Reciprocal competitive effects of congeneric invaders, Trapa natans L. and Trapa bispinosa Roxb. var. iinumai Nakano, in established freshwater plant cultures.
Plant diversity is a known indicator of healthy ecosystems, therefore understanding interspecific relationships that structure plant communities is critical for invasive species management. A cryptic introduction of invasive macrophyte water chestnut (Trapa bispinosa var. iinumai) into the northeastern United States prompted investigation into potential competitive interactions between it and two common submersed species in the Chesapeake Bay watershed, Vallisneria americana and Hydrilla verticillata. Because the congeneric invader Trapa natans co-occurs in the same watershed with a documented history of negative environmental impacts, it was included for comparison. Reciprocal pairwise competitive effects between water chestnut and resident species were determined by additive reciprocal design, plants were grow in monoculture and biculture under controlled greenhouse conditions for one year. Competitive reciprocal effects between species were evident. Vallisneria americana produced 1.7 X more ramets in monoculture than in combination with Trapa spp. and tuber production was reduced by 37% in biculture, independent of Trapa competitor. Trapa bispinosa var. iinumai fruit production was reduced by 80% when grown in biculture with V. americana, and no fruits were produced when grown with H. verticillata. Biculture T. natans fruit production was impacted with less production observed for H. verticillata (54%) than V. americana (25%). Trapa bispinosa var. iinumai produced nearly 150% more biomass in monoculture than T. natans, suggesting increased impacts in species-poor or disturbed areas. Findings suggest established monocultures of V. americana and H. verticillata may be vulnerable to invasion by either species of Trapa, but long-term competitive effects of invasion may not preclude co-existence.