September 26, 2011
The Amazon rainforest can recover from logging, but has a far more difficult time returning after repeated burning, reports a new study in mongabay.com's open-access journal Tropical Conservation Science. In areas where the Amazon had been turned to pasture and was subject to repeated burning, Visima trees become the dominant tree inhibiting the return of a biodiverse forest. The key to the sudden domination of Visima trees, according to the study, is that these species re-sprout readily following fires; a capacity most other Amazonian trees lack.
"Throughout much of the Amazon Basin, abandoned pastures are often dominated by species of Vismia because it is the only tree genus capable of regenerating shoots from below ground tissues. Repeated burning of pastures kills other advance regeneration," the paper's authors write.
When left to regenerate, logged Amazon areas see the return of Cecropia trees, which is similar to what happens when the Amazon sees natural disturbance. However, repeated fires suppress Cecropia trees, and favor instead Visima trees: researchers found that 100 percent of Visima trees re-sprouted after fires. But, where Cecropia trees dominate, so does biodiversity: twice as many species were found in these regenerating forests than in the Visima dominated. Visima forests therefore become what researchers dubbed a 'wasteland', while Cercopia forests held the potential to return to biodiverse rainforest.
The scientists further found that seed dispersers did not play a significantly different role in Visima over Cercopia secondary forests, as both forests saw very few species brought in by dispersers.
"Seed dispersal of mature forest species into Vismia-dominated stands is close to nil, but this is no different from dispersal into Cecropia-dominated stands where succession is not arrested. Therefore, the arresting mechanism lies in the early years following abandonment when Vismia, surviving pasture burns, becomes dominant by default," the authors explains. Visima forests can arrest any forest recovery for decades.
Given this knowledge the authors recommend new policies to dissuade burning of the forests. Instead, a logged forest should be allowed to regenerate without additional burning to turn the area into pasture.
"As most of the forest value lies in the timber extracted, clearcuts should be abandoned without conversion to pasture," the authors write, adding that, "in order to avoid extensive forest conversion into unproductive Vismia wastelands in the Amazon Basin, forestry permits for harvesting timber should include restrictions on subsequent anthropogenic degradation, such as conversion to pasture and prescribed burning."