Dynamics of competition in populations of carrot (Daucus carota)

Populations of carrot (Daucus carota) were raised over a wide range of densities (79-5763 plants m(-2)) to examine the dynamics of competition in terms of yield-density relationships and size variability, and to investigate the effects of nutrient supply on competition. While the relationship between shoot yield and density was asymptotic, the relationship between root and total yield and density tended to be parabolic. For a given time and density series the relationship between yield per unit area and density could best be described by the model: where y is the yield per unit area, D is density, w(m), a and b are fitted parameters. The parameters w(m) and a increased over time but nutrient availability affected only w(m). An extension of the basic yield-density model is proposed to describe the dynamics of the yield-density relationship over time: y=kD/[1 + c exp(-rt)]{1 + alpha[k/(1 + c exp(-rt))](beta) D}(b) in which t is time, k, c, r, alpha and beta are fitted parameters, and the other parameters are as previously defined. Size variability of individuals, measured by the coefficient of variation (CV), was influenced by both density and time after sowing. The general relationship between CV and the logarithm of mean weight per plant, after first harvest, could be described by a linear regression model, with a slope of approximately -0.2. A theoretical justification for a monotonically declining relationship between CV and mean plant weight is proposed. The magnitude of size variability was ranked in the order: root weight > total weight > shoot weight. The differences in the magnitude of size variability in yield components were due in part to allometric growth of plant parts. Nutrient availability, however, had no influence on size variability. (C) 1996 Annals of Botany Company