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 CO 2 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 CO 2 and nitrogen enrichment in P. australis by evaluating stomatal shifts under experimental conditions and relating them to maximal stomatal conductance ( g wmax ) and photosynthetic rates. METHODS Plants were grown in situ in open‐top chambers under ambient and elevated atmospheric CO 2 (eCO 2 ) and porewater nitrogen (N enr ) in a Chesapeake Bay tidal marsh. We measured light‐saturated carbon assimilation rates ( A sat ) and stomatal characteristics, from which we calculated g wmax and determined whether CO 2 and N enr altered the relationship between g wmax and A sat . RESULTS eCO 2 and N enr enhanced both g wmax and A sat , but to differing degrees; g wmax was more strongly influenced by N enr through increases in stomatal density while A sat was more strongly stimulated by eCO 2 . There was a positive relationship between g wmax and A sat that was not modified by eCO 2 or N enr , individually or in combination. CONCLUSIONS Changes in stomatal features co‐occur with previously described responses of P. australis to eCO 2 and N enr . 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.