By Richard Black
Environment correspondent, BBC News website
The grassland plots were grown in DRI's "Ecocell" facility
Plants are unlikely to soak up more carbon dioxide from the air as the planet warms, research suggests.
US scientists found that grassland took up less CO2 than usual for two years following temperatures that are now unusually hot, but may become common.
The conclusion parallels a real-world finding from Europe's 2003 heatwave, when the continent's plant life became a net producer, not absorber, of CO2.
The latest study is published in the scientific journal Nature.
Researchers extracted four intact segments of grassland, about 3 sq m in area and weighing about 12 tonnes each, from the prairies of Oklahoma, and placed them in special chambers at the Desert Research Institute (DRI) in Reno, Nevada.
Conditions in the chambers, such as temperature, moisture and sunlight, could be precisely controlled.
Two of the four chambers were given a set of conditions mimicking what actually happens, on average, on the wild prairies. Temperatures rose and fell with days and nights and seasons, and "rainfall" was injected in a realistic pattern.
The other two chambers received the same prescription with the exception that for a whole year, temperatures were always 4C higher.
Grassland "monoliths" were excavated from the Oklahoma prairie
The warmer plots saw a shortfall in carbon dioxide uptake of about 30% during the warm year and the one following.
DRI's Jay Arnone, who led the study, said two different mechanisms appeared to be responsible.
"So in the warm year, the temperature goes up and causes more evapotranspiration from the plants," he told BBC News.
"But plants have evolved to 'know' that when it gets dry they should curb their water loss, so they reduce the apertures of their stomata (pores) to conserve water, and that constrains the amount of CO2 they can take up (by photosynthesis)."
This response has been understood for some time. But what happened in the following year, when temperatures returned to "normal", was not so familiar.
Even during the warm year with its meagre amount of photosynthesis, plants had put carbon in the soil.
So during the normal year following, soil microbes had extra carbon to process, which they did, emitting more carbon dioxide into the air.
By complete coincidence, the study mimicked fairly closely events on the other side of the Atlantic.
As DRI researchers were turning up the heat in 2003 in their experimental plots, in Europe it was happening for real, with temperatures in some places reaching 6C above normal.
An analysis led by French researchers, published in 2005, showed that as the continent became hotter, Europe's plants changed from being net overall absorbers of CO2 to net producers.
A lot of faith is being placed in some circles in the capacity of trees and plants to maintain absorption of CO2 as concentrations of the gas rise, or even to use the extra CO2 to grow faster and absorb more of it.
It is one of the reasons behind the recent upsurge of interest in having western governments pay to protect tropical forests.
But the DRI research is one of a number of pieces of evidence suggesting it will not always work. Some ecosystems might continue to absorb carbon dioxide, and perhaps increase the rate of absorption; others may react to warming by releasing the greenhouse gas.
"We conducted this study under current ambient levels of CO2 so we don't know for sure what'll happen in the future," said Professor Arnone.
"But we don't anticipate a huge effect of [elevated] CO2 on these systems. As high temperatures become more commonplace, you might expect a persistent reduction in the uptake of CO2 by natural ecosystems, and that may mean that the net rate of CO2 elevation may increase."