Explaining hydrological niches: the decisive role of below-ground competition in two closely related

1. Evidence from plant-community structure suggests that niche segregation between plant species is widespread, but the mechanisms are still generally obscure. We used experimental mesocosms to investigate the role of above- and below-ground competition in defining the distinct niche distributions of two Senecio species that separate along a water-table gradient in meadow habitats. In a target-border design, Senecio target plants were surrounded by six fence-sitting plants of Phleum pratense and fully factorial, randomised treatments for above-ground and below-ground competition, water level and nitrogen were applied. 2. Below-ground competition was found to be the most influential factor for plant biomass and seed production, whereas above-ground competition had negligible effects. Judging from their per-formances under different combinations of water level and nitrogen fertilization, the Senecio species showed different types of niche differentiation. Senecio aquaticus showed a preference for water-logged over dry soils irrespective of the presence or absence of competition. Senecio jacobaea showed no preference for any hydrological condition, as long as below-ground interaction was prevented. In the presence of competing roots, it showed the expected preference for dry soils, especially under N-fertilized conditions