem length and leaf condition) to define treatments. This was in addition to measuring the percentage of biomass removed from total plant biomass as is typical, and is therefore fundamentally different from many other shrub defoliation studies (Huntsinger 1996;Tappeiner et al 2001).…”