Credit: David Gray/Reuters
are predicted to expand their range
as the climate heats up.
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 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.
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.
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.
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.
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.
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.
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.
And Brazil has been working for several years to establish the first such experiment in a native tropical forest, although Richard 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.
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.