DEMOGRAPHY OF CIRSIUM-VULGARE IN A GRAZING EXPERIMENT

1. The complete demographies of 16 populations of Cirsium vulgare were followed in a replicated experiment. The experiment was a factorial combination of two intensities of sheep grazing in each of three seasons - winter (grazed or ungrazed), spring (grazed or ungrazed), and summer (light or heavy grazing) giving eight treatments in two blocks 2 For 6 years from 1987 to 1992 the population sizes of C. vulgare were monitored in each of the 16 paddocks. After 1989 grazing in spring or winter or increased grazing in summer all increased population sizes. Population sizes fluctuated widely between years. 3 The effects of the grazing treatments and plant sizes on the transitions between nine life-history stages were determined. There were no grazing effects on seed numbers per flowerhead, postdispersal seed survival or between year seed survival in the seed bank. 4 More-intense summer or winter grazing increased seedling emergence by increasing the proportion of microsites with no canopy or with no litter. Emergence was found to respond positively to these factors. Seedling survival was increased by winter or spring grazing and winter grazing increased the year-to-year survival of small and medium-sized rosettes. These effects probably occurred through selective grazing decreasing competition from the dominant grasses. Larger rosettes had lower mortality rates which were unaffected by the grazing treatments. 5 There was a minimum rosette size threshold for flowering above which flowering probability increased with size. Because winter grazing increased the survival of smaller rosettes it increased the proportion of smaller rosettes in the populations and thus decreased the proportion of rosettes flowering. By the same process winter grazing decreased the average size of flowering rosettes and, because smaller rosettes produced fewer flowerheads, this treatment decreased the flowerhead number of flowering rosettes. 6 The lepidopteran seed predator Eucosma cana attacked a larger proportion of heads on plants with more flowerheads. By decreasing the average number of flowerheads per flowering plant winter grazing decreased the proportion of flowerheads on a plant attacked. 7 Matrix analysis of life tables for each paddock showed that winter and spring grazing and increased summer grazing all significantly increased the population growth rate lambda. The population sizes of each paddock correlated significantly with the estimated lambda values. 8 Elasticity analysis was performed on each matrix and elasticity values within each matrix were summed to give a value for fecundity (e(F)), Stasis (e(L)) and growth (e(G)). Triangular ordination of e(F), e(L) and e(G) showed that lambda became more sensitive to small changes in fecundity and less sensitive to small changes in stasis as grazing intensity increased.