Forests recover in a few decades when deforestation stops, study shows

Deforested forests in the tropical regions of the Americas and Africa manage to recover a good part of the characteristics they had in a few decades, shows the most extensive study ever carried out on the subject.

The data indicate that, in many cases, reforestation can be a very simple and natural process, without requiring large investments — it is enough to interrupt the agricultural use of deforested areas and let nature do the rest of the work.

The conclusions are in an article in the latest issue of the specialized journal Science. The work is the result of a large international collaboration, coordinated by Lourens Poorter, from the University of Wageningen, in the Netherlands, and with the participation of scientists from several Brazilian institutions.

The team followed the forest regeneration processes in 77 different locations, ranging from Mexico to Rio Grande do Sul (in the case of the American continent) and, on the other side of the Atlantic, covering West Africa, including countries like Ivory Coast and to Nigeria.

As it was not feasible to monitor the recovery of the forest over several decades in all these places, the researchers adopted a methodology that, in a way, is like simulating a time machine.

Rather than monitoring individual sites over time, they chose areas that were known to be undergoing forest regeneration during different periods, both shorter and longer. This allowed them to estimate the duration of the phases of this process.

“It’s an imperfect measure, but it helps to understand what happens in each period,” he told leaf Pedro Brancalion, professor at the Department of Forest Sciences at USP de Piracicaba and one of the Brazilian co-authors of the research. He explains that the work focused on deforested areas that suffered from mild to moderate agricultural impacts.

“You can think, in the case of the Amazon, for example, in an area that started to receive pasture, but still has an ecological memory of what the forest used to be. There is still a seed bank [as sementes das espécies nativas que ainda ficam no solo mesmo após o desmate], there are still seedlings [sementes já germinadas]. It’s different when you carry out intensive soil preparation and apply herbicide so that nothing else will grow”, he says.

The team compared the different stages of regeneration in secondary forests (that is, those that re-emerge after the land is no longer intended for agricultural use) with what is seen in primary, or “virgin” forests. On average, they found that regenerated forests recover almost 80% of the attributes of primary forests after 20 years.

Within this average, however, different attributes of recovering forests function at very different paces. Interestingly, the easiest thing is to get back to adequate levels of nutrients in the soil (measured by the presence of carbon and nitrogen), something that happens in less than ten years.

Some of the so-called functional aspects of vegetation, such as its ability to produce wood and foliage during the tree growth process, also recovered relatively quickly —in less than three decades—to form a proper forest again.

“The recovery of biodiversity, on the other hand, is the slowest, of course”, says biologist Fernando Periotto, a professor at UFSCar (Federal University of São Carlos) who commented on the research at the request of leaf.

In this case, although the plant species richness of the regenerated forest grows relatively quickly, it takes more than a century to become similar to that of a primary forest.

What happens is that, in the first three or four decades of forest recovery, both species of trees that “like” a lot of light, typical of more open areas, and those that are adapted to growing in shaded places coexist in the same place.

As the new forest matures, however, the former lose space, while the rarer species, whose seeds need to be brought from more distant intact forests, take a long time to establish themselves again in the regenerated forest.

For Brancalion, the data indicate that even regions that have been historically explored by agriculture, such as the Brazilian Southeast, could naturally recover at least part of their forest areas. “Forest cover in the Paraíba valley [interior de São Paulo] has doubled in the last 20 years”, he exemplifies.

The process tends to be easier in steeper and wetter areas, he said. In drier areas, one of the big problems is the presence of invasive grasses of African origin, originally brought in to form pastures, which aggressively compete with native plants for water and also tend to increase the incidence of fire.