Mechanisms of Phragmites australis invasion: feedbacks among genetic diversity, nutrients, and sexual reproduction

Abstract

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.

Genetic and epigenetic changes during the invasion of a cosmopolitan species (Phragmites australis)

“…This assumption is also supported by the little genetic and epigenetic differentiation between the DELTA and MED groups. Genetic diversity seems to be a key factor explaining its broad ecological adaptation (Kettenring, McCormick, Baron, & Whigham, 2011; Lambertini, Mendelssohn et al., 2012). Compared with LAND group, the DELTA group can adapt to wetter and slightly more saline habitats.…”

Local and regional disturbances associated with the invasion of Chesapeake Bay marshes by the common reed Phragmites australis

“…Management strategies that reduce nutrient pollution, preserve natural shorelines, and limit nearshore disturbance of soils and vegetation may enhance the efforts to increase the resilience of shorelines to new invasion. These efforts are not likely to succeed unless they consider both regional and local factors and can address disturbance, nitrogen, and propagules together (Hazelton et al 2014;Kettenring et al 2011;McCormick et al 2016).…”

“…Elevated nitrogen increases P. australis sexual reproduction and expansion in Chesapeake Bay (Kettenring et al 2011); increases P. australis density, height, and above-ground shoot biomass (Bastlova et al 2004;Engloner 2009); and allows seedlings to rapidly escape from a vulnerable life stage (Saltonstall and Stevenson 2007;Kettenring et al 2015;Hazelton et al 2014). P. australis stands have larger nitrogen pools than other marsh communities (Meyerson et al , 2000Windham and Meyerson 2003).…”

“…australis stands have larger nitrogen pools than other marsh communities (Meyerson et al , 2000Windham and Meyerson 2003); and anthropogenic nitrogen can promote the expansion of the invasive P. australis strain (King et al 2007;Chambers et al 2008;Kettenring et al 2011). Compared to the native subspecies, the invasive one demands four times more nitrogen to support its biomass than the native subspecies (Mozdzer and Zieman 2010), but the invasive strain can exploit high nitrogen levels to achieve higher growth rates and biomass than the native strain and other species of marsh plants (Mozdzer et al 2013).…”

Reproductive Systems

“…In the few such taxa that have been studied in detail, however, outcross pollen leads to higher seed set and germinability than self pollen (Fang et al 2004;Kettenring et al 2011). Even though barriers to self-pollination are rarely absolute, estimated rates of outcrossing are strongly bimodal, with self-incompatible species generally above 80 % outcrossing and self-compatible ones mostly well under 15 % (Connor 1987;Raduski et al 2012).…”