Did trees fall in the forest?

YaleNews

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In the category of “relatively good news,” Africa’s tropical forests aren’t disappearing as rapidly as assumed. While long-standing estimates have pegged the continent’s forest loss since 1900 at somewhere between 35 and 55 percent, new research cuts it down to 21.7 percent.

“All of the old work on tropical Africa used climate data to model and predict where forests ought to have been in the past,” says Carla Staver, an assistant professor of ecology and evolutionary biology at Yale. These models rest on the widely held, and usually accurate, assumption that from climate you can predict vegetation. “But there are places where this relationship breaks down,” she says. The margin between savanna and forest is one of these places—and the old models suggested that much tropical forest in Africa had been supplanted by savanna.

Staver and two postdocs, Julie C. Aleman and Marta A. Jarzyna, set out to calculate the extent of recent deforestation more reliably. Given the scarcity of data about Africa, which is grossly understudied, they developed a methodological bricolage: pollen, phytoliths, and charcoal residue collected from lake bottoms; carbon isotopes pulled from soil; old maps. “We had to be creative,” Staver says.

The result: much of the savanna previously thought to have replaced forests lost due to global warming or to land-use changes, like clearing trees, has been there far longer. The new deforestation estimate of 21.7 percent should elicit cheers.

But, as with most environmental findings, tugging on the good news reveals new concern. One of the core proposals for mitigating climate change is to plant more trees—restoring forests while capturing carbon—especially in tropical Africa. “Now I don’t think I’d go so far as to say we shouldn’t be doing that, but if we want to plant trees we ought to do it with our eyes open,” Staver says. We won’t be restoring lost forest, she notes; we’ll be destroying natural savanna. “This isn’t the win-win that we’ve been telling ourselves it is.”