Grassland plants show surprising appetite for carbon dioxide

Grasslands in warm and dry climates could grow faster as carbon dioxide levels rise, according to data from a long-term ecological field experiment in Minnesota. The finding, which runs counter to long-established ideas about how plants will respond to the greenhouse gas, suggests that grasslands could provide a buffer against climate change.

The research, published on 19 April in Science, delves into a longstanding question about how Earth’s two major groups of plants will respond to the growing level of CO₂ in the atmosphere. The biggest group, known as C₃ plants, comprise 97% of all plant species. These species make energy through photosynthesis, using sunlight to synthesize sugars from CO₂ and water. In theory, giving these plants extra CO₂ would rev up their energy production.

The other group of plants – so-called C₄ species – use a two-step process to boost their internal CO₂ levels before photosynthesis takes place, making energy production more efficient. For decades, scientists have thought that C₄ plants would not benefit from additional CO₂ in the atmosphere because they are already turbo-charged. But the Science paper suggests that the opposite might be true.

“The main message is don’t count out the C₄ grasslands,” says Dana Blumenthal, an ecologist with the US Department of Agriculture in Fort Collins, Colorado. Because C₄ plants evolved to live in hot and arid conditions, scientists have long projected that the species will expand their range as the climate warms. Now, it turns out that they also might pull more CO₂ out of the atmosphere.

Growing questions

The latest findings come from the Biodiversity, CO₂ and Nitrogen (BioCON) experiment. Beginning in 1997, researchers planted C₃ and C₄ plants on 88 open-air plots about 50 kilometres north of Minneapolis, Minnesota. The team then pumped in enough CO₂ to some of those plots to raise the average atmospheric concentration of the gas to around 550 parts per million – about double the level present in the air before the industrial era. For the first 12 years, the growth rate of the C₄ plants exposed to extra CO2 did not increase. But over the next eight years, that group outperformed C4 plants that weren’t grown in the high-CO2 environment.

It’s not yet clear why this has happened, but scientists note that as CO₂ levels rose, the amount of nitrogen available to the plants also increased. Nitrogen is an essential nutrient that is crucial to photosynthesis. One possibility is that changes in the composition of soil microbes drove the increase in nitrogen. “It’s a huge surprise,” says Peter Reich, an ecologist at the University of Minnesota in Saint Paul who heads the experiment. “I don’t think any scientist in the world would have predicted it. ”

Scientists estimate that plants absorb roughly one-quarter of humanity’s carbon emissions each year, and the Minnesota experiment is one of several that have sought to determine whether that trend will continue as atmospheric CO₂ levels rise. Much of the research has focused on C₃-dominated forests, which absorb large amounts of CO₂ from the atmosphere.

Scientists had thought that the forests would be likely to grow faster when exposed to higher CO₂ levels in the atmosphere. But that’s not what experiments have shown. When C3 plants are exposed to higher Co2 levels, their rate of growth increases for a period – but eventually the plants are hobbled by the limited availability of nutrients such as nitrogen and phosphorus.

Reich and his colleagues saw a similar effect in the C3 plants that they exposed to high CO₂ levels: the initial boost in productivity disappeared entirely around the same time that the fertilized C4 plants began to grow more quickly.

“The lesson is that photosynthesis doesn’t equal growth,” says Richard Norby, an ecologist at the US Department of Energy’s Oak Ridge National Laboratory in Tennessee. He says that if scientists want to understand how an ecosystem’s plants will respond to increased CO₂, they need to look at how nutrient cycles change over time. “You can’t get at that with short experiments. ”

Global perspective

In addition to the Minnesota site, two similar CO₂ field experiments are operating in forests outside of Birmingham, UK, and Sydney, Australia. And Brazil has been working for several years to establish the first such experiment in a native tropical forest, although Norby says that the project has encountered funding troubles. The US Department of Energy has been conducting field work in tropical ecosystems in Puerto Rico, Panama and Brazil, but plans to shut that research down early.

Reich and his colleagues working at the Minnesota site are now focusing on how the microbial community there might be changing below ground. That includes fungi that inhabit plant roots, and microbes that decompose dead plants and release nutrients such as nitrogen. Pinning those details down will help scientists to understand more about why C₄ plants seem to thrive as CO₂ levels rise, but Reich says that the results of his experiment won’t be enough to pin down what’s happening.

“The reason we can’t say exactly what this means globally is that we really need a dozen of these experiments, not just one,” he says.