The Atlantic forest originally covered over 1.2 square kilometres (red); today, just 10% of this remains in small pockets along the coast (black) (Image: Conservation International)
The Atlantic forest originally covered over 1.2 square kilometres (red); today, just 10% of this remains in small pockets along the coast.
We all know it takes a long time for cleared rainforests to regenerate, but how long exactly? According to a study focusing on the Brazilian Atlantic forest, certain aspects can return surprisingly quickly – within 65 years. But for the landscape to truly regain its native identity takes a lot longer – up to 4000 years.
The Atlantic forest originally stretched along the southern half of Brazil's Atlantic coast, covering some 1.2 million square kilometres. Once lush, the forest has been continually exploited for food, wood and space.
Today, land it used to occupy is home to most of the country's population, including Brazil's two largest cities, São Paulo and Rio de Janeiro, and only 100,000 square kilometres of forest remain. In 1993, however, the government created several protected areas to conserve the forest's remnants.
To determine how long it would take for the forest to regenerate, Marcia Marques and colleagues at the Federal University of Paraná collected data on different parcels of forest that had been virtually cleared and left to recover for varying amounts of time.
They then plugged the data into a computer model to calculate how long it would take for the forest to recover entirely.
Animal dispersal
The researchers looked at four different measures of forest regrowth: the proportion of tree species whose seeds are dispersed by animals, the proportion of species that can grow in shade, tree height, and the number of native species.
"Animal-dispersed trees sustain a large number of fruit-eating animals, that sustain other animals including large carnivores," says Marques. "Thus, from the proportion of animal-dispersed trees we can estimate how complex the forest's ecological web has become."
Animals are key to the successful regeneration of cleared areas and, typically, 80% of the tree species in a mature tropical rainforest are animal-dispersed. The researchers found that it took just 65 years for a forest to recover to this level.
Isolated species
Another indication of forest regeneration is the existence of a high proportion of shade-loving trees.
This is because immediately after trees are felled, the land tends to be repopulated by opportunistic species that thrive in the Sun. Shade-lovers take considerably longer to find their way back into the forest – about 160 years according to Marques.
But it's recovering the proportion of native species that are unique to the original forest which takes the longest time – the model predicts this will take up to 4000 years.
Native species that are unique to the Atlantic forest have been isolated over the years into separate plots of forest. As a result, their seeds take a long time to disperse to protected areas. "If endemic species do not germinate and grow, the forest cannot recover fully," says Marques.
Unusually resilient?
"This is a very interesting study," says Nick Brown of the University of Oxford.
"What is intriguing is that everyone is led to expect a rapid decline if you fragment a tropical forest," he says. "But this study shows the Atlantic forest has a surprising resilience."
Brown's own research on one parcel of Atlantic forest has come to similar conclusions.
"It causes me to wonder whether this is something that is unique to the Atlantic forest," he told New Scientist, adding that, because it has a long history of natural and human disturbances, the Atlantic forest may be populated by species that are naturally resilient.
So the rapid regeneration in Brazil's Atlantic forest unfortunately might not apply to other fragmented tropical forests.
