The establishment, expansion and ecosystem effects of Phragmites australis, an invasive species in coastal Louisiana

As biological invasions have become a common phenomenon throughout the world, ecologists have intensified efforts to understand why natural communities are susceptible to invasion. Invading species can cause shifts in community structure that result in irreversible changes to ecosystem function. Phragmites australis has rapidly spread in North American coastal wetlands during the past 50 years and has become a dominant feature in Northern Gulf of Mexico brackish marshes. The rate at which Phragmites is spreading or the mechanisms controlling its establishment in these marshes is unknown. My research objectives were to: (1) determine the spatial and temporal patterns of Phragmites invasion and expansion; (2) evaluate how disturbance and nutrient enrichment controls brackish marsh invasibility and Phragmites establishment, and (3) identify the ecosystem impacts occurring within a brackish marsh during Phragmites invasion. I found substantial increases in the abundance and size of clones of Phragmites during the past 75 years. Annual increases of 11-23% occurred in area covered by clones, which had intrinsic rates of increase in size of 0.07 - 0.23 yr-1. To test marsh invasibility, I manipulated both nutrient levels and disturbance regimes in conjunction with purposeful introductions of Phragmites seed and rhizome material. Phragmites demonstrated the potential for active growth and spread when rhizomes were introduced into brackish marsh. To examine the ecosystem impacts of Phragmites invasion, I located three isolated Phragmites invasions and identified four distinct community types along a transect from the center of each invasion to adjacent un-invaded marsh. My results demonstrate for the first time that Phragmites increases marsh surface elevation relative to un-invaded marsh. Phragmites invasion resulted greater aboveground biomass, increased organic matter accumulation and peat development and lower cellulose decomposition rates relative to un-invaded marsh. The numbers and sizes of Phragmites invasions are increasing without apparent restriction in this Louisiana brackish marsh. These communities remain vulnerable to future Phragmites invasions if rhizomes are transported to new locations. Furthermore, Phragmites has an obvious affect as an ecosystem engineer and may allow invaded marshes to better tolerate increasing water levels due to sea-level rise/land subsidence than native short-stature graminoids