Rising CO₂ levels might not be as good for plants as we thought

Two major groups of plants have shown a surprising reversal of fortunes in the face of rising levels of carbon dioxide in the atmosphere.

During a 20-year field experiment in Minnesota, a widespread group of plants that initially grew faster when fed more CO₂  stopped doing so after 12 years, researchers report in the April 20 Science. Meanwhile, the extra CO₂ began to stimulate the growth of a less common group of plants that includes many grasses. This switcheroo, if it holds true elsewhere, suggests that in the future the majority of Earth’s plants might not soak up as much of the greenhouse gas as previously expected, while some grasslands might take up more.

“We need to be less sure about what land ecosystems will do and what we expect in the future,” says ecosystem ecologist Peter Reich of the University of Minnesota in St. Paul, who led the study. Today, land plants scrub about a third of the CO₂ that humans emit into the air. “We need to be more worried,” he says, about whether that trend continues.

The two kinds of plants in the study respond differently to CO₂ because they use different types of photosynthesis. About 97 percent of plant species, including all trees, use a method called C3, which gets its name from the three-carbon molecules it produces. Most plants using the other method, called C4, are grasses.

Both processes ultimately feed plants by pulling carbon dioxide from the air. But C4 plants use CO₂ more efficiently, so they’re less hungry for it. As a result, it has long been dogma that when CO₂ increases in the air, C3 plants gobble up more of it — and thus grow faster — while C4 plants ignore it.

And that’s what experiments on plants grown in elevated CO₂ have always shown — until now. For 20 years, scientists at the Cedar Creek Ecosystem Science Reserve in Minnesota have grown both C3 and C4 grasses in 88 plots, pumping extra CO₂ into half of them to increase concentrations by 180 parts per million. That amounts to about 50 percent more CO₂ than was in ambient air at the experiment’s beginning, and double preindustrial levels.

For the first 12 years, the plants hummed along as expected, with C3 plants responding more strongly to extra CO₂ — a 20 percent boost in growth compared with plants grown in ambient air — and C4 plants largely ignoring the difference. But then something unexpected happened: The pattern reversed. Over the next eight years, C3 plants grew on average 2 percent less plant material if they received extra CO₂, while C4 plants grew 24 percent more.

“I’m not at all surprised that an experiment like this would produce the unexpected,” says forest ecologist Rich Norby of Oak Ridge National Laboratory in Tennessee. Norby led a different project that tested a forest’s response to elevated CO₂ for 12 years, and says the new results highlight the importance of such long-term experiments.

In particular, Norby says, soil fertility can affect how plants respond to CO₂ in the long run.

In fact, soil nutrients may have been key to the flip-flop in Minnesota. Without the nitrogen they need, plants can’t take advantage of extra CO₂ no matter how much there is. Over the course of the experiment, nitrogen grew to be in shorter supply for C3 plants, but in greater supply for C4 plants. The team suspects that differences in decomposing plant material might have led to changes over time in the community of microbes that process nitrogen in the soil and make it available to plants.

Since grasslands cover 30 to 40 percent of Earth’s land area, Reich says it’s important to learn how they could store carbon in the future. If grasslands worldwide behave as in the experiment, C4 grasslands — found in warm, dry regions — may absorb more CO₂ than thought, while more abundant C3 plants could soak up less. As for crops, which can be either C3 like wheat or C4 like corn, the future is even less clear since farmlands are highly managed and often fertilized with nitrogen.

More studies are needed to figure out whether, and how, the world’s plants could shift in their response to increasing CO₂. In the meantime, says Reich, “this means we shouldn’t be as confident we’re right about the ability of … ecosystems to save our hides.”