Common Reed Grass, Phragmites australis, Expansion into Constructed Wetlands: Are We Mortgaging Our Wetland Future?

The use of constructed wetlands to replace natural wetlands has become a widespread management tool. Because of the inherent disturbed nature of these sites, constructed wetlands are susceptible to colonization by undesirable plant species. Vegetated communities in 15 constructed wetland sites ranging in age from 1 to 12 yr and in size from 0.4 to 5.3 ha were surveyed using differential global positioning system (GPS) technology in 1994. These sites were re-surveyed in 2000. Colonization of the sites by Phragmites australis expanded from 73% of the sites in 1994 to 80% of the sites in 2000. The total area colonized by P. australis within the sites increased from 3.47 to 4.96 ha. In some sites, the area of P. australis decreased, which appears to be correlated with an increase in scrub-shrub vegetation (0.986, p = 0.014). Similar to results from the previous study, sites that are surrounded by subtidal perimeter ditches have significantly less P. australis than those sites without perimeter ditches (p = 0.019). P. australis expansion rates within the sites varied from 0.1 to 5.6 yr-1. Colonization of constructed wetland sites by P. australis should be a continued concern of resource managers. Activities such as planting scrub-shrub species on the upland-wetland berm and construction of subtidal perimeter ditches should be considered as methods to reduce the probability of invasion.