Savanna vegetation predictions best done by continent

Jan 30, 2014
Savanna vegetation predictions best done by continent
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. Credit: Dr. William Hoffmann, NC State University

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 , says Dr. William Hoffmann, associate professor of plant and microbial biology at North Carolina State University and 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."

The researchers examined more than 2,100 sites in savannas across the three continents. They found that tree density was influenced by a number of different factors, including moisture availability, temperature, soil fertility and frequency of fires. Yet the power of these relationships differed significantly among the three continents.

"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.

The researchers also modeled what would happen to tree density if the mean annual temperature increased by 4 degrees Celsius. Because tree density is controlled differently on the three continents, the study predicts different responses to global warming. In Africa, tree density is predicted to climb in hotter temperatures, while it is expected to decline in Australia and Africa. These differences, Hoffmann says, could not be predicted if are assumed to behave identically across the tropics.

"Climate modelers examining atmospheric carbon dioxide levels use these types of models to make projections on carbon storage, which has major global implications," Hoffmann says. "We're trying to make these models better."

Caroline E.R. Lehmann of Macquarie University in Australia led the study. Researchers from some 20 universities around the globe contributed to the work.

Explore further: Temperature found to be most significant driver of the world's tallest trees

More information: "Savanna Vegetation-Fire-Climate Relationships Differ Between Continents," by C.E.R. Lehmann et al. Science, 2014.

Related Stories

Forest and savanna can switch quickly

Oct 14, 2011

(PhysOrg.com) -- Two recent studies have found that environmental changes can bring previously stable forests and grasslands to tipping points that produce sudden large-scale and sometimes irreversible changes ...

Recommended for you

Education is key to climate adaptation

6 hours ago

Given that some climate change is already unavoidable—as just confirmed by the new IPCC report—investing in empowerment through universal education should be an essential element in climate change adaptation ...

India court slams Delhi's worsening air pollution

16 hours ago

India's environment court has slammed the government over the capital's horrendous air pollution, which it said was "getting worse" every day, and ordered a string of measures to bring it down.

User comments : 0

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.