Interactive effects of fragment size, nutrients, and interspecific competition on growth of the floating, clonal plant Salvinia natans

Groups of connected ramets produced by clonal growth in plants are particularly susceptible to fragmentation in aquatic ecosystems. Both clonal fragment size and nutrient availability are likely to affect the competitive ability of clonal aquatic plants, but the effects of fragment size, nutrients, and competition on plant performance have rarely been studied in combination. We hypothesized that larger fragments have greater competitive ability and that this effect is greater when nutrients are lower. To test this and other interactive effects, we grew fragments of the widespread, floating, clonal pteridophyte Salvinia natans initially containing 1, 2, 3, or 6 ramets at two levels of nutrients with and without a co-occurring, floating pteridophyte, Azolla pinnata, for 7 weeks. Floating mass of S. natans was mostly higher and submerged mass lower at high than at low nutrients, summing to a 0-30 % increase in total mass with higher nutrients at different initial fragment sizes. The presence of A. pinnata reduced total mass of S. natans by 30-60%. In partial agreement with the hypotheses, response of S. natans to competition was less negative in larger fragments in the low nutrient treatment but did not vary consistently with initial fragment size in the high nutrient treatment. Results show that growth of aquatic clonal plants can be determined by complex interactions and suggest that eutrophication may increase the competitive ability of very small clonal fragments.