The Role of Propagule Type, Resource Availability, and Seed Source in Phragmites Invasion in Chesapeake Bay Wetlands

Abstract

The relative importance of sexual vs. asexual propagules in wetland plant invasions is poorly understood, particularly for their importance in increasingly disturbed, resource-rich environments. Using the invasive wetland grass Phragmites australis as a model, we evaluated the resource needs of its seeds and rhizome fragments. In a greenhouse experiment where we manipulated nitrogen and propagule type, we found that Phragmites seedlings, but not rhizome fragments, responded positively to nutrient enrichment. In a second experiment, where we focused on seedling resource needs and manipulated light, nitrogen, and seed source, we found that seedlings were inhibited by heavy shade with no additional benefit of nutrient additions. Different seed sources (wetlands surrounded by different dominant land-use types, a proxy for disturbance) had variable leaf production and stem height (biomass did not vary). Understanding the mechanisms of Phragmites recruitment via seeds and rhizomes is critical for informing how a clonal species—now understood to disperse largely by seeds—can invade increasingly disturbed, resource-rich habitats. Our study suggests that Phragmites invasion by seed is being fueled, in part, by the positive effects of increased nutrients and light on Phragmites seedling performance and potentially by varying performance of seedlings derived from different watersheds.