Heat waves stunt grassland growth

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 Reno, Nev.

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 of Oklahoma grassland, took place in climate-controlled chambers larger than railroad boxcars. During the four-year-long experiments, half of the prairie plots suffered a one-year-long spike in temperature. The grassland samples that experienced heat-wave conditions took up only one-third the carbon dioxide of those experiencing normal climate, Arnone and colleagues report.

Environmental conditions inside the chambers mimicked those in central Oklahoma, where the samples — each 2.44 meters long, 1.22 meters wide, 1.8 meters deep and weighing 12 metric tons — were obtained. Each sample was extracted from the ground in one piece, minimizing adverse effects on resident plant roots, soil microbes and other subterranean creatures, says Arnone.

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 Oklahoma climate, with one exception: In year two of the experiment, the temperature in chambers housing six of the samples was cranked up an additional 4 degrees Celsius, says Arnone.

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 ecologist at ColoradoStateUniversity in Fort Collins. “It’s a nice demonstration of what many of us had already suspected,” he notes.

The decreases in carbon uptake seen in heat-wave-afflicted plants “are not unexpected,” says Clenton Owensby, an agronomist at KansasStateUniversity in Manhattan. He notes that his team’s field tests indicate that “whenever grassland plants start out with a lowered amount of food reserves stored from the previous season, their growth never catches up.”

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 Blacksburg, agrees. “These plants are highly adapted, and they’ll eventually recover.”

Although the heat-wave experiment was intended only to simulate an abnormally warm year — the kind that Oklahoma already suffers occasionally — the new findings may have implications for how grasslands might respond as the planet’s climate warms. By later this century, Earth’s average temperature might have increased to the point where today’s extremes might be considered normal, says Arnone.

“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.

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