Mechanisms of <i>Phragmites australis</i> invasion: feedbacks among genetic diversity, nutrients, and sexual reproduction

Summary 1. A fundamental challenge to invasion ecology is to determine what factors cause an exotic species to spread rapidly long after the initial introduction. The increase of a resource (e.g. nitrogen) could trigger an expansion, but in other instances, species have overcome biological limitations (e.g. an Allee effect) like accumulating sufficient genetic diversity for successful reproduction. Understanding the ecological mechanisms governing plant invasions, such as nutrient enrichment or Allee effects, can be used to improve invasive plant management. 2. We used the invasive, introduced grass Phragmites australis as a model to evaluate the role of nutrient enrichment and Allee effects in invasion. Based on recent studies that demonstrated the importance of sexual reproduction in this plant’s spread, we chose to focus our efforts on reproductive output. We examined the effects of patch‐level genetic diversity on viable seed production across watersheds of the Chesapeake Bay, USA, with differing levels of anthropogenic development (a proxy for nutrient enrichment). In an outdoor mesocosm experiment, we treated Phragmites plants originating from forested and developed watersheds with elevated vs. ambient nutrients and cross vs. self‐pollination and determined the effects on viable seed, floret and inflorescence production. 3. The proportion of viable seeds produced at field sites varied widely and was not directly related to watershed development. Instead, seed viability was positively related to patch‐level genetic diversity, and patches in more developed watersheds had higher genetic diversity. Also, plants in larger patches produced a higher proportion of viable seeds. In the mesocosm experiment, seed viability was substantially higher for out‐crossed plants. Elevated nutrients resulted in greater floret and inflorescence production, particularly for plants originating from developed vs. forested watersheds. 4. These findings have important management implications: small populations should be controlled before they accumulate sufficient genetic variation for prolific viable seed production, and landscape‐scale nutrient management could further limit reproductive output. 5. Synthesis and applications. Our research shows how nutrient enrichment and a weak Allee effect can interact across multiple scales to impact invasion success and how understanding the ecological mechanisms governing plant invasions can be used to better inform invasive plant management.