Sediment type, population density and their combined effect greatly charge the short-time growth of two common submerged macrophytes

Most wetland re-vegetation projects involve seedling transplantation. Sediment type and population density are of primary importance because they determine the fate of those seedlings. In order to ensure successful re-vegetation, an investigation was undertaken into the effect of three sediment types (sand, clay and their 50:50, v/v mixture), three densities (3, 9, 15 initial plants per tray) and their interaction on the growth of two common submerged macrophytes, Myriophyllum spicatum and Potamogeton crispus. Clay sediment resulted in a faster plant growth, including height, stem diameter and individual biomass than sand sediment for both species at low density. On clay, higher density induced faster growth in height for both species at first, and then slower growth at a later stage. In terms of root growth, M. spicatum and P. crispus had different responses to sediment type. On sand, M. spicatum allocated more biomass to roots while P. crispus allocated less, which resulted in a different root: leaf mass ratio. Density results showed different regulation patterns on individual biomass and shoot number, which were important in population establishment, on different sediment types for both species. On sand, medium density had the smallest individual biomass, while on clay, biomass increased with the decrease of density. On sand, little variation of shooter number showed among densities, while on clay, low density increased shoot number. All these results suggested that density had a feedback regulation on the effect of environmental resources, and combined consideration of these two factors is absolutely necessary before seedling transplantation.