Nutrients, arbuscular mycorrhizas and competition interact to influence seed production and germination success in Achillea millefolium

1. Environmental variables that positively affect one aspect of plant fitness, may have no effect, or even negatively affect some other component of fitness. 2. Using individuals of Achillea millefolium grown under field conditions in Michigan, USA, the hypothesis was tested that seed number was determined largely by plant biomass, while seed germination success depends on tissue nutrient concentrations. The impact was assessed of four biotic and abiotic factors on seed number and germination success: root competition, shoot competition, fertilizer, and removal of fungi by fungicide application. 3. Using a path analysis, it was found that total plant biomass positively affected both seed number and germination success, while inclusion of other variables did not greatly affect the amount of variation the model was able to explain. Fertilizer and fungicide increased, while root and shoot competition decreased both seed number and germination. 4. Fungicide applied to the maternal plant increased biomass but decreased tissue phosphorus concentrations. In species where germination responds to nutrient concentrations, the potential exists for opposing impacts of environmental treatments on different components of fitness. 5. This study suggests that environmental impacts on seed number will outweigh impacts on germination success under field conditions, and that biomass is an adequate surrogate for fitness in herbaceous plants.