Above- and belowground competition between two submersed macrophytes

Above- and belowground competition between two submersed plants with similar growth forms, Hydrilla verticillata and Myriophyllum spicatum, was studied in a controlled experiment. Plants were grown with and without above- and belowground partitions in monocultures and mixtures. Biomass accumulation and partitioning were significantly affected by competition in relation to species identity, with H. verticillata accumulating more biomass than M. spicatum. The root-to-shoot ratio of M. spicatum significantly increased in response to inter-specific competition, whereas that of H. verticillata increased with intra-specific competition. Hydrilla verticillata had a competitive advantage with regard to light by shading and simultaneously reducing root growth of M. spicatum. These results suggest that despite their similar ecology, H. verticillata may outcompete M. spicatum due to interaction between above- and belowground competition, whereas aboveground partitions can not stop submerged macrophytes competing for waterborne variables such as bicarbonate ions. We concluded that competitive interactions between neighboring plants depend on species-specific biomass allocation strategies. However, it is difficult to transfer physical partitioning techniques previously used in terrestrial plant studies to aquatic systems, suggesting that the mechanism of competition between aquatic plants may be more complicated.