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A vital fuel of global dryland greening is thick atmospheric carbon dioxide, explains a new study by researchers at Indiana University. Recent satellite images show the trend in dryland vegetation greening.

But it is only now that scientists are understanding the reason behind the trends. Dryland zones have a mean annual precipitation less than two-thirds of potential evaporation. They also signify the "largest terrestrial biome" on the planet, with over two million people inhabiting them.

The study was published in the Feb. 12 issue of Scientific Reports.

After examining 45 separate studies from eight nations, the team concluded that due to higher levels of atmospheric carbon dioxide, there is more greening in the world, due to plant water savings and the resultant increase in available soil water.

"We know from satellite observations that vegetation is greener than it was in the past," Lixin Wang, co-author of the study, said in a press release. "We now understand why that's occurring, but we don't necessarily know if that's a good thing or not."

The main cause of the greening in many regions is species change, even as the greener invasive plant species replace the indigenous ones, or bushes give way to grasslands used to feed cattle.

Wang and his team believe that greening is due to higher atmospheric carbon dioxide levels that lead to a reduction in "plant stomatal conductance". This assesses the rate of carbon dioxide entering or water going out through the stomata, and also the increase in soil water, which stimulates vegetation growth.

Wang's earlier work indicates that even small changes in soil moisture in drylands can make a significant impact on vegetation productivity.

"Importantly, the observed response lends weight to the hypothesis that any additional soil water in the root zone is then available to facilitate vegetation growth and greening under enhanced carbon dioxide," said Wang. "Future studies using global-scale process-based models to quantitatively assess the carbon dioxide impact on soil moisture is needed to further validate the hypothesis."