Nitrogen status regulates morphological adaptation of marsh plants to elevated CO2

Coastal wetlands provide valuable ecosystem services that are increasingly threatened by anthropogenic activities 1 . The atmospheric carbon dioxide (CO 2 ) concentration has increased from 280 ppm to 404 ppm since the Industrial Revolution and is projected to exceed 900 ppm by 2100 (ref. 2 ). In terrestrial ecosystems, elevated CO 2 typically stimulates C 3 plant photosynthesis and primary productivity leading to an increase in plant size 3 . However, compared with woody plants or crops 4 , the morphological responses of clonal non-woody plants to elevated CO 2 have rarely been examined. We show that 30 years of experimental CO 2 enrichment in a brackish marsh increased primary productivity and stem density but decreased stem diameter and height of the dominant clonal species Schoenoplectus americanus . Smaller, denser stems were associated with the expansion of roots and rhizomes to alleviate nitrogen (N) limitation as evidenced by high N immobilization in live tissue and litter, high tissue C:N ratio and low available porewater N. Changes in morphology and tissue chemistry induced by elevated CO 2 were reversed by N addition. We demonstrate that morphological responses to CO 2 and N supply in a clonal plant species influences the capacity of marshes to gain elevation at rates that keep pace with rising sea levels. A 30-year dataset shows that marsh plants increased primary productivity and stem density with CO 2 enrichment, but diameter and height decreased under nitrogen limitation. The addition of nitrogen reversed these changes, which is important to allow marshes to keep pace with rising sea levels.