An Australian study has found an expected effect of rising CO2 on plants may not work as expected.

Increasing atmospheric carbon dioxide could lead to plants using less water, because plants close the pores (called stomata) on their leaves and take in less water from the soil.

Previous studies have found that by taking in more carbon dioxide, plants can close their stomata earlier and lose less water than they would otherwise.

But there has been little research on this effect in warm, dry ecosystems that cover much of the world in the tropical, subtropical and dry temperate regions including most of Australia.

New research testing this theory on Australian grass species has shown that it is the presence of water that controls whether plants open their stomata more and not because of the extra CO2 in the air.

This is the reverse of what scientists expected to find based on experiments from international research and is another example of the importance of tailored experiments specific to Australia’s unique ecosystems.

“This research demonstrates that water availability in Australia has a big impact on increasing plant photosynthesis together with increased carbon dioxide”, said lead researcher Professor David Ellsworth.

“Results from similar experiments running in cold temperate climate grasslands are quite different in their response to the results here in Australia. Here in Australia … we essentially show that there is no water-savings effect from rising CO2.”

“Currently, global climate change prediction models are based on data that indicates that grasslands will increase their rate of photosynthesis under rising CO2, whereas in fact changes to ecosystems such as increased growth, increases in woody seedling establishment or establishment of different types of plants in the ecosystem are more likely to be the result of fluctuations in water and not as a result of extra CO2.”

Previous research from satellite imagery has demonstrated a trend towards ‘global greening’, attributed to increased CO2 thought to enable plants to use less water and therefore stay greener.

“Satellite imagery can tell us about what has happened in the past up to today”, explains Professor Ellsworth.

“These results indicate that the big changes in carbon absorption by CO2 happen when there is enough rainfall and moisture.

“In Sydney, the climate frequently swings from wet to dry and back again so often that this has much more impact than in other more consistently wet or dry regions. We can expect in the future that the changes in rainfall brought on by rising CO2 as well as the direct effect of CO2 on plants will interact.”

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