Influence of intraspecific interaction and substrate type on initial growth and establishment of Hydrilla verticillata

Both substrate type and plant–plant interaction can greatly influence the growth and establishment of plants. In order to assist the re-vegetation of submerged macrophytes, the growth of Hydrilla verticillata with increasing equi-distance neighboring plant density on two substrate types (sediment and sand, representing high- and low-nutrient level, respectively) was assessed in monoculture stands. The results showed that substrate type greatly changed the biomass allocation patterns of the target plants, with a smaller root mass ratio on sediment compared to sand (0.70 vs. 3.11%). However, interaction between substrate type and neighboring density was observed. At low density, growth on sediment greatly increased plant height (43.90 vs. 22.10 cm), leaf biomass (216.63 vs. 68.41 mg), and total biomass (298.39 vs. 121.77 mg) when compared to growth on sand. However, at high density, no significant effect of the substrate type was found in those parameters. On sediment, high neighboring density greatly decreased the height, root number, total root length, root mass, and total biomass, implying large intraspecific plant–plant competition. However, such competition can be greatly reduced in infertile environments. Therefore, when the plants were grown on sand, neighboring density showed little effect on the height (22.10–26.53 cm), total root length (21.34–40.50 cm), and root biomass (3.14–6.27 mg). Total biomass and root number significantly increased by 50% and 115%, respectively, at high density compared to low density on sand, suggesting that facilitation rather than competition was occurring. Therefore, plant–plant interaction can vary from competition in fertile environments to facilitation in infertile environments. In summary, neighboring density should be manipulated according to the environmental nutrient level, so as to reduce intraspecific competition or increase intraspecific facilitation, and finally enhance the initial growth and establishment of H. verticillata in re-vegetation activities.