Cropping Africa's wet savannas would bring high environmental costs

A press release from Princeton University (the Woodrow Wilson School): With the global population rising, analysts and policymakers have targeted Africa's vast wet savannas as a place to produce staple foods and bioenergy groups at low environmental costs. But a new report published in the journal Nature Climate Change finds that converting Africa's wet savannas into farmland would come at a high environmental cost and, in some cases, fail to meet existing standards for renewable fuels.

Led by researchers from Princeton University's Woodrow Wilson School of Public and International
Affairs and Department of Ecology and Evolutionary Biology, the study finds that only a small percentage of Africa's wet savannas (2-11 percent) have the potential to produce staple crops while emitting significantly less carbon dioxide than the world’s average cropland. In addition, taking land conversion into account, less than 1 percent of these lands would produce biofuels that meet European standards for greenhouse-gas reductions.

"Many papers and policymakers have simply assumed that Africa's wetter savannas are expendable from an environmental standpoint because they aren’t forests,” said co-lead author Tim Searchinger, a research scholar at Princeton's Program in Science, Technology and Environmental Policy (STEP), which is based at the Woodrow Wilson School. "Governments have used this assumption to justify large leases of such lands to produce food for the outside world and large global targets for bioenergy. But when you actually analyze the realistic potential to produce food or bioenergy relative to the losses of carbon and animal biodiversity, the lands turn out not to be low cost."

Even if these lands are converted for agricultural use, the only way Africa could become an exporter of crops is by depriving its own people of food, the researchers report. Farming a large expansion of Africa's savannas — nearly half of the world's remaining savannas — would also have negative impacts on the rich and diverse population of tropical birds and mammals....

On the Cameroon savanna, shot by Amcaja, Wikimedia Commons via Flickr, under the Creative Commons Attribution 2.0 Generic license 

Savanna vegetation predictions best done by continent

North Carolina State University Newsroom: A “one-size-fits-all” model to predict the effects of climate change on savanna vegetation isn’t as effective as examining individual savannas by continent, according to research published in Science this week.

Savannas – grasslands dotted with trees – cover about 20 percent of the earth’s land and play a critical role in storing atmospheric carbon, says Dr. William Hoffmann, associate professor of plant and microbial biology at North Carolina State University and co-author of the study.

A single global model can't predict savanna tree density as well as continent-specific models, according to research published in Science this week.

A single global model can’t predict savanna tree density as well as continent-specific models, according to research published in Science this week. Photo of a South African savanna courtesy of Dr. William Hoffmann, a co-author of the study.

“We wanted to find out what controls savanna vegetation – essentially the density of trees within the savanna – and whether we can use a single global model to predict what will happen to savannas if global temperatures rise,” Hoffmann said. “We found that the rules determining tree density are fundamentally different among the three continents studied – Africa, Australia and South America. That means a ‘one-size-fits-all’ approach won’t work.”

…“For example, greater moisture availability – a combination of rainfall, rainfall seasonality and drought indices – meant greater tree density in Africa and Australia, but it had almost no relationship with tree density in South America,” Hoffmann said.

Not surprisingly, he added, the study showed that fire reduces tree density. But the researchers found some strong counter-intuitive relationships between rainfall and fire frequency, namely that more moisture meant more fires. Hoffmann explained that more rainfall in a savanna meant faster-growing grasses, which meant any fires in that savanna would have ample fuel to spread quickly and easily…

A grassland in Slovenia, shot by Eleassar, Wikimedia Commons, under the Creative Commons Attribution-Share Alike 3.0 Unported license

Climatic tipping points for tropical forest and savanna: satellite data reveal where they are most vulnerable

Science Daily: Tropical tree cover will jump sharply between a forested state and savanna or treeless conditions rather than respond smoothly to climate change, according to a study by scientists from Wageningen University published in the journal Science.

The study shows that the resilience of forest and savanna is related to rainfall in an almost universal way on the three continents studied, Africa, Australia and South America. The authors used the resulting empirical model to predict where existing forest is most fragile, but also to reveal where a treeless situation may be most easily tipped into a savanna, or where savanna may be turned into forest with little effort.

"Tree cover is one of the most defining aspects of landscapes," says Milena Holmgren, one of the authors and a specialist on plant ecology. "It is therefore remarkable how poorly we understand what determines where we have forest, and where savanna or desert. Obviously rain is important. Deserts are found in the driest places and rainforest in the wettest. However, what happens in between has been very puzzling for scientists." The study based on massive satellite data now reveals an intriguing global pattern. Instead of a gradual increase in tree cover with rainfall, there appear to be 'forbidden' states around 5% and around 60% tree cover. Thus the system jumps between three contrasting alternative states: forest, savanna (with roughly 20% tree cover) and a treeless state.

"This is one of the most convincing lines of evidence for the existence of large scale alternative stable states in nature," says Marten Scheffer, who leads the research program on tipping points. "We were surprised ourselves how well the data supported this influential, but radical theory." While the proof of the existence of tipping points is of great scientific importance, the practical power of this study is that it also uses the data to reveal where on the globe we have the greatest risks of collapse, and where the greatest opportunities for recovery of forest....

Elephant reaching for leaves on a savanna, in Kenya. Photograph taken by Sandra Fenley while on safari. Wikimedia Commons, under the Creative Commons Attribution ShareAlike 3.0 License