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Collapse of forest elephant populations in central Africa rainforests reduces aboveground carbon stocks   
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Référence :

" Carbon stocks in central African forests enhanced by elephant disturbance ". Fabio Berzaghi, Marcos Longo, Philippe Ciais, Stephen Blake, François Bretagnolle, Simone Vieira, Marcos Scaranello, Giuseppe Scarascia-Mugnozza & Christopher E. Doughty. Nature Géosciences, 2019.

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The forest elephant is a species of African elephant that was once widespread in the forests of central and west Africa. Forest elephant populations have been in decline since the colonization of west Africa by Europeans due to unsustainable hunting for ivory and habitat destruction. Recent decades have seen the rate of loss of forest elephants accelerate and today forest elephant populations have been decimated to less than 10% of their original size. Previous studies hypothesized that forest elephants engineer forests by dispersing seeds, moving nutrients and trampling small trees. Trampling small trees had been hypothesized to reduce competition among trees and increase space allowing for trees to grow larger and hold more aboveground biomass. More carbon in these forests means less atmospheric carbon causing climate change.

To solve this ecological puzzle, scientists collected field measurements in the Congo Basin and compared the number and size of trees between different forests, some affected by elephants and some in which elephants had been eradicated. These new data confirmed that when elephants are present, the forest contains larger trees and higher abundances of hard-wood species (trees with higher carbon content). As a consequence, forests with elephants contain fewer trees but higher carbon storage in biomass compared to forests where elephants are absent.  However, forest elephants affect long-term forest processes (>100 years), which are difficult to observe in field experiments.

Therefore, this group used a computer model which can simulate the long-term effect of elephants on forests.  In this model, different abundances of elephants impact how different types of trees compete with each other for light and water. These simulations allowed to study how elephants can affect forest structure, biomass, and potential for carbon storage.

“Forest elephants are natural forest-managers that thin forests by “pruning” or removing small trees which increases the growth of large trees and the production of wood” explains Fabio Berzaghi from the Laboratory for Climate and Environmental Sciences (CEA / CNRS / UVSQ) near Paris, who led the study.

The grim implication of this study is that the ongoing decline of forest elephants in central African forests due to the illegal international ivory trade will lead to forests becoming crowded with small, soft-wood trees with lower carbon content, provoking a significant loss of carbon storage in biomass. Without elephants, central African forests could lose up to 3 billion tons of carbon. “Our simulations suggest that if elephant loss continues unabated, central African forests may release the equivalent of multiple years of fossil fuel CO2 emissions from most countries, thus potentially accelerating climate change.  Therefore, their loss could have a drastic impact both locally and on global climate.” says co-author Chris Doughty. As the largest land mammal on the planet is systematically wiped out so are its myriad roles in forest function, which will have ramifications throughout tropical ecosystems.

The good news is that this trend in carbon loss can be stopped and reversed by protecting forest elephants and allowing their populations to recover. A comeback of forest elephant populations can only be positive for carbon storage and for the ecosystem integrity and biodiversity value of African rainforests. Forest elephants not only thin the forest but disperse seeds widely through the forest assisting the germination of over 100 tree species, which provide food and habitat for primates, birds and insects.  “Our study shows that even at high population densities, forest elephants continue to improve the carbon storage potential of central African forests, so there is no ecological concern for their comeback” said Fabio.

“Forest elephants are the gardeners and guardians of biodiversity in the Congo Basin” says Stephen Blake, one of the paper’s authors. Protecting and expanding the remaining populations of forest elephants in Africa will be a win-win option for wildlife and biodiversity and also for fighting against climate change.

 
A. Mazaud, dépêche du 17/07/2019
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