Description

The first experiment on the marsh, and the longest-running of its kind in the world, began in 1987 under the guidance of plant physiologist Bert Drake. By 1987, carbon dioxide (CO₂) levels in the atmosphere had reached 350 parts per million. It was common knowledge that rising carbon dioxide stimulated plant growth, but it was unclear how this could shift the species makeup of a wetland—and how it would impact photosynthesis and plant growth over the long term.

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The Question

Most elevated CO₂ studies before 1987 were in greenhouses. Dr. Drake performed his study in a real ecosystem, to find out how plants behaved in situ and discover how the wetland as a whole would respond to this single, but crucial, component of climate change. He centered his study around the wetland's two major plant species: the sedge Schoenoplectis americanus (then called Scirpus olneyi), with C3-type photosynthesis, and the grass Spartina patens, with C4-type photosynthesis. Based on previous research, Bert Drake expected elevated carbon dioxide to increase the growth of C3 plants, but not C4 plants. However, he questioned whether this growth benefit would persist over many years. The only previous study of elevated CO₂ had taken place in an Arctic tundra, where the growth benefits of CO₂ disappeared in a few years.

Methodology

Dr. Drake's team built 30 open-top chambers, each surrounding a small area (0.8m²) of marsh plants. Inside half of the chambers, they raised the carbon dioxide concentration from 350 to 700 parts per million, effectively simulating predictions of carbon dioxide worldwide by the end of the 21^st century. In total, the experiment contained six types of experimental chambered plots: two CO₂ treatments (elevated vs. ambient) around three plant communities (C3 Schoenoplectus, C4 Spartina, and mixed). To control for chamber effects, the team also monitored 15 plots with no chambers.

Discoveries

Past experiments had shown that higher carbon dioxide stimulates C3 plants, but not C4 plants. Scientists expected Schoenoplectus to respond well to rising carbon dioxide because it is a C3 plant. This initial hypothesis proved correct: Schoenoplectus grew roughly 30 percent more biomass, and absorbed 32 percent more carbon dioxide, under the elevated CO₂ treatment. Moreover, the benefit of elevated CO₂ for plant growth has persisted over this long-term experiment.

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The Future

Smithsonian scientists also expected the C3 Schoenoplectus to displace the C4 Spartina over time, because rising carbon dioxide favors C3 plants. However, this has not happened in nearly three decades. By running this experiment over many years, our team is beginning to understand the reasons for this unexpected result. We suspect that changes in global change factors other than carbon dioxide, such as sea level and nitrogen loading, favor the C4 plants. These insights have inspired new experiments at the Global Change Research Wetland.