Wetlands throughout North America are under siege by an invasive genetic lineage of the common reed, Phragmites australis.  Although populations of the native genetic lineages exist, they are becoming displaced along the North American Atlantic coast by a vigorous introduced lineage from Eurasia.  A C₃ plant and vigorous invader, little is known about how rising CO₂ and nitrogen (N) pollution may exacerbate Phragmites growth and invasion.  Testing global change effects on genetically distinct conspecific lineages allows us to determine which phenotypic traits make an invasive plant successful, and provides insights as to how these traits are affected by global change.  To evaluate the effects of elevated CO₂ and N, clones of native and introduced Phragmites lineages were grown in CO₂ controlled chambers at ambient and elevated (ambient + 340ppm) CO_2, with half the individuals receiving an elevated N treatment approximating current N-loading rates in coastal wetlands.  We assessed growth responses above and belowground to better understand how this already invasive plant may respond to predicted global change.

Results/Conclusions

Both native and introduced lineages responded positively to the CO_2, N, and CO₂ + N treatments in both biomass and relative growth rates.   Although the overall growth response in the introduced lineage was consistently more than two times greater than the native lineage within any given treatment, biomass was stimulated equally in both lineages.  N availability, more so than CO₂ enrichment alone, limited the growth and invasive potential of Phragmites in our study.  Under low-N availability, CO₂ enrichment stimulated Phragmites growth by 47-70% in both lineages, whereas N enrichment stimulated biomass by over 150%.   Our data suggest that even in a high CO₂ environment, it may be possible to limit future Phragmites invasions if wetland ecosystems can be restored to their pre-industrial oligotrophic state.  However, if anthropogenic N inputs and CO₂ levels increase as predicted, introduced Phragmites invasion will likely intensify in the future as evidenced by the 290% stimulation in biomass production.