Debunking benefits of rising carbon dioxide

Flaws in theory that CO2 will boost plant drought tolerance, Australian research reveals


Australian research contradicts the expectation there would be widespread water savings for grassy ecosystems under elevated atmospheric CO2.

Australian research contradicts the expectation there would be widespread water savings for grassy ecosystems under elevated atmospheric CO2.

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Flaws in theory that CO2 will boost plant drought tolerance, Australian research reveals

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Australian research debunks the assumption that the elevated atmospheric carbon dioxide (CO2) always makes plants more drought tolerant

It was thought that carbon emissions, which continue to add to the level of atmospheric CO2, would boost plant photosynthesis and allow them to use water more efficiently. That would be expected to allow them to tolerate drought.

Research that developed the drought tolerant theory was conducted in North America and Europe but assumed to apply to drought tolerance in warm and dry ecosystems, such as Australian grasslands and grassy woodlands.

But new findings from research in Australian conditions, at Western Sydney University shows increased CO2 does not necessarily benefit plant drought tolerance.

“People believed there would be really large water savings for grassy ecosystems in elevated CO2 that is a result of plants using less water, so there is more left in soil," said lead scientist David Ellsworth.

“Currently, global climate change prediction models are based on data that indicates that grasslands will increase their rate of photosynthesis under rising CO2 because of this,” Prof. Ellsworth said.

These kinds of grassy woodlands are extensively grazed. That makes this research relevant for agricultural community, and whether they can expect benefits (from increased CO2) through extended periods of greening for grazing - Professor David Ellsworth

Prof. Ellsworth lead the research project in grassy woodlands at the University’s Hawkesbury Campus.

“These kinds of grassy woodlands are extensively grazed. That makes this research relevant for agricultural community, and whether they can expect benefits (from increased CO2) through extended periods of greening for grazing,” said Prof. Ellsworth.

Western Sydney University's grassy woodland trial site, located at the Hawkesbury Campus.

Western Sydney University's grassy woodland trial site, located at the Hawkesbury Campus.

The CO2-drought tolerant theory says plants need CO2 to photosynthesise, and with more CO2 in the atmosphere, plants do not work as hard to get it.

To suck in the necessary CO2, plants open pores, called stomata. The more a plant opens its stomata, the greater its water loss from evaporation.

Scientists expected elevated C02 in the atmosphere would make plants open their stomata less.

In turn, plants would lose less water to evaporation, which in turn reduces demand on water in the soil and an increases drought tolerance.

“Plants try to balance the need to take in CO2 against losing water to the atmosphere,” Prof. Ellsworth said.

“Plants have a hardwired censor for atmospheric CO2. Even when you blow on them they start closing stomata.

“You could say they have landed at an optimal place to balance evaporation with the need for water and c02.

“Atmospheric CO2 can short circuit that process and change the way that balance works.”

But Western Sydney University’s research on Australian grass species showed it is the presence of water that controls whether plants open their stomata regardless of extra CO2 in the air.

“This research demonstrates that water availability in Australia has a big impact on increasing plant photosynthesis,” Prof Ellsworth said.

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