Impact of competition on plant performances along a flooding gradient: a multi-species experiment

1. How do the competitive response and the importance of competition vary between species and along a flooding gradient? 2. How does the role of competition in constraining species distribution limits along the gradient vary between lower and upper limits? A 1-ha meadow within the Alzette floodplain in Luxembourg. Competitive response and importance of competition were assessed on seven meadow species differing in their tolerance to flooding. Species were cultured in monocultures and in mixtures, in three water treatments reflecting the wet, the middle and the dry end of a natural flooding gradient. We developed two models based on a multiple regression in order to express each component of competition as a function of the neighbour biomass. Five species showed variations in their competitive response across water treatments; however, these species achieved either their highest or their worst competitive response in their optimal water treatment (i.e. the treatment in which the species had the highest biomass in monoculture). Competition was more important for the flood-tolerant species in the dry treatment than for the flood-intolerant species in the wet treatment. 1. Variations in species competitive responses along flooding gradients may be the result of either an amplified effect between competition and hydrological stresses, or a hierarchical effect of stress over competition. 2. The role of competition is more important in constraining the upper distribution limits of the flood-tolerant species than the lower limits of the flood-intolerant species along flooding gradients.