DAILY SCIENCE

Not everything needs to be a forest
Too often viewed as degraded forests rather than valuable grasslands, savannas are threatened by carbon-storing afforestation programs that might not even work.
December 4, 2019

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It was in Africa’s savannas that humanity’s ancestors evolved to walk upright—yet these savannas are threatened by a human-dominated age, one in which they’re officially designated as degraded forests and scheduled for replacement.

The plight of savannas is the subject of two recent papers, one about Asia and one about Africa, but both sharing an essential concern: that people are quick to see tree-dotted grassy plains as mere ecological placeholders for the forests that ought to be there. “Asian savannas have been misinterpreted as degraded forest since the colonial period,” write Dushyant Kumar, an ecologist at Germany’s Senckenberg Biodiversity and Climate Research Centre, and colleagues in the journal Biological Conservation. “There is an urgent need for a correct interpretation.”

Tropical savannas presently cover about one-fifth of Earth’s land surface, write Kumar’s team, but they are threatened. By suppressing naturally-occurring fire and eradicating shrub-munching large herbivores, people have allowed forests to encroach. Savannas are also the target of afforestation programs that promise to sequester carbon in newly-grown forests—but whether these will work as intended is a subject of ongoing scientific debate, and “these agendas may omit the potential negative consequences for biodiversity.”

The researchers modeled the vegetative future of South Asia’s savannas from now until the century’s end under a variety of climate scenarios. Forest area is expected to increase by about 44 percent; grasslands are expected to contract by nearly 40 percent. And that’s not even taking into account climate-oriented afforestation projects. “The continuous effort to afforest savanna areas poses major threats to their biodiversity,” write the researchers, depriving species adapted to savanna life of their only home. “Ecosystem management policies in South Asia should adopt a grass-centric perspective and prioritize grassland and savanna conservation.”

Their sentiments are echoed in the journal Trends in Ecology & Evolution, where researchers led by biologist William Bond of the University of Cape Town lament that one million square kilometers of African savanna—an area roughly the size of France and Germany combined—is now targeted for so-called restoration by the year 2030. “The target is based on the erroneous assumption that these biomes are deforested and degraded,” they write. Among the landscapes formally mapped as degraded are the ancient savanna landscapes of the Serengeti and Kruger National Park, which have not been forests for several million years.

More than a billion dollars have already been pledged by Germany and the World Bank; 28 African countries have signed up for the AFR100, an offshoot of the Bonn Challenge, the international forest restoration and carbon sequestration initiative launched in 2011 with a goal of putting trees on 3.5 million square kilometers by 2030. Yet Bond’s team, like Kumar’s, also points to growing scientific debate over the climate impacts of afforestation—especially if new growth is, as will likely be the case in Africa, plantations rather than diverse forests.

They highlight recent research suggesting that the Bonn Challenge’s 3.5 million square kilometers would, if covered by natural forests, sequester 42 gigatons of carbon, but that figure falls to a mere 1 gigaton if the forests are the pine and eucalyptus plantations expected in much of Africa. Forests may also absorb more solar radiation than do grasslands, thus offsetting the extra carbon they store. And when eucalyptus and pine plantations, which are particularly vulnerable to high-severity fires, burn, most of the carbon they store is released back into the atmosphere. In grasslands, argues Bond’s team, most carbon is stored below ground and persists through fire. “Converting African savannas to plantations is pointless as a mitigation measure,” they write.

Bond’s team stresses that truly degraded forests ought to be restored and existing forests protected. But large-scale afforestation “is based on the wrong assumptions,” they argue. “Far from being deforested and degraded, Africa’s savannas and grasslands existed, alongside forests, for millions of years.” Rather than covering them with trees, people might “promote energy efficient cities in this rapidly urbanizing continent so that Africa follows a less carbon-intensive trajectory of development than other emerging economies.”

Sources: Bond et al. “The Trouble with Trees: Afforestation Plans for Africa.Trends in Ecology & Evolution, 2019.

Kumar et al. “Misinterpretation of Asian savannas as degraded forest can mislead management and conservation policy under climate change.” Biological Conservation, 2019.

Image: Pixabay

 

About the author: Brandon Keim is a freelance journalist specializing in animals, nature and science. He is now writing Meet the Neighbors, a book about what animal personhood means for our relationships to animals and to nature. Connect with him on Twitter, Instagram and Facebook.

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