The introduced haplotype of Phragmites australis is an aggressive invader and has a negative effect on many aspects of wetland ecosystems in North America. In Chesapeake Bay subestuaries it was more abundant and had higher foliar N in wetlands with developed vs. forested watersheds. Little is known, however, about factors responsible for its spread and success. We addressed the following questions: Are within- and among-patch genetic variation an indicator of spread by seeds vs. rhizomes and do they vary by the level of watershed development? Does seed viability vary across wetlands in different watersheds? Is there a relationship between viable seed production and Phragmites seedling emergence from the seed bank? Do nutrients and cross pollination influence viable seed production? Do environmental and stand conditions affect recruitment of Phragmites from seeds or rhizome?
High levels of genetic diversity were found both within and among patches, particularly in developed watersheds, indicating the importance of sexual reproduction in Phragmites spread. Viable seed production varied greatly within and among subestuaries but was highest in patches in developed watersheds. High viable seed production resulted in a greater number of seedlings emerging from the seed bank. Patches with >1 genotype produced more viable seeds and viable seed production was greater for out-crossed plants. In the field, Phragmites seed germination increased in disturbances but few seedlings survived (<1%). In contrast, emergence from rhizomes was low overall (<10%) and was not influenced by stand or environmental conditions. In the greenhouse, nutrient levels had a strong positive effect on seedlings grown in full sun but no effect on emergence from rhizomes. Under heavy shade, seedling growth was negligible as were the effects of nutrients. Our study illustrates the importance of land-use, disturbances, nutrients, genetic diversity, and viable seed production on Phragmites invasion. Phragmites is more likely to invade by seed in high-light, disturbed areas, and seedling growth is enhanced by higher nutrient availability, conditions more likely to occur in developed watersheds. Patches formed from seedlings contain multiple genotypes and produce more viable seeds. As Phragmites spreads by seed it can further increase local genetic diversity and thus, viable seed production and seed bank formation.