Responses of stomatal features and photosynthesis to porewater N enrichment and elevated atmospheric CO₂ 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 (g_wmax ) and photosynthetic rates.

Methods: Plants were grown in situ in open-top chambers under ambient and elevated atmospheric CO₂ (eCO₂ ) 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 CO₂ and N_enr altered the relationship between g_wmax and A_sat .

Results: eCO₂ 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 eCO₂ . There was a positive relationship between g_wmax and A_sat that was not modified by eCO₂ or N_enr , individually or in combination.

Conclusions: Changes in stomatal features co-occur with previously described responses of P. australis to eCO₂ 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.