Responses of stomatal features and photosynthesis to porewater N enrichment and elevated atmospheric CO 2 in Phragmites australis , the common reed

PREMISE Biological invasions increasingly threaten native biodiversity and ecosystem services. One notable example is the common reed, Phragmites australis, which aggressively invades North American salt marshes. Elevated atmospheric CO2 and nitrogen pollution enhance its growth and facilitate invasion because P. australis responds more strongly to these enrichments than do native species. We investigated how modifications to stomatal features contribute to strong photosynthetic responses to CO2 and nitrogen enrichment in P. australis by evaluating stomatal shifts under experimental conditions and relating them to maximal stomatal conductance (gwmax ) and photosynthetic rates. METHODS Plants were grown in situ in open-top chambers under ambient and elevated atmospheric CO2 (eCO2 ) and porewater nitrogen (Nenr ) in a Chesapeake Bay tidal marsh. We measured light-saturated carbon assimilation rates (Asat ) and stomatal characteristics, from which we calculated gwmax and determined whether CO2 and Nenr altered the relationship between gwmax and Asat . RESULTS eCO2 and Nenr enhanced both gwmax and Asat , but to differing degrees; gwmax was more strongly influenced by Nenr through increases in stomatal density while Asat was more strongly stimulated by eCO2 . There was a positive relationship between gwmax and Asat that was not modified by eCO2 or Nenr , individually or in combination. CONCLUSIONS Changes in stomatal features co-occur with previously described responses of P. australis to eCO2 and Nenr . Complementary responses of stomatal length and density to these global change factors may facilitate greater stomatal conductance and carbon gain, contributing to the invasiveness of the introduced lineage.