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 c ombination of two intensities of sheep grazing in each of three season s - winter (grazed or ungrazed), spring (grazed or ungrazed), and summ er (light or heavy grazing) giving eight treatments in two blocks 2 Fo r 6 years from 1987 to 1992 the population sizes of C. vulgare were mo nitored in each of the 16 paddocks. After 1989 grazing in spring or wi nter or increased grazing in summer all increased population sizes. Po pulation sizes fluctuated widely between years. 3 The effects of the g razing 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 s eed survival in the seed bank. 4 More-intense summer or winter grazing increased seedling emergence by increasing the proportion of microsit es with no canopy or with no litter. Emergence was found to respond po sitively to these factors. Seedling survival was increased by winter o r spring grazing and winter grazing increased the year-to-year surviva l of small and medium-sized rosettes. These effects probably occurred through selective grazing decreasing competition from the dominant gra sses. Larger rosettes had lower mortality rates which were unaffected by the grazing treatments. 5 There was a minimum rosette size threshol d 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 th us 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 de creased the flowerhead number of flowering rosettes. 6 The lepidoptera n seed predator Eucosma cana attacked a larger proportion of heads on plants with more flowerheads. By decreasing the average number of flow erheads 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 summ er grazing all significantly increased the population growth rate lamb da. The population sizes of each paddock correlated significantly with the estimated lambda values. 8 Elasticity analysis was performed on e ach matrix and elasticity values within each matrix were summed to giv e a value for fecundity (e(F)), Stasis (e(L)) and growth (e(G)). Trian gular 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 c hanges in stasis as grazing intensity increased.