Battle of the biomes: Savannas vs. forests

PRINCETON (US) —Large stretches of South American and African forest and savanna could begin to encroach on each other due to factors such as climate change and land use—much to the detriment of the people and animals that rely on them.

Savanna wildfires keep tree cover low and prevent forests from encroaching on the grassland. When tree cover is high, as in a forest, fires cannot spread as easily, halting the savanna’s advance into the forest.

But new research published in Science suggests that savanna wildfires could be heavily influenced by factors such as climate change, road construction, and fire-prevention measures. Less rainfall can result in an uptick in fires that can transform a forest into a savanna, just as breaking up the landscape through road construction and fire control disrupt natural blazes and allow a forest to sprout where there once was a savanna.

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Because of these factors, forests and savannas in South America and Africa could degenerate into chaotic mutual encroachment. The changeover from one biome to the other—which can happen within several decades—can be extremely difficult to reverse once it has happened, explains Carla Staver, a doctoral student in the laboratory of co-author Simon Levin, professor of biology at Princeton University.

Staver and Levin worked with co-author Sally Archibald, a senior research scientist at the Council for Scientific and Industrial Research in South Africa.

Plants and animals that thrive in a forest or savanna often cannot transition from one habitat, or biome, to the other, Staver says. The Science paper illustrates that the loss of savanna to forest is just as ecologically traumatic—though less well known—as deforestation.

“Savanna and forest are definitely not locally compatible,” she says. “There is a risk of losing plants and animals endemic to one or the other, which would affect the people who depend on those species.

“Savannas, for instance, are useful to people as cattle rangeland. When forests encroach, the grass productivity decreases dramatically and the land becomes much less useful. In terms of livelihood, that would have a huge impact.”

Fire feedback

The team’s work provides among the first experimental evidence that the ecological effect of fires—known as fire feedback—is the dominant force in maintaining the division between forests and savannas, and that it can determine where habitats flourish.

Using satellite data of fire distributions—combined with climate and soil data, as well as satellite data of tree cover—to survey the tropical and subtropical regions of Africa, Australia, and South America, the researchers found that the frequency of fires determines whether forest or savanna will dominate an area more than other factors such as rainfall, seasons, and soil texture, especially in areas with moderate precipitation.

Regular fires prevent trees from establishing and savannas from turning into forest. A lack of fires allows a forest to develop, which in turn excludes future fires. Human alterations to the climate and landscape, however, may disrupt the natural spread of fire and lead to rapid changes in biome distribution.

Direct actions such as building roads and deploying methods such as controlled burning that prevent the natural spread of wildfires could break up savannas, altering wildfires and allowing forests to take root. At the same time, drier conditions—particularly in areas now experiencing diminished monsoons—rob forests of their primary safeguard against fire and rain.

Under these circumstances, a forest can overtake a savanna, or vice versa, in a matter of decades, and a return to the original terrain would be exceedingly difficult, even if the original climate conditions return, Staver says.

“If a savanna were to turn into a forest, for instance, that change would be quite sudden, much quicker than we might expect, and it would be hard to reverse. You’d cross a threshold where fire cannot spread anymore. Conversely, if a forest dried out and fire started to spread, it could turn into a savanna, maintained by fire. The magnitude of change needed to return a biome to its original state would be much more than it needed to change in the first place.”

Losing a savanna

The research could be significant in determining the “future trajectory” of global forest cover, and also illustrates the natural obstacles to restoring cleared forests, says Brian Walker, who studies ecological sustainability and resilience as a research fellow at the Commonwealth Scientific and Industrial Research Organization in Australia.

“Savanna systems are very resilient across a range of climatic and herbivore variation, in regard to fire. Forest systems are less so, except under very high rainfall where fire cannot be regular,” says Walker, who was not involved in the current study.

“In the case of rainforests, once they are in a state where fire can play a role and therefore keep the system in a savanna state, it is extremely difficult to prevent fires from recurring, and so the chances of a savanna state getting back to rainforest are small.  In the original forest state, the amount of dry fuel in the ground layer is insufficient for fire to take hold and run.”

In a broader sense, the Princeton findings stress that encroachment is not a threat unique to forests, Staver says.

“There’s a sense among savanna ecologists that the loss of savanna is considered secondary to deforestation as a conservation concern, but it really shouldn’t be. The loss of functionality and diversity in the savanna is just as important as in forests.

“At the moment, we can’t say that one is winning out over the other,” Staver says. “We can come up with examples where savannas are encroaching into forests and forests are encroaching into savannas. Both are happening extensively, and both are really huge issues that are likely to become even more important.”

This research was supported with funds from the Andrew W. Mellon Foundation.

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