Competition between Hydrilla verticillata and Vallisneria americana as influenced by soil fertility

The influence of soil fertility on competitive interactions between dioecious hydrilla {Hydrilla verticillata (L.f.) Royle] and American eelgrass {Vallisneria americana Michx) was investigated. Addition series experiments were conducted with mixed plantings of Hydrilla : Vallisneria grown at two levels of soil fertility. Competitive abilities of each plant species were determined using the reciprocal-yield model of mean plant weight. In monocultures, Hydrilla biomass averaged six times higher at high fertility as compared to biomass at low fertility, whereas only a two-fold increase in biomass was obtained with Vallisneria grown in similar fertility treatments. In mixed cultures at high fertility, Hydrilla was the stronger competitor relative to Vallisneria, with one Hydrilla plant being competitively equivalent to 7.2 Vallisneria plants in terms of their respective abilities to reduce Hydrilla biomass. Under nutrient limiting conditions, however, Hydrilla growth and its competitive advantage over Vallisneria were both depressed, and Vallisneria was the dominant species. These data indicated that differential response to increased nutrient inputs is one major determinant of competitive success in mixtures of dioecious Hydrilla and Vallisneria.