- A study carried out by scientists in 18 countries found tropical forests to be more resilient than once believed and largely capable of regenerating over just a few decades.
- The study analyzed 2,200 patches of forest in West Africa and Central and South America, including areas of the Atlantic and Amazon rainforests.
- In the areas studied, soil richness was restored about 10 years after deforestation; after 25 years, the forests’ structure and function had fully returned.
- However, biodiversity took longer to fully return, at an average of 120 years.
Tropical rainforests are more resilient than previously thought, a new study shows, with a high capacity for natural regeneration in areas that are only slightly degraded adjacent to patches of native vegetation.
In the space of less than 20 years, these patches of secondary forest can regain important characteristics and functions of the original forest, such as soil fertility and a significant amount of carbon stock.
But enabling this low-cost regeneration and supporting restoration and conservation projects requires three things: understanding each area’s different characteristics; halting the deforestation; and keeping nearby healthy primary forests standing.
These were the conclusions of the unprecedented study carried out by 2ndFOR, a collaborative research network focusing on secondary forests, which involves more than 100 scientists in 18 countries. The study was published at the end of 2021 in the journal Science.
The study developed by the international group, which includes Brazilian scientists, was innovative because of the two main characteristics of its investigative strategy. The first was an approach seeking to understand the development of each forest function and the relationships between these attributes over the recovery period. Most previous observations had focused on recuperation of one specific ecosystem service or other, such as carbon sequestration, biodiversity or soil improvement.
The second characteristic was the study’s breadth. Previous projects had focused on specific individual locations, while the 2ndFOR study used a database including dry and damp areas of tropical and subtropical forests the world over, including more than 2,200 swaths of forest in Central and South America — including the Atlantic and Amazon rainforests — and West Africa.
“We studied different forests with diverse climates, soils and landscapes,” said study co-author Pedro Brancalion, professor of forest sciences at the University of São Paulo’s Luiz de Queiroz Graduate School of Agriculture (Esalq-USP). “As our sampling was broad and well distributed, our results should be quite close to the ‘true field,’ representing a general average pattern for tropical forest regeneration.”
Brancalion said it’s important to evaluate other types of forests aside from secondary ones in order to define goals and models for regeneration according to each one’s characteristics.
“We noticed that knowledge wasn’t moving forward on all the new types of forests that exist, like planted restoration forests, agroforests and abandoned eucalyptus monocultures. People invest in restoration because they have goals: to sequester carbon, conserve biodiversity or protect wetlands and the soil. Our study gives a general idea of how and when these different goals can be achieved depending on the forests’ characteristics and locations,” he said.
Soil regenerates itself in 10 years
The 2ndFOR team analyzed 12 forest attributes related to four groups of forest characteristics: soil (apparent density, carbon and nitrogen); ecosystem function (nitrogen-fixing tree species, wood density and specific leaf area); forest structure (topsoil biomass, maximum trunk diameter and structural heterogeneity); and biodiversity (richness, diversity and species composition).
They found that soil fertility had recuperated after 10 years, and that ecosystem function was restored in less than 25 years. Between 25 and 60 years, forest structure and diversity returned. And after 120 years, topsoil biomass and composition of species had recovered, which is considered to be the most sensitive attributes of the fallout from deforestation.
“It was surprising to find that soil characteristics returned in less than 10 years,” said co-author Catarina Jakovac, professor of plant sciences at Santa Catarina Federal University’s (UFSC) Agricultural Sciences Center. “The soil has more resistance to disturbances, like when mature forest is transformed into agricultural land, as long as there is no extreme degradation like mining.”
On the other hand, Jakovac said the research team was surprised at how long it takes to recuperate topsoil biomass — things like stems, trunks and branches — which play a fundamental role in the sequestering of atmospheric carbon dioxide.
Meanwhile, the sluggish rate at which species recovered — a parameter used to define an environment’s biodiversity — was no surprise. Many vulnerable plants have large seeds, and as the animals that spread them have gone extinct in some places, their prospects of becoming more widely distributed dwindle.
“The exact same species may never return after deforestation because many are rare and this makes it hard for these plants to become established in a secondary forest,” Jakovac said.
While the study’s findings show the prospects for forest recovery to be good under certain conditions, Brancalion said it should not be taken out of context and misrepresented. The potential for this kind of disinformation is especially rife during the current U.N.-declared Decade of Ecosystem Restoration (2021-2030).
Brancalion, who is also vice coordinator of the Atlantic Rainforest Restoration Pact, said there are many interests at play and people who want to destroy protected forests for intensive farming. Justifications include the claim that they will compensate for damages in other locations.
“The message has to be read with responsibility because it could give the impression that forests can be destroyed and later recuperated,” Brancalion said. “One analogy that I use is the use of oxygen masks in an airplane: adults should place their masks first before helping children. The same is true with forests. The mature forests must be protected so that others have good ecological conditions to be able to recuperate. Any secondary forest is the offspring of the forests surrounding it. If we protect what is left over, there will be a chance in the future to promote regeneration. But if we promote regeneration without protecting leftover standing forest, we won’t get anywhere.”
Poorter, L., Craven, D., Jakovac, C. C., Van der Sande, M. T., Amissah, L., Bongers, F., … Hérault, B. (2021). Multidimensional tropical forest recovery. Science, 374(6573), 1370-1376. doi:10.1126/science.abh3629