Competition does not differ between mono and polycultures of two Persicaria species under varying nutrient levels

Abstract

Natural environments are subject to nutrient fluctuations that can influence interspecific interactions and shape species distribution. Emergent macrophytes of the genus Persicaria co-occur in shallow habitats in Neotropical freshwater ecosystems, and their distribution may be influenced by both nutrient availability and competition. In this study, we tested the hypothesis that Persicaria ferruginea (Wedd.) Soják exhibits higher growth than Persicaria acuminata (Kunth) M. Gómez, resulting in stronger interspecific versus intraspecific competition. We predicted that nutrient availability would not alter these patterns. We conducted an experiment in which stem fragments of each species were planted in monocultures and polycultures, and exposed to low and high nutrient concentrations in the sediment. After 60 days, we measured growth parameters such as shoot length, total biomass, relative growth rate (RGR), and root-to-shoot biomass ratio. Additionally, we calculated relative competition indices (RCI). Our results showed that shoot length varied between species and was significantly influenced by nutrient levels, while total biomass was affected solely by nutrients. RGR was influenced by the interaction of species and nutrient levels, with P. acuminata exhibiting greater growth than P. ferruginea under high nutrient conditions. The RCI values were near zero, indicating that the growth of both species did not differ between intra and interspecific competition, and it was not affected by nutrient addition. We conclude that neither differences in morphological traits (shoot length and biomass growth) between the species nor nutrient amendments changed the competitive relationship between both species.