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Forage selection by California bighorn sheep and the effects of grazing on an Artemisia-Agropyron community in southern British Columbia Wikeem, Brian Michael


This study was undertaken to determine the interrelations among forage production and utilization, forage quality, forage availability, diet selection, and the subsequent impact of grazing by captive California bighorn sheep under controlled experimental conditions on an Artemisia - Agropyron plant community in southern interior British Columbia. Fourteen grasses, 58 forbs plus 18 trees and shrubs were available for grazing by the captive herd within a 42 ha experimental enclosure. Agropyron spicatum, Bromus tectorum and Artemisia tridentata were the dominant species on the site equaling 22.1, 9.5 and 6.7% of the cover respectively. Both annual and seasonal variations in plant species phenology, cover and botanical composition were evident among phenological groups, forage classes and individual species. Cover of grasses (51.4%) and forbs (24.1%) was highest in 1978 which was the wettest year, and it was lowest in 1979 which was the driest year equaling 34.2 and 12.8% for each group respectively. Cover of shrubs, and both cover and botanical composition of Agropyron spicatum varied little among years. Yields for total standing crop varied from 40.83 to 62.95 g/m² in 1977 and 1978 on the ungrazed areas in response to annual weather patterns. Fall regrowth was prevalent in 1978 and 1979 equaling 8.18 and 19.42 g/m² or 11.5 and 27.0% of the total herbage produced in each year respectively. Maximum availability of nutrients occurred in March and April each year, but from September through to November fall regrowth re-established nutrient levels to those recorded in the early growth stages for all plant species sampled. A total of 79 species consisting of 14 grasses, 47 forbs and 18 shrubs were observed in the diet of the experimental herd from 1977 to 1979. Grasses, forbs and shrubs comprised 66.6, 18.9 and 14.5% of the diet respectively over the 28 month study period. Agropyron spicatum was generally the most common plant found in the bighorn diet in all seasons, but this plant species typically occurred more frequently on the range than in the diet. Other grasses such as Festuca scabrella, Festuca idahoensis, Koeleria cristata and Stipa comata were preferred. Forbs were grazed most in summer (26.5 and 36.2%) and spring (19.2 and 18.4%) in 1977 and 1978 respectively. Perennial forbs were generally preferred but annual forbs were selected against by the bighorn sheep in all seasons. Browse was utilized most in winter and spring each year averaging 18.7 and 17.6% of the diet in each season respectively over the two year period. The dominant shrub, Artemisia tridentata was not preferred but most other browse species were. No consistent regressions could be established between forage consumption and corresponding nutritive quality of selected forage species. Balsamorhiza sagittata and Agropyron spicatum were least affected from grazing despite their importance in mountain sheep diet. Indeed, only leaf lengths differed among grazed and un-grazed Balsamorhiza sagittata plants and no significant differences were observed in basal diameters or the number of culms produced on grazed and ungrazed Agropyron spicatum plants after three years of grazing. Reductions in nearly all measured parameters were observed on grazed Koeleria cristata, Poa sandbergii, Stipa comata, Castilleja thompsonii, Lupinus sericeus and Erio- gonum niveum plants compared to ungrazed plants. Utilization of Amelanchier alnifolia was determined with three methods in 1977 and five methods in 1978. It was concluded that weight methods do not evaluate the impact of mountain sheep browsing any better than methods based on stem lengths. The long term effects of grazing by California bighorn sheep was evaluated from 1976 to 1983. Total cover remained virtually the same on the grazed (71.1 and 85.0%) and ungrazed (70.3 and 86.0%) areas between these two years respectively. Annual, perennial and individual plant species all reacted differently to grazing by mountain sheep. Cover of perennial grasses increased only slightly on both the grazed and protected areas between 1976 and 1983 but these differences were not statistically significant. No differences in either cover or botanical composition were observed between the grazed and ungrazed areas for Agropyron spicatum, Koeleria cristata or Poa sandbergii. Cover and botanical composition of forbs remained the same in 1983 compared to 1976 on the grazed areas. On the ungrazed areas, both cover and botanical composition of forbs increased over the same time period. Botanical composition of shrubs increased slightly on both the grazed and' ungrazed areas over the seven year period but cover for this group remained the same. Both Eriogonum niveum and Eriogonum heracleoides declined significantly on the areas grazed by bighorn sheep. Declines in both species were attributed to grazing pressure by the captive herd.

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