The Hot Zone

Grass gone wild

Posted by Alison Hawkes on July 1, 2010
Category : Climate Science and Scientists

Buying carbon offsets may, indeed, be a sin tax. But there are more reasons than dubious moral rectitude to question the practice.

Offsets are based on the belief that doing things like replanting forests and setting aside undeveloped land will mitigate the effects of global warming because more greenery will draw down CO2 from the atmosphere. Some research has even shown that plants do well under higher CO2 concentrations, growing faster and more lush, a kind of carbon sequestration known as “CO2 fertilization.”

The Amazon rainforest is the site of carbon offsets.

The Amazon rainforest is the site of carbon offsets.

But a paper in the journal Nature published today paints a fuzzier picture. First of all, to make good on all the extra CO2, plants also need more Nitrogen to grow, and that may or may not be available. There’s a lot of Nitrogen floating around from human-made sources (fertilizers), so we should be good, right?

Researchers led by the Smithsonian Environmental Research Center and Villanova University’s Department of Biology experimented with plants in the brackish marsh of a tributary off the Chesapeake Bay. They found that the two types of highly responsive grasses that dominate the plant ecosystem there had differing responses to changes in Nitrogen and CO2 levels.

Both types of grasses shot up under higher Nitrogen and CO2 levels during the first year, but over time one type — C4 grasses, more prevalent in warmer, drier climates — pulled ahead and began encroaching on their C3 plant neighbors. C4 grasses have a special type of turbocharged photosynthesis that gives them a distinct advantage (others C4 species include switchgrass and corn).

Indian grass, a tall North American prairie grass, is a C4 plant. Photo courtesy of St. Olaf College

Indian grass, a tall North American prairie grass, is a C4 plant. Photo courtesy of St. Olaf College

That spells trouble because while C4 grasses may be better adapted to higher CO2 levels, they don’t actually absorb as much of it. Ultimately their dominance in the ecosystem suppresses CO2 fertilization – not exactly what we want. They could also alter plant communities in ways that impact the rest of the ecosystem, the researchers conclude.

The biggest problem, from the standpoint of planning for climate change, is that we may be overestimating how much nature will be able to work in our favor. Storing carbon is no easy matter.

-Alison Hawkes

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