Phragmites australis - cryptic invasion of the Common Reed in North America
Phragmites australis (Cav.) Trin. ex Steudel is a tall, perennial wetland grass that grows in temperate and some tropical regions and is one of the most widely distributed flowering plants on Earth. Commonly called Phragmites or Common Reed, hereafter it will be referred to as P. australis. Phragma is Greek for ‘fence,’ which aptly describes the plant’s fence-like growth along waterways such as brackish and freshwater marshlands, riparian areas, lakeshores and areas that experience seasonal flooding.
This large grass consists of a rigid, hollow stalk 2.5 cm in diameter and 2-4 meters tall. Its long, narrow leaves grow along the length of the stalk, which terminates in a bushy flowering panicle. Flowers are purplish to tan, turn golden and finally take on a silver, fluffy appearance when its hairy seeds set in the fall. P. australis develops an extensive subterranean network of rhizomes and roots that may grow to 2 meters in depth and constitute a substantial proportion of the plant’s biomass. Once established, it reproduces primarily vegetatively through horizontal growth of the rhizomes and it also germinates from seed. In the fall, nutrients are shunted from the leaves and culms down to the rhizosphere for winter storage. Desiccated leaves fall to the ground while the stalks typically stay upright through much of the dormant period.
Historical and Contemporary Distribution
Paleobotanists have uncovered preserved remains of P. australis in the American Southwest that date back 40,000 years. Other researchers studying core samples of marsh beds discovered that P. australis has been present along the Atlantic coasts for 4,000 years. Historically the plant has been a minor component of the upper border community of marsh ecosystems and has lived in mixed associations with sedges, cattails, forbs and woody shrubs.
Today, however, P. australis is more often seen growing in dense, expansive monocultures, especially along the Atlantic coast and the Mississippi River delta. Core samples from New England suggest that P. australis monocultures are a new community type, emerging within the past 100-150 years. The plant can also be found growing along roadsides and railroad tracks across North America, ostensibly dispersed by seed or pieces of rhizome stuck to vehicle tires. Its astonishing rate of increase in distribution and abundance has alarmed many people, including scientists and natural resource managers. Eighteen states have officially declared P. australis an invasive species and it is widely considered an indicator of wetland disturbance.
Genetic Evidence of a Cryptic Invader
If this native plant has been present in North America for 40,000 years, how did it become a threatening invasive species over the course of one century? Genetics is providing some revealing answers. Kristin Saltonstall of the Smithsonian Tropical Research Institute has conducted a series of groundbreaking genetic analyses on P. australis. Her research has identified 29 unique genetic types, or haplotypes, of the grass globally. Of these, 13 are native to North America and historical pre-1910 samples indicate a wide distribution of these native haplotypes across the continent. Modern sampling has revealed the widespread presence of a non-native haplotype growing throughout North America. This newcomer’s DNA matches that of a Eurasian haplotype that is the most common P. australis haplotype in the world. The presence of the non-native P. australis in the Midwest and Pacific Coast regions of the U.S. hasn’t, to date, changed the native haplotype’s distribution from before 1910. In New England, however, the distribution and abundance of native populations has declined dramatically and genetic data indicate that some haplotypes found historically may no longer be present.
Scientists surmise that seeds and rhizomes of this non-native haplotype probably arrived in America in the ballast of ships sailing from Europe in the 18th century. Because the invasive haplotype looks so similar to its native relative, it took a long time for humans to detect its arrival. Scientists call this kind of invasion by a genetically unique but morphologically similar subspecies a cryptic invasion. The genetically unique native haplotypes of P. australis have been given the new name Phragmites australis subspecies americanus.
In general, both subspecies are highly adaptable and may look different depending on growing site and conditions. There are some features, however, that may distinguish the two but a diagnostics laboratory should confirm identification. The non-native P. australis can grow up to 5 meters tall, has a rough, entirely green stalk, dark blue-green foliage, a dense flower panicle and often grows in dense monocultures. Alternatively, the native subspecies americanus is typically shorter, has a smooth, often reddish stalk, lighter yellow-green foliage, a sparser flower panicle and often grows in association with other plants. The best method for determining a plant’s origin is with genetic analysis and a new, rapid, low cost method has been developed that uses a restriction fragment length polymorphism (RFLP) assay to distinguish P. australis subspecies.
Factors Influencing Invasiveness
Colonies of both P. australis subspecies are often stable and may pose no threat to ecosystem health. The plant is considered invasive if it begins to spread rapidly and negatively impact the growth and persistence of other plants. Invasiveness is difficult to predict as the phenomenon is caused by a convergence of plant traits, plant ecology, site conditions and human activity. Plant traits that give the non-native a competitive edge may include extended photoperiod, greater leaf turnover and reduced susceptibility to herbivory.
Habitat conditions also influence the invasiveness of P. australis. Many stressors in the natural marsh environment, such as high salinity levels, anoxic soils, sulfide concentrations and high plant densities keep its growth in check. Human alteration of marshlands can release introduced P. australis from one or more of these stressors and enable it to spread. Successful dispersion can occur with the transport and burial of large rhizome fragments. If rhizomes become established on a good site then rapid growth into suboptimal areas can occur. Colonization is especially likely if large rhizome fragments are buried in well-drained areas where salinity is low. Human activities that decrease site-level salinity and sulfide concentrations increase invasion rates and extent. Such activities include ditching for mosquito control and flow-alteration, bulkhead construction and tidal restrictions, which are installed in order to drain marshes for salt haying, mosquito control and, most recently, protecting coastal development from flooding during storms.
Coastal development encourages the establishment and spread of P. australis in New England marshes for a number of reasons. Development projects destroy woody upland borders and eliminate their important functions of filtering nutrients, collecting sediments and absorbing freshwater runoff. Nitrogen and phosphorous nutrient loading, or eutrophication, caused by fertilizer and waste runoff, gives P. australis a competitive advantage by fueling increased aboveground growth. Because it is considerably taller than its high marsh plant associates it quickly shades out competitors. Increased sedimentation from erosion creates the bare-soil conditions the plant prefers to colonize. Freshwater runoff that would normally be absorbed by the deep roots of trees and shrubs now flows directly into the marsh, decreasing salinity and promoting the growth of P. australis.
Across the Atlantic in coastal Europe another interesting ecological phenomenon is unfolding involving P. australis. For thousands of years stands of the grass have been farmed commercially for thatch and now concern is growing over their unexplained decline. Possible forcing factors include habitat destruction and manipulation of hydrologic regimes by humans, grazing, sedimentation and eutrophication, which are, ironically, many of the same explanations for P. australis expansion in North America.
In North America, rapidly expanding monocultures of P. australis commonly threaten native plant diversity. Populations of Spartina species in salt marshes and Typha species in freshwater marshes can be severely reduced. It is unclear what additional ecological impacts may be attributed to P. australis-dominated wetlands. Numerous studies have examined fish, macro-invertebrate and avian species diversities in the monocultures and the data are highly variable with regards to the direction and magnitude of impact of P. australis on habitat suitability for these wildlife species. Amongst birds, for example, there is evidence that ecological specialists and rare species such as the willet, seaside sparrow and sharp-tailed sparrow are less abundant in P. australis stands than non-P. australis stands. Other studies show, however, that the same dense stands of P. australis provide critical nesting habitat for wading birds and that red-winged black birds and other sparrow species thrive there.
Control Methods and Future Prospects
Managers of conserved marshlands along the Atlantic coast and Mississippi River delta achieve varying degrees of success in their frequent attempts to control and eradicate P. australis monocultures. Methods include chemical control, water level alteration, mowing, cutting, burning and physical destruction and removal of the plants by disking, bulldozing and dredging. Several stalk and rhizome boring insects have been suggested as candidates for biological control but no field trials have been attempted to date. In some cases, simply encouraging shrub and tree growth at the upland border of marshes can create enough shade to discourage P. australis growth and spread.
Some scientists suggest rethinking the plant’s assumed negative impact and recommend considering its potential role in managed systems. Studies show P. australis can provide important ecosystem services such as improving water quality, stabilizing banks and preventing erosion. These services may be especially valuable in highly disturbed coastal areas where other native plants can no longer survive. As coastal ecosystems become increasingly stressed due to human activity and climate change, some of the most severely impacted marshlands may be best managed for sustained ecological function by permitting P. australis stands to grow locally.
This article was researched and written by a student at the University of Massachusetts, Amherst participating in the Encyclopedia of Earth's (EoE) Student Science Communication Project. The project encourages students in undergraduate and graduate programs to write about timely scientific issues under close faculty guidance. All articles have been reviewed by internal EoE editors, and by independent experts on each topic.
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