A DENSITY-DEPENDENT MODEL OF CIRSIUM VULGARE POPULATION-DYNAMICS USING FIELD-ESTIMATED PARAMETER VALUES

Two versions of a stage-structured model of Cirsium vulgare population dynamics were developed. Both incorporated density dependence at one stage in the life cycle of the plant. In version 1 density dependence was assumed to operate during germination whilst in version 2 it was i ncluded at the seedling stage. Density-dependent parameter values for the model were estimated from annual census data in a factorial grazin g experiment. Version 1 of the model produced significant estimates of density dependence under field conditions. The estimated values, when included in a simulation of the dynamics, produced two-point limit cy cles under conditions of hard grazing. The limit cycles were most pron ounced at the early rosette stage. Comparison of the effects of densit y dependence at the two different stages in the life cycle revealed a strong difference in predicted dynamics. This emphasizes the importanc e of determining where density dependence operates under field conditi ons and the potential problems of arbitrarily assigning it to particul ar life-history stages. Version 1 of the model produced a good predict ion of observed mean plant density across the different grazing treatm ents (r(2)=0.81, P<0.001).