Biogeographic gradients in ecosystem processes of the invasive ecosystem engineer Phragmites australis

Latitudinal gradients in ecosystem patterns arise from complex interactions between biotic and abiotic forces operating at a range of spatial and temporal scales. Widespread invasive species, particularly invasive ecosystem engineers with large effects on their environment, may alter these gradients. We sampled 3–5 stands of the invasive common reed, Phragmites australis, in eight coastal wetlands ranging from Massachusetts (42°N) to South Carolina (32°N) to document geographic variation in P. australis primary production, associated plant and animal species diversity, and sediment carbon storage and to examine how local-, regional-, and large-scale environmental factors contribute to these patterns. Latitude best explained variation in P. australis density, but contrary to expectations, density increased with increasing latitude across our sites. Latitude also predicted macroinvertebrate species richness, which increased with latitude in a manner similar to P. australis density. In addition to latitude, P. australis leaf carbon:nitrogen ratios, distance to the open coast, and sediment oxygen levels were most important for explaining variation in P. australis production, as well as community (plant or animal species richness) and ecosystem (carbon storage) variables. The percent of developed land was positively associated with P. australis density, yet this variable had relatively low predictive power in our study. Our study provides an important biogeographic perspective for documenting and understanding variation in invasive P. australis that is fundamental both for managing the invasion and for understanding latitudinal gradients in ecosystem structure and function.