Effects of physical space and nutrients on the growth and intraspecific competition of a floating fern

Abstract

Physical space, defined by its volume and shape, is considered a resource for plant growth, as a plant can be limited by physical space even when other resources (e.g., light, water and nutrients) are unlimited. However, the effect of physical space limitation on intraspecific competition of plants, especially floating plants, is not well understood. Here we tested the hypothesis that physical space affects the growth and intraspecific competition of floating plants, which is further influenced by the volume and surface area of the containers in which these plants are grown. We grew either one or four clonal fragments of a floating clonal fern, Azolla imbricata, in cylindrical containers differing in diameter and height (and thus surface area and volume) and filled with solutions containing the same or different nutrient concentrations. Biomass and number of clonal fragments of A. imbricata were higher in the container with the larger diameter and thus water surface area, but were not significantly affected by the height/volume of the container. Biomass and number of clonal fragments were reduced by intraspecific competition and tended to increase first and then decreased with increasing nutrient concentration. Increasing nutrient concentration inhibited the growth and then reduced intraspecific competition of A. imbricata, but the diameter or height/volume of the container had no effect. Our findings suggest that nutrient levels can alter intraspecific competition of plants, but physical space may not.