Competition along a nutrient gradient: A case study with Daucus carota and Chenopodium album

A binary competition experiment between carrot (Daucus carota L.) and Chenopodium album L. was conducted in a greenhouse at seven population densities and eight nutrient concentrations to investigate the effects of a nutrient gradient on plant competition in both monocultures and mixtures. The patterns of carrot biomass allocation (measured as root : shoot ratio) in monocultures and mixtures with C. album were affected by both nutrient availability and population density. Chenopodium album had a broader response to nutrient concentrations than carrot. The maximum yield of carrot in both monocultures and mixtures occurred at fourfold the standard concentration of nutrients, while C. album in both monocultures and mixtures had the maximum yield at 16-fold the standard nutrient concentration. The yield–density relationship of carrot tended to be increasing or asymptotic at lower nutrient concentrations but parabolic at higher concentrations, whereas that of C. album was little affected by nutrient availability. Nutrient availability had a profound influence on the competitive relationships between the two species: at both low and high nutrient concentrations, C. album tended to be more competitive than carrot, while at intermediate levels of nutrients, carrot was more competitive than C. album. Our results suggest that in relation to competitive performance, the weed has a greater ability to adapt itself to varying environments than does the crop. Additionally, the relative merits of the quantitative measures of competitive ability are briefly discussed.