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

The influence of soil fertility on competitive interactions between dioecio us hydrilla [Hydrilla verticillata (L.f.) Royle] and American eelgrass (Val lisneria americana Michx) was investigated. Addition series experiments wer e conducted with mixed plantings of Hydrilla : Vallisneria grown at two lev els of soil fertility. Competitive abilities of each plant species were det ermined using the reciprocal-yield model of mean plant weight. In monocultu res, Hydrilla biomass averaged six times higher at high fertility as compar ed to biomass at low fertility whereas only a two-fold increase in biomass was obtained with Vallisneria grown in similar fertility treatments. In mix ed cultures at high fertility, Hydrilla was the stronger competitor relativ e to Vallisneria, with one Hydrilla plant being competitively equivalent to 7.2 Vallisneria plants in terms of their respective abilities to reduce Hy drilla biomass. Under nutrient limiting conditions, however, Hydrilla growt h and its competitive advantage over Vallisneria were both depressed, and V allisneria was the dominant species. These data indicated that differential response to increased nutrient inputs is one major determinant of competit ive success in mixtures of dioecious Hydrilla and Vallisneria. (C) 1999 Els evier Science B.V. All rights reserved.