Battle of the giants: Clonal expansion rates, effects on wetland plant communities and competition between introduced <i>Phragmites australis australis</i> and native <i>Phragmites australis americanus</i>

Abstract The success of non‐native species in novel environments has received worldwide attention, resulting in dozens of hypotheses trying to explain this demographic success. Unfortunately, most hypotheses are built on little empirical data, and evaluations often come to contradictory conclusions, even when exploring the same mechanisms or species in different contexts. The focus on species as the unit of inquiry may impede progress because intraspecific diversity (including variation among populations) can be as profound and of a similar magnitude as interspecific variation. We explored how origin, residence time and population may affect competitive ability, spread (i.e. invasiveness), and impact (on local plant communities) using non‐native Phragmites australis australis and native Phragmites australis americanus in long‐term experiments (8–12 years) in a common garden and at field sites in New York State, USA. Non‐native P. australis australis spread faster and, despite inter‐population variation, all populations were competitively superior to native P. australis americanus . Non‐native and native genotypes were taller and competitively superior to other herbaceous wetland plants, confirming well‐established generalizations about wetland plant assembly and traits (height, biomass) associated with competitive dominance. However, non‐native genotypes more strongly reduced wetland plant diversity. While traits associated with competitive ability and ecological dominance are well known, the mechanisms that allow non‐native genotypes to be more invasive and to outcompete both other wetland plants and native Phragmites genotypes remain unclear. Improved photosynthetic capacity and below‐ground resource capture are likely mechanisms. Synthesis . Non‐native P. australis australis is considered one of the most troublesome species in North American wetlands while native genotypes are declining. Our work confirms evidence from across North America indicating non‐native P. australis australis superior competitive ability and inter‐population variation. While native genotypes are competitively superior to other herbaceous wetland plant species, they are unable to persist once non‐native genotypes establish. Read the free Plain Language Summary for this article on the Journal blog.