Effects of elevated CO2 and nitrogen pollution on Phragmites australis invasion
Coastal plant communities are very sensitive to environmental change. Recent evidence suggests that global change-induced species shifts in natural communities may alter the ability of wetland ecosystems to persist in a rapidly changing climate. While earlier global change studies provide clear insights into the responses of native plant communities to predicted changes in both carbon dioxide (CO2) concentrations and nitrogen (N) pollution, little is known about how another global change factor, invasive species, may alter the response and stability of coastal wetlands to predicted global change. For example, coastal wetlands in North America are increasingly dominated by an introduced lineage of the common reed, Phragmites australis, which has the potential to re-engineer the structure and function of tidal wetlands. To evaluate how the process of invasion will be altered by both CO2 and N pollution, we began a factorial open top chamber (OTC) elevated CO2 x N experiment in a brackish Chesapeake Bay tidal marsh at the Smithsonian Global Change Research Wetland, Edgewater, MD, USA, in May 2011. OTCs were deployed along the invading edge of Phragmites into a native Chesapeake Bay marsh community and exposed to factorial treatments of CO2 (ambient or elevated CO2) and N (ambient or +25g N m-2 yr-1). We also analyzed population structure of Phragmites with microsatellite analysis of the invading clonal plant.
After four years of treatment, we found that ecosystem productivity of our mixed community was significantly greater with N enrichment (p>0.001). We also found that Phragmites density responded strongly to both elevated CO2 (p=0.007) and N pollution (p<0.001), which facilitated invasion into the native plant community. We also report Phragmites genotype frequencies changed significantly when exposed to a single global change factor, either CO2 or N. However, no genotype significantly changed in frequency under the control or combined CO2+N treatment. Together, our data suggest that both N pollution and elevated CO2 will increase rates of Phragmites invasion in brackish marshes under current and near-future conditions. Our results also suggest that genetically diverse populations may respond particularly strongly to interacting global change factors, given that no single genotype was able to respond positively to multiple global change factors. Our data highlight the need to combine population genetics with global change studies to improve our ability to predict invasion responses at the ecosystem level.