Competition between two wetland macrophytes under different levels of sediment saturation

Plant-plant interactions have been widely studied under various environmental conditions. However, in wetland ecosystems how plant interactions change in response to variation in sediment saturation remains largely unclear, even though different levels of sediment saturation play important roles in determining plant growth performance in wetland ecosystems. To this end, a competition experiment with two typical wetland species, Carex brevicuspis (neighbor plant) and Polygonum hydropiper (target plant), was conducted in a target-neighbor design. Two water levels (0 cm and -40 cm water levels representing waterlogged and drained sediments, respectively) and three neighbor plant densities (0 plants m^-2, 400 plants m^-2, and 1600 plants m^-2) were tested in a factorial design. Biomass accumulation of P. hydropiper decreased along with enhanced C. brevicuspis density in the waterlogged treatment. However, in the drained treatment, biomass accumulation did not change under two C. brevicuspis densities. Above-ground relative neighbor effect index (ARNE) and relative neighbor effect index (RNE) of C. brevicuspis on P. hydropiper increased along with enhanced C. brevicuspis density only under waterlogged conditions. The below-ground relative neighbor effect index (BRNE) was not affected at the different water level and density treatments. The below-ground mass fraction of P. hydropiper was much higher in the waterlogged treatment than it was in the drained one, especially with no C. brevicuspis treatment. However, the leaf mass fraction displayed the opposite pattern. The longest root length of P. hydropiper was much shorter under waterlogged treatment than under the drained treatment. These results suggest that the competition intensity of C. brevicuspis to P. hydropiper increased along with increasing C. brevicuspis density only under waterlogged conditions. Moreover, this study also confirms that P. hydropiper can acclimate to water stress mainly through modulating its root architecture, for example reducing the root system length or increasing the root mass fraction.