One hot year can create a scenario that stifles prairie plants even after the heat wave
An abnormally hot year can significantly suppress the amount of atmospheric carbon dioxide that grasslands can absorb, new experiments suggest.
And the effect can linger for months after temperatures return to normal, the researchers report in the Sept. 18 Nature.
The long-term effects on carbon uptake seen in this
experiment “are a dramatic reminder of the fragility of ecosystems that are key
players in global carbon sequestration,” says lead author and ecosystem analyst
Jay Arnone of the Desert Research Institute in
Grasslands and their soils are considered a major sink for excess atmospheric carbon dioxide. Such natural carbon sponges, if they continue to thrive, could help alleviate the warming effects of manmade CO2 emissions.
The tests, which evaluated the carbon uptake by a dozen samples
Environmental conditions inside the chambers mimicked those
A sprinkler system in the chambers provided
rainfall in amounts typical of the region. Temperature and humidity in the
environmental chambers were adjusted every five minutes to replicate the
average daily and seasonal variations of
Those samples still experienced daily and seasonal variations, a pattern intended to simulate an extended heat wave like those occasionally experienced in the region. Researchers constantly monitored atmospheric concentrations of carbon dioxide in the chambers and weighed the samples to track the amount of water and carbon taken up and lost by plants.
Although the grasslands exposed to heat-wave conditions in year two stored carbon during that time, they stored, on average, about 250 grams of carbon per square meter less than the grassland samples that experienced normal climate — a 63 percent decrease, says Arnone. In year three of the experiment, when the heat-afflicted plots returned to normal climate conditions, carbon storage in those plots still lagged that measured in the unheated samples, though the discrepancy was less than in year two. The heat-wave plots took up about 100 grams of carbon per square meter less than the plots that continually grew in normal climate conditions.
Test data suggest that decreased plant productivity was the main cause for the large reductions in net carbon dioxide uptake during the warm year, the researchers note.
The lack of complete recovery in carbon uptake the following year largely stemmed from suppressed microbial activity in the soil, they speculate.
“This is a nicely controlled experiment that documents the
legacy effects of an extended heat wave,” says Alan K. Knapp, a grassland
The decreases in carbon uptake seen in heat-wave-afflicted
plants “are not unexpected,” says Clenton Owensby, an agronomist at
Nevertheless, he adds, he doesn’t buy into all of the findings of the Arnone team. For one thing, nutrients that weren’t used during the years of stifled growth would build up and eventually act as fertilizer. “The system will compensate later by having a period of above-average growth,” he says.
David J. Parrish, an agronomist at Virginia Tech in
Although the heat-wave experiment was intended only to
simulate an abnormally warm year — the kind that
“It’s tough to extrapolate what a long-term change in climate would do to these grasslands,” says Knapp. For one thing, he notes, these tests didn’t address how repeated heat waves would affect carbon storage. Also, in a drastically altered climate, the grassland’s balance of species might change to restore carbon uptake — a phenomenon that this experiment was too short to have considered.
Arnone, J.A. III, et al. 2008. Prolonged suppression of ecosystem carbon dioxide uptake after an anomalously warm year. Nature 455(Sept. 18):383.