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 (CO2) 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 CO2 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 CO2 have rarely been examined. We show that 30 years of experimental CO2 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 CO2 were reversed by N addition. We demonstrate that morphological responses to CO2 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.