The effects of leaf litter and competition from hybrid cattails (Typha × glauca) on the seed germination and seedling performance of its parental species

Invasive species affect native vegetation through combinations of direct interactions (e.g. competition) and indirect interactions (e.g. altered pollination regimes) mediated through the occupation of shared environments. Hybrid cattails (Typha × glauca) are rapidly displacing their parental species, the native broadleaf cattail (T.latifolia) and the introduced narrow-leaf cattail (T. angustifolia), but the mechanisms involved are poorly understood. We evaluated the potential for direct and indirect interactions among cattails using a series of experiments. First, we evaluated whether seed germination was affected by hybrid leaf litter using a factorial experiment in which we tested for indirect effects of leaf-litter leachate and shading. Second we tested whether these indirect effects of hybrid leaf litter affected seedling performance using a factorial common-garden experiment. Finally, we tested for direct competitive interactions between hybrid and native cattail seedlings. One or more components of hybrid leaf-litter leachate strongly reduced the germination of the parental cattail species, but not that of the hybrid. There was no evidence that direct competitive interactions affected seedling performance. Because regular recruitment by seeds appears to be important in shaping patterns of genetic diversity in natural populations, our findings provide a mechanism for explaining the dominance of hybrid cattails in eastern North America.