Clonal integration increases relative competitive ability in an invasive aquatic plant

PREMISE OF THE STUDY: Physiological integration between connected ramets is well known to increase performance of clonal plant species. However, no direct evidence appears to exist that integration can increase the ability of clonal species to compete with other species within mixed communities. We tested this hypothesis using two floating, invasive, aquatic species in which fragmentation-and thus extent of integration-is likely to vary between habitats and times. METHODS: Individual ramets of Pistia stratiotes and Eichhornia crassipes were grown in monoculture or in mixture, and new stolons bearing new off spring were severed or left intact. After 6 wk, the numbers of off spring and second-generation (2 degrees) off spring produced by each original ramet, or parent, were counted; and the final dry mass of each parent, its stolons, its off spring, and its 2 degrees off spring were measured. KEY RESULTS: Fragmentation decreased the relative competitive ability of Pistia, but not that of Eichhornia. This was mainly because Pistia accumulated similar to 30% less dry mass of off spring when fragmented and grown with Eichhornia than in other treatments. Off spring of Pistia were smaller than those of Eichhornia in all treatments. CONCLUSIONS: Our results show that clonal integration can increase competitive ability in some clonal species. In this case, integration appeared to enable the small off spring of Pistia to compete more effectively with the large off spring of Eichhornia. Lower rates of fragmentation may select for production of more numerous, smaller vegetative off spring in clonal species.