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 prove exceedingly difficult, even if the original climate conditions return, Staver said.
"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," Staver said. "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."
The Princeton research could be significant in determining the "future trajectory" of global forest cover, and also illustrates the natural obstacles to restoring cleared forests, said 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," said Walker, who had no role in the Princeton research but is familiar with it.
"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 encro
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