Wetlands fringe the edges of land adjacent to the sea and, although they represent a small proportion of Earth’s terrestrial habitats, they provide important ecosystem services. They protect communities against the worst effects of hurricanes and typhoons and the plants sequester large amounts of carbon. But, as sea levels rise, the fate of wetlands hangs in the balance. Will they adapt or will they drown?
For decades, scientists have hoped that rising concentrations of carbon dioxide (CO2) would stimulate increased plant growth in general. In the case of coastal wetlands, this scenario would enable them to grow fast enough to outpace the rise in sea level. A new study, conducted by researchers from the Smithsonian Environmental Research Center (SERC), has now shed light on this question, and the findings do not look as hopeful.
The study took place at SERC’s Global Change Research Wetland, a research site run by biogeochemist Pat Megonigal on the western shore of Maryland. The research on wetland plant growth there represents the longest-running field experiment in the world on the impact of rising CO2 levels on plants. Started in 1987, the design involves enclosing 15 small areas of wetland within open-top chambers, and then modifying the concentration of CO2 in the chambers and monitoring the growth responses of the plants as sea levels have slowly risen. A further 15 chambers serve as experimental controls that did not have any CO2 enrichment.
Over time, the scientists have raised the concentration of CO2 within the experimental chambers by an additional 340 parts per million, roughly doubling the atmospheric levels of 1987. In monitoring plant growth, they have focused particularly on the 10 chambers that contain “C3” plants that use a specific metabolic pathway to incorporate carbon from the atmosphere into sugars in their cells. These plants include roughly 85 percent of all plant species on Earth and they are known to respond vigorously to CO2 enrichment.
The results, published today in the journal Science Advances, show that plant growth in the higher CO2 chambers flourished for about the first two decades of the study. Above ground, plants in the high-CO2 chambers grew on average 25 percent more than plants in the untreated chambers. The effect was even more powerful underground, where higher levels of CO2 triggered about 35 percent more root growth. Root growth is especially critical for wetland survival, as roots help wetlands trap soil and keep the foundations growing upward even as seas continue to rise.
“Even though elevated CO2 contributes to sea-level rise, it also enhanced the marsh’s ability to accrete vertically during the early years of the experiment,” said Don Cahoon, a co-author and research ecologist, emeritus, with the U.S. Geological Survey.
“In some ways, this is a race, a race between what CO2 can do and what sea level can do,” said study co-author Lewis Ziska, a plant physiologist at the Columbia University Mailman School of Public Health.
But after 2005, the growth effect declined and then stopped altogether. For the past 14 years of data in the study, there was no average difference in plant growth between the high-CO2 and the normal chambers.
“The CO2 effect has always been one of the silver linings of climate change,” said study co-author Adam Langley, an ecologist with Villanova University. “Well, at least plants are going to grow more. But we see here that they didn’t. So the silver lining to me just got a little cloudier.”
The team investigated several possible explanations for the return to normal growth rate: precipitation, temperature, the saltiness of the water during growing season, and the presence of critical soil nutrients, like nitrogen. Only sea-level rise showed any link to plant growth. Once the sea level at the wetland rose by 15 centimeters from where it was in 1987, the benefits of higher CO2 disappeared.
Sea water contains far less oxygen than air does and so, when wetlands flood, plants are deprived of oxygen. Plants need oxygen as well as CO2 for growth, and wetland plants are adapted to get most of their oxygen from the air rather than extracting it from water. As sea levels rise, the frequency of flooding increases and plants are unable to obtain the oxygen they need, so growth rates slow down.
“Plants are aerobic, oxygen-breathing organisms,” explained Megonigal. “And that includes their roots. And so they’re fundamentally faced with this problem of having their root system in an environment that doesn’t have any oxygen in it.”
Some wetlands may still be able to escape drowning. If wetlands cannot rise higher by building soil, migrating inland is another possibility. However, that can only happen if they have enough space to move into. For many communities, allowing room for wetlands to move inland would require a shift in how they use and value the land.
In the meantime, Earth’s climate accountants will need to rethink the planet’s carbon budget because it is possible that wetlands will not be around to absorb carbon from the atmosphere, as was originally thought. Now that scientists know extra CO2 does not always stimulate wetland growth as much as they thought, the amount of carbon that can be sequestered in wetland plants in the coming decades is no longer certain.