Shrinking stems of marsh plant under elevated carbon dioxide

Background/Question/Methods

Coastal wetlands are considered as one of the most valuable ecosystems on Earth, yet sea level rise, acidification, pollution and climate change are threatening these fragile systems. Atmospheric carbon dioxide (CO₂) concentration has evidently increased 43% since 1850 and is projected to reach 985 ± 97 ppm by 2100. It is well established that CO₂ fertilization typically stimulates C3 plant photosynthesis and primary productivity in terrestrial ecosystems, basically leading to the increase in plant body size. However, whether the enhanced C uptake under rising CO₂ concentration may increase the dimension of mash plant in coastal wetlands, has not been well examined through manipulative experiment. The morphology of marsh plant plays critical roles in coastal wetlands because it shields habitats and homes from floods and hurricanes, sustains nests for waterfowls, and supplies nurseries for nektons. To investigate the effects of elevated CO₂ on the morphology of marsh plant, open top chambers (OTC, double the CO₂ concentration in the elevated chambers compared to the ambient chambers) were set up at Smithsonian Global Change Research Wetland (GCREW) in Kirkpatrick Marsh, Rhode River Estuary, Chesapeake Bay. C3 plant shoot dimension, biomass, carbon (C) and nitrogen (N) concentration were measured annually.

Results/Conclusions

Here after 30 years CO₂ enrichment we found a significant and 18.5% decrease in the stem leaf area but an increase of 51.9% in stem density of C3 sedge Schoenoplectus americanus (Pers.). The CO₂-induced stimulation of sedge productivity and litter accumulation, the depletion of soil N availability, and plant C:N shift all suggest N-limitation of the CO₂ effect. The soil N availability becomes increasingly limiting as the shift of plant C:N may not compensate CO₂ induced N deficiency. Marsh plant trends to allocate more biomass to rhizome and root for nutrient uptake so that the N limitation can be alleviated. The extending rhizome system leads to more tiller recruitment of stems, while the shrinking stem occurs along with the N depletion. Moreover, an increase of 9.6% in stem leaf area of S. americanus was found in another 10 years elevated CO₂ plus N addition experiment, suggesting the mitigation of N limitation can reverse the effect of elevated CO₂ on stem dimension. The results from two experiments indicate that the response of marsh plant morphology to elevated CO₂ is regulated by soil N availability and the trade-off between stem dimension and density of marsh plant.