The grass is greener only when the rain falls in spring

For a planet beset by the negative impacts of climate change, it has long been assumed there would be at least one significant benefit: rising carbon dioxide levels would lead to huge growth in the world’s grasslands.

Grasslands cover one-third of the world’s land area. They are worth more than $22 billion a year to Australia’s economy and account for more than 40 per cent of the country’s agricultural productivity.

A ‘bonanza’ of rising productivity has been widely expected, with suggestions of a 20 per cent increase in grassland growth rates thanks to the fertilisation effects of increased CO2 in the atmosphere.

But new research published today in the journal Nature Plants shows that for these expectations to be met it needs to rain in the right places at the right times. And that is not going to happen.

Study lead Mark Hovenden, an Associate Professor at the University of Tasmania’s College of Sciences and Engineering, said the research revealed that the increase of the growth rates in the world’s grasslands would be closer to 6 per cent than 20 per cent.

“We found the fertilisation effect of high carbon dioxide increases as the amount of spring rainfall increases,” Associate Professor Hovenden said.

“But the fertilisation effect decreases as the amount of rain at other times of the year goes up.

“Growth is therefore stimulated more in sites that have most of their rain in spring.”

Combining the results of 19 different experiments on the effect of carbon dioxide and precipitation on grasslands, from Tasmania to Denmark, the research shows that any increases in plant growth in response to rising atmospheric carbon dioxide levels will always be restricted by the natural distribution of rainfall.

The findings also showed the opposing effects of rainfall in different seasons may account for why previous studies had been unable to explain why the carbon dioxide fertilisation effect was strong in some sites and negligible in others.

The results were uniform around the globe.

“Importantly, these results are from actual experiments distributed across the world and show that the response is uniform across all grasslands, despite large differences in climate and local conditions,” Associate Professor Hovenden said.

“Therefore, this is something fundamental about the way this system will respond to the rising CO2 concentration.”

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