Phragmites australis dieback in the Mississippi River Delta, USA: Vegetation restoration

Abstract

The Mississippi River Delta (MRD) is one of the largest wetlands in the world and is experiencing widespread dieback of its most prevalent plant species, Phragmites australis. The potential for revegetation of dieback sites was assessed by planting three genetically distinct lineages of P. australis at nine sites in the MRD that varied in water depth. These lineages include Delta, the most prevalent in the MRD, EU, a highly invasive lineage, and Gulf, a common lineage of the Gulf Coast and southern United States. Four additional wetland species were planted at a subset of these sites. Plantings were monitored for 32 months during which time, we surveyed plantings for survival, growth and infestation by a scale insect that has been implicated in P. australis dieback. Survivorship differed considerably among plant types and by 6 months, Delta, Gulf and California bulrush (Schoenoplectus californicus) had twice the survivorship of the other plant types. However, by the end of the study, Delta, Gulf and bulrush were the only plant types alive, with Delta persisting in more than twice as many sites and having > 6 times more area of coverage as Gulf and bulrush. For all plant types, water depth was negatively related to survivorship. Finally, although scale insects achieved twice the abundance on Delta than the other lineages, we found no evidence that scales were inhibiting the growth and survival of P. australis. Despite potential negative legacy effects of dieback, Delta can be used revegetate shallow-water sites within a year following dieback.