Short-term nutrient cycling responses following control of an invasive wetland plant

Background/Question/Methods

Because invasive species can dramatically alter ecosystem structure and function, their control is often a key focus of land managers.  In wetlands of the Great Lakes region, considerable effort has been devoted to large-scale control of the common reed, Phragmites australis.  Phragmites is a large, fast-growing plant that can dramatically alter carbon and nutrient cycling in the wetlands it invades.  Along the Huron Erie corridor in Michigan and Ohio, a cooperative weed management program coordinates large-scale treatment of Phragmites with aerial spraying of herbicide (a mixture of glyphosate and imazapyr).  When possible, treated areas are burned the following year to remove the large quantities of standing dead Phragmites biomass.  This study examined the short-term ecosystem impacts of Phragmites control on carbon and nutrient cycling by comparing changes in nutrient availability and plant productivity before and after treatment in coastal Great Lakes wetlands.  Nearby pre-invaded sites were also monitored to serve as controls.  We measured plant diversity, biomass production, and annual rates of carbon and nutrient uptake.  We also sampled sediment pore waters and surface waters bi-monthly throughout the ice-free season for dissolved nutrients, dissolved organic carbon (DOC), and dissolved CO₂ and CH₄.    

Results/Conclusions

Plant species richness increased in the year following Phragmites removal at some but not all of the sites.  Aboveground net primary production and plant nutrient uptake of nitrogen (N) and phosphorus (P) were significantly reduced (by 81%, 88%, and 95% respectively) in the year following treatment (p < 0.05).  Compared to the year prior to Phragmites control, plant nutrient uptake was reduced by 13.1 g N m^-2 y^-1 and 1.9 g P m^-2 y^-1.  We also observed changes in sediment pore water chemistry following Phragmites control.  Ammonium and DOC concentrations were elevated, but soluble reactive P concentrations were lower and pH declined following spraying and burning (p < 0.05 for all four comparisons).  No differences in nitrate concentrations were detected.  Our results suggest that sites may differ in their capacity to recover following Phragmites control and that nutrient and carbon cycling are substantially altered in the year following herbicide application and burning.