UNC-TV Science: Savanna Ecosystems

UNC-TV Science: February 4, 2014
Complex Controls on Savanna Ecosystems

If you watch Animal Planet or the National Geographic Channel long enough to get through the shows about Doomsdays and Bigfoot, you’ll eventually see a savanna. Or you could check out this white lions film from Nature. These dryish, grassy areas with the occasional tree poking up are the popular settings for shows about lions, cheetahs and hyenas, who all seem to be stalking the same hapless antelopes.

South African savanna courtesy of Dr. William HoffmannSavannas take up 20% of the Earth’s land surface and sprawl across Africa, Australia and South America and they all have that same grassy-with-occasional-tree look to them, so you might think that their ecosystems behave and adapt to climate changes in pretty much the same way. But according to a new study of more than 2000 savanna sites around the world, you would be wrong.

North Carolina State University ecologist William Hoffman and Wake Forest University ecologist T. Michael Anderson, along with 20 other scientists, found that African savannas respond differently than Australian savannas, and Australian savannas respond differently than South American ones.

Savannas are home to enormous amounts of grass, shrubs and trees, all of which draw carbon dioxide from the atmosphere as they grow. So understanding how these ecosystems respond to environmental and climatic pressures is important for determining how our planet will adapt for future climate changes.

Hoffman, Anderson and their colleagues measured the changes in above-ground woody biomass (AWB) or the number and size of those occasional trees in the savanna landscape. The big factors that change AWB are ones you might predict: temperature, soil fertility, rainfall and fire.

But those factors don’t have the same effect on every continent. In Africa and Australia, more rain generally meant more AWB. In South America, however, changes in moisture did not cause the same increases in the number and size of trees. Furthermore, at the upper limits of rainfall in Africa, AWB tends to level off, meaning that adding more and more water will still keep roughly the same AWB. This is not so in Australia where the data suggests that more and more water will bring more and more trees.

Across all three continents, fire was one of the largest controls over AWB. But the researchers also found that increased rainfall meant more fires. Rainfall had a different effect on the number of fires in each continent, but the general trend was the same. One reason that rain may contribute to fires is that more rainfall causes more grass to grow, and savanna grasses burn easily. So more rain provides more fuel for natural fires.

The researchers also used temperature data to predict how AWB would change if global temperature increased by 4 degrees Celsius and predictably, the three continents did not respond the same way. In South America and Australia, AWB decreases with a greater temperature. In Africa, however, at higher temperatures, more trees will grow.

So what does this study tell us? Since each of these savannas responds differently to climate and the atmosphere, scientists can’t draw up a one-size-fits-all model to describe all the changes to savanna ecosystems. Further, when determining how to preserve savannas worldwide and allowing them to continue drawing carbon from the atmosphere, a different strategy will be necessary for each continent.

The findings are published in the journal Science.

- Daniel Lane

Daniel Lane covers science, medicine and the environment as a reporter/writer. He is currently pursuing a master's degree in medical and science journalism at UNC - Chapel Hill.