Competitive ability of Rumex obtusifolius against native grassland species: above- and belowground allocation of biomass and nutrients

Understanding the competitive ability of Rumex obtusifolius against grassland species is essential for developing efficient control strategies against docks. Here we present results from a greenhouse experiment where we tested shoot and root competition between Rumex regenerating from rhizome fragments and grassland species (grasses, herbaceous and leguminous species). Competitive ability was expressed as the relative above- and belowground biomass partitioning and, concentrations and allocation of carbon and nitrogen of the plant species studied. Regeneration of Rumex was significantly affected by competition through neighbouring native grassland species. Rumex responded to concurrent shoot and root competition of grassland species with 50% more investment in root biomass compared to shoot competition through those species alone. Aboveground biomass allocation of Rumex was unaffected by competition. Grassland species on the other hand responded to concurrent shoot and root competition through Rumex with 20% more shoot biomass compared to no competition treatment. Belowground biomass allocation of grassland species was unaffected by competition through Rumex. Rumex roots also showed a significantly higher C:N ratio when concurrent shoot and root competition through grassland species occurred compared to only shoot or no competition through grassland species. C:N ratios of Rumex shoots and of grassland shoots and roots remained unaffected by competition. Despite changes in biomass allocation of Rumex into the root system, this did not translate into changes in the amount of carbon and nitrogen stored in the biomass. However, grassland species allocated significantly more carbon into aboveground organs when only shoot competition and significantly more nitrogen when both shoot and root competition through Rumex occurred. Our data indicate that the competitive ability of Rumex obtusifolius regenerating from rhizome fragments could be significantly altered by management strategies focussing on improved grassland species performance.