Responses of two macrophytes of the genus Polygonum to water level fluctuations and interspecific competition.
We assessed the zonation of two emergent macrophytes, Polygonum ferrugineum and Polygonum acuminatum, and experimentally tested the effects of water level fluctuations and competition on the growth of these two species. We modelled the abundance of both species as a function of depth using a Bayesian approach based on a 16-year dataset from a subtropical floodplain. Then, we designed a factorial experiment in which species were planted in monocultures and mixtures and subjected to three water level treatments (low, high and variable) for 30 days. All response variables and relative yields were analysed with a two-way ANOVA. The modelled data showed that the two species colonize different depths, with P. acuminatum colonizing shallower areas than P. ferrugineum. Biomass accumulation did not differ between species in monocultures, but P. acuminatum showed significantly higher shoot and internode lengths when it grew submerged, indicating different survival strategies between the two species. P. acuminatum resisted inundation through an escape strategy, elongating above the water surface, while P. ferrugineum did not show this strategy. In mixtures, P. acuminatum showed a significantly higher competitive ability than P. ferrugineum, indicating that the former species has advantages over the latter during the colonization phase. These results are consistent with our observational results because they show that P. acuminatum can dominate in shallower areas by having morphological adaptations to intermittent fluctuations in water level and a competitive advantage over P. ferrugineum.