Invasive Phragmites

A native subspecies of Phragmites australis (ssp. americanus) has long been a component of North American wetlands, but approximately 200 years ago a non-native subspecies (ssp. australis) was introduced from Europe and has become a major invader of brackish wetlands in the Chesapeake Bay region and throughout North America. Unlike native Phragmites, the non-native subspecies forms dense monocultures that exclude other plants and alter the nutrient dynamics, hydrology and function of invaded wetlands.

Along with the Plant Ecology Lab, we are working to investigate the combined effects of multiple stressors on nearshore habitats throughout the Chesapeake Bay region as part of a $5 million grant from the National Oceanic and Atmosphere Administration (NOAA). Our role in this project focuses on using population genetics to understand how human activity could be impacting the spread of Phragmites on shorelines and wetlands.

A major key to understanding why Phragmites has become such a significant invader has resulted from understanding why, over 200 years after invading the Chesapeake Bay, Phragmites has only recently begun to spread explosively. In a collaboration with Karin Kettenring (Utah State University), Eric Hazelton (Ph.D. student, Utah State University) and Dennis Whigham (SERC), we demonstrated that Phragmites invasion was linked to the level of genetic diversity contained within patches. We found that Phragmites needed to have more than one genotype in a patch to produce significant numbers of viable seeds. Only after multiple genotypes became established close together, which appears to have taken many years to happen after initial invasion, were enough seeds produced to fuel further spread. The accumulation of genetic diversity was also influenced by anthropogenic development and happened more quickly in disturbed, eutrophic subestuaries. This work has been published in Journal of Ecology, Wetlands and Journal of Applied Ecology.

Many factors can contribute to how quickly newly formed patches accumulate enough genetic variation to produce the abundant seeds that allow them to fuel further invasion. As part of the larger NOAA grant, we are studying how different types of shoreline hardening affect the accumulation of multiple genotypes in Phragmites patches and how the age of patches affects their function in a subestuary.

In collaboration with Tom Mozdzer (Bryn Mawr College) and Pat Megonigal (SERC), funded by Maryland Sea Grant, we are studying how the genetic diversity of Phragmites patches affects their ability to grow and spread in response to elevated CO2 and nitrogen. We are using this information to understand how the spread of Phragmites is likely to change in the future and to find ways to improve management.