Heterosis is more important than propagule pressure for the establishment of invasive hybrid cattail (Typha x glauca) populations

Abstract A notable proportion of invasive plant taxa are interspecific hybrids, and their success can be influenced by both the frequency with which hybrids are formed (propagule pressure) and their ability to outcompete their parent species. A vast cattail hybrid zone in central Canada and the USA comprises T. latifolia, T. angustifolia, and their hybrid T. x glauca. The maternal parent is T. angustifolia, which in some regions is less common than T. latifolia or T. x glauca; whether this translates into low T. x glauca propagule pressure will depend partly on whether T. angustifolia produces a high proportion of hybrids. The success of hybrids also depends on seedling establishment, and although T. x glauca exhibits heterosis at later life stages, little is known about its competitive ability at the seedling stage. We tested whether propagule pressure and/or competitive ability can help to explain the successful establishment of invasive T. x glauca. We collected fruit from 14 maternal T. angustifolia plants across 12 sites in and around Peterborough, Ontario, Canada, and grew seedlings from each plant both singly (without competition) and in groups (with competition). We used genetic data to assign a subset of seedlings to taxon, and found that overall, most seedlings (78%) were T. angustifolia, suggesting relatively low propagule pressure for hybrids. However, significantly more T. angustifolia seedlings (86%) grew singly - and thus without competition - compared to those grown in a group, competitive environment (71%). Typha hybrids dominate wetlands across a substantial area including the Laurentian Great Lakes and Prairie Pothole regions, and our data suggest that strong competitive ability is more important than propagule pressure for the establishment of these successful invaders.