Multiple Potential Stressors and Dieback ofPhragmites australisin the Mississippi River Delta, USA: Implications for Restoration

In 2016, widespread dieback ofPhragmites australiswas reported in the Mississippi River Delta (MRD), Louisiana. We conducted two common-garden experiments to investigate several potentially important factors associated with this dieback: scale insects, water depth, fertilization andP. australisgenetics (three lineages: Gulf, Delta and invasive EU). Predictions tested were scale abundance is lower in high water, at low fertilization, and for EU; plant biomass is negatively impacted by scales, high water and high fertilization; and EU suffers the least damage from the three potential stressors. Scale abundance was 41% lower in high water and decreased 2.7 fold as fertilization increased. Also, EU had 1.5-2.6 times fewer scales than Gulf, but had similar scale abundance to Delta. Impacts of scales, water depth and fertilizer on plant biomass depended strongly on lineage. Scales reduced biomass of Delta, EU and Gulf by 38%, 32% and 10%, respectively. In comparison, biomass was 30% higher for EU, 46% lower for Gulf and unchanged for Delta in high versus low water. Finally, at high fertilization levels, Gulf produced 57% more biomass than EU. Owing to its greater tolerance to scales and high water, EU may be most suitable for use in restoration of the MRD.