Unusually hot temperatures slow plants' carbon uptake, scientists find

Tall grasses exposed to an unusually warm year of weather lose some of their ability to capture carbon dioxide from the atmosphere the following year, according to a U.S. study to be published Thursday.

The findings, to be published in the journal Nature, suggest that as unusually warm years become more frequent, it could increase the amount of carbon dioxide in the atmosphere, which could in turn raise temperatures.

Plants and soils help regulate the amount of carbon dioxide in the atmosphere, with plants absorbing CO2 and returning the carbon to the soil when they die, and bacteria in the soil returning the carbon to the atmosphere after they feed on the dead plants.

Determining what role a single factor like temperature has, however, is difficult given the number of factors at play in even the simplest ecosystem.

Scientists at the Desert Research Institute in Reno, Nev.,and four American universities, however, based their findings on an ambitious four-year study that sought to isolate the impact of temperature on prairie tall grasses.

"This is the first study to quantitatively track the response in carbon dioxide uptake and loss in entire ecosystems during anomalously warm years," lead author Jay Arnone, a research professor at the Desert Research Institute, said in a statement.

The researchers studied 12,000-kg containerized grassland plots in four living-room-sized sealed environments at a research facility near Norman, Okla.

The chambers replicated sunlight and annual rainfall and temperatures, but six of the 12 plots were exposed to an unusually warm year in the second year of study.

They found that in the unusually warm year, the net ecosystem carbon dioxide exchange decreased by 63 per cent and only partially recovered in the following year.

Two years after the unusually warm year, the ecosystem returned to its usual exchange of carbon dioxide, the researchers found. They said this was likely due to the decreased supply of carbon in the soil decreasing the rate at which soil bacteria returns the carbon to the atmosphere.

"The 'lagged' responses that carry over ... are a dramatic reminder of the fragility of ecosystems that are key players in global carbon sequestration," said Arnone.

Arnone said the findings should be factored into future climate models and that "the response of ecosystems to climate variability seems to be more complex than can easily be inferred from traditional experimental and observational approaches."