Coexistence of Typha latifolia, T. angustifolia (Typhaceae) and their invasive hybrid is not explained by niche partitioning across water depths

Spatial separation between hybridizing taxa can limit hybridization and introgression when mating occurs between near neighbours. Water depth can be an important factor structuring wetland plant communities, which often include cattails (Typha spp., Typhaceae). In eastern North America, native Typha latifolia regularly hybridizes with introduced T. angustifolia to produce the invasive F1 hybrid T. × glauca, which can backcross to parental species. Hybridization in this wind-pollinated species complex may be facilitated by overlapping niches among taxa, but previous studies have yielded contradictory results, possibly because these studies were limited to shallow sites or single locations. We investigated patterns of depth occupancy in ditches (mean depth 3.4 cm) and permanent wetlands (mean depth 25.0 cm) with various mixtures of T. latifolia, T. angustifolia, and T. × glauca across a 1700 km transect from Michigan, USA, to Nova Scotia, Canada. Overall, we found that water depth cannot on its own explain patterns of cattail occupancy. We therefore conclude that niche partitioning by water depth does not promote mating isolation in Typha. Previous findings of habitat segregation by depth may be attributable to a combination of idiosyncratic features of individual sites, multi-dimensional niches, and phenotypic plasticity, which could prevent consistent niche segregation among Typha taxa. A lack of spatial segregation between Typha taxa should promote hybridization and introgression. This, coupled with the competitive superiority of hybrids, appears to be facilitating the displacement of native T. latifolia across a wide geographical range.