Recent satellite images revealed a trend in dryland vegetation greening, although the reasons behind it have been unclear - until now. A new study by researchers at Indiana University revealed that one of the key drivers behind global dryland greening is high levels of atmospheric carbon dioxide. Drylands are zones that possess a mean annual precipitation that is less than two-thirds of potential evaporation and represent the largest terrestrial biome on the planet, harboring more than two million people.
The team analyzed 45 separate studies from eight different countries and came to the conclusion that rising levels of atmospheric carbon dioxide are causing the greening process due to the effects on plant water savings and the increases in available soil water that result from this process.
"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."
In many regions, the main cause of the greening is likely species changes, with many of the greener invasive plant species replacing indigenous ones, or bushes making way onto grasslands that are used to feed cattle.
Wang and his team believe that greening is a response to higher atmospheric carbon dioxide levels causing a decrease in plant stomatal conductance, which is the measure of the rate of carbon dioxide entering or water exiting through the stomata of the leaf, as well as increases in soil water, thus stimulating vegetation growth.
Wang's earlier work points to even small changes in soil moisture in drylands having significant impacts 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."
The findings were published in the Feb. 12 issue of Scientific Reports.
