There is usually only bad news from the tropical rainforest. For example, according to the United Nations, most of the rainforest is lost in Africa: 39,000 square kilometers per year. In tropical America it is the gigantic area of 26,000 square kilometers annually. An international team of tropical ecologists recently wrote in the science journal Science but finally to report something positive. According to this, tropical rainforests can recover surprisingly quickly after they have been completely cut down. After just 20 years, they would have regained on average almost 80 percent of their characteristic features.
“It is important to actively protect old forests and to stop further deforestation,” says Lourens Poorter from Wageningen University, who is the first author of the study. “But tropical forests have the potential to naturally grow back on abandoned land in areas that have already been deforested.” To arrive at this conclusion, the scientists analyzed for 77 locations in America and Africa how the forest gradually recovered. On more than 2,200 plots, they assessed the progress of regeneration using twelve parameters.
After ten years the soil is fertile again
“We actually expected the vegetation to recover first and then the soil,” the ecologists write in their study. But to their surprise, it was the other way around: According to the data, the fastest regeneration is the soil. After less than ten years, he has regained 90 percent of the fertility of the soil in an intact forest. After 25 years, the regrown plants can fulfill many functions of a forest, such as nitrogen fixation, up to 90 percent. And it takes the longest before as many different plants grow again in the new forest as in the old one: 120 years. It takes about the same amount of time for the total aboveground biomass of the plants to be the same as it was before the deforestation.
“We also looked at how the recovery of the various forest characteristics is interrelated,” says Dylan Craven of the Chilean Universidad Mayor. According to this, only three indicators can be used to estimate how well a forest has already regenerated: the height of the trees, the structure of the forest and the number of different tree species that grow in it. This is important because these parameters can be measured comparatively easily.
According to the authors of the study, these so-called secondary forests, which naturally grow back on deforested and then abandoned areas, can play an important role in the fight against climate change and the extinction of species. They cover huge areas: in tropical Latin America about more than a quarter of the total land area.
There are various causes for deforestation in the rainforest, which differ from region to region. In Africa it is shifting agriculture: smallholders cut down trees and plant fields in the open space. When the soil has become depleted and sterile, they move on and clear a new plot. In South America, forests are mostly cleared to make pastures for livestock. In Southeast Asia, on the other hand, mainly plantations are created on the cleared areas, for example to produce palm oil.