| By Richard Black Environment correspondent, BBC News website |
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.
Drying out
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.
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.
Mixed forest
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.
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