Quality not Quantity: Seedlings of the Invasive Hybrid Cattail Typha x glauca Outcompete the more Abundant Seedlings of their Maternal Parent T. angustifolia

Numerous invasive plant taxa are hybrids that outcompete their parent species, thus exhibiting heterosis. Invasive hybrids can also have an advantage if the genotypes of one or both parent species are replaced by hybrid genotypes in a process known as genetic swamping, which can remove or reduce the parental competitors. A vast cattail hybrid zone in North America comprises T. latifolia, T. angustifolia, and their hybrid T. × glauca. Genetic swamping may be reducing the abundance of the maternal parent, T. angustifolia, across the hybrid zone, although competition could also be important for hybrid success. We tested two non-mutually exclusive hypotheses: 1) Genetic swamping means that T. angustifolia is producing a high proportion of hybrid offspring, and 2) T. × glauca seedlings outcompete T. angustifolia seedlings. We collected fruit from 14 maternal T. angustifolia plants across 12 sites in 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 most seedlings (78%) were T. angustifolia, which does not support our hypothesis of genetic swamping. However, a significantly higher proportion of seedlings were hybrids in the competitive (29%) versus the non-competitive (14%) environment, which supports our hypothesis that T. x glauca seedlings outcompete T. angustifolia seedlings. Typha hybrids dominate wetlands across a substantial area in North America, and our data suggest that strong competitive ability is more important than genetic swamping for the establishment of these successful invaders.