Fragment growth performance of the invasive submerged macrophyte Myriophyllum spicatum under conditions of different water depths and sediment types

In wetlands, many invasive plants, especially submerged macrophytes, are propagated by vegetative fragments. Fragment colonization and growth, the first step of plant invasion, strongly depend on the species and environment. In this study, the influences of water depth and sediment type on fragment growth performance of Myriophyllum spicatum were evaluated in an outdoor experiment. The experiment consisted of two sediment types (mud and sand) and three water depths (20, 40, and 60 cm), with two fragment sizes (6 and 12 cm), in a factorial design. The relative growth rate (RGR), relative elongation rate (RER), shoot diameter, branching number, and total N and total P contents of plants derived from fragments of two different sizes were investigated. We hypothesized that the larger fragment size, lower water depth, and higher sediment nutrient would aid the growth of M. spicatum fragments. The RGR of M. spicatum was considerably higher for plants growing in mud than in sand. However, water depth and fragment size did not significantly influence RGR. The RER was considerably higher in plants growing in mud than in sand, and it was significantly lower when larger fragments were used. The influence of water depth on RER was found to depend on sediment type. Branching number was only affected by sediment type, and it was higher in the mud treatment than in the sand treatment. Shoot diameter was significantly larger in plants derived from the larger fragment size and grown in mud. Total N content was significantly lower in the higher water depth and was markedly higher in plants grown in mud compared to sand; total P content only decreased in the higher water depth. The results indicate that fragment size, water depth, and sediment type affect the growth performance of M. spicatum in different ways, providing insights into the invasiveness of M. spicatum under various environmental conditions.