In 20 years, natural regeneration of tropical forests recoups 80% of old-growth carbon and soil fertility

A study published in Science shows that secondary forests are more resilient than previously thought and can recover quickly, helping to mitigate the adverse effects of climate change and conserve biodiversity.

By Luciana Constantino  |  Agência FAPESP – A study published in the journal Science shows for the first time that natural forest regeneration can recoup almost 80% of the stored carbon, soil fertility and tree diversity of old-growth forests within 20 years. 

Using one of the world’s largest tropical forest databases, the analysis concluded that natural regeneration is a low-cost solution to mitigate the effects of climate change and contribute to biodiversity conservation. The process is due to regeneration of degraded areas and not merely to the passage of time in these places.

“Tropical forests can regenerate naturally in accordance with human expectations, but destruction happens much faster than recovery. Our findings should be seen as optimistic, but they also call for responsibility,” Pedro Brancalion, a professor of tropical forestry at the University of São Paulo’s Luiz de Queiroz College of Agriculture (ESALQ-USP) in Brazil, told Agência FAPESP.

Brancalion and Frans Bongers, a professor at Wageningen University in the Netherlands, are co-authors of the study and co-principal investigators for the project “Understanding restored forests for the benefit of people and nature – NewFor”, supported by FAPESP under the aegis of its Research Program on Biodiversity Characterization, Conservation, Restoration and Sustainable Use (BIOTA-FAPESP). 

“Another interesting point is that the study establishes a ‘chronological order’ for the restoration of several tropical forest functions, which should be very useful for the management of the restoration and sustainable development projects planned for the coming decade,” said Brancalion, who heads the Tropical Forestry Laboratory at ESALQ-USP and is vice-coordinator of the Atlantic Rainforest Restoration Pact. 

In June 2021, the United Nations launched the UN Decade on Ecosystem Restoration as a rallying call to governments, business organizations and civil society to foster initiatives that halt environmental degradation and protect nature. 

In November 2021, at the end of the 26th UN Climate Change Conference (COP-26), the 197 attending countries signed an agreement reaffirming their determination to try to limit global warming to 1.5 °C and committing to a reduction in global carbon dioxide emissions to net zero by 2050, with financing for reforestation, technologies to capture carbon from the atmosphere, and regulation of carbon markets.

“Knowing how fast forests recover is important, for example, to assessments of the feasibility of investing in projects to capture carbon by regenerating degraded areas,” Brancalion said. “We need to understand the dynamics of restoring each forest function in order to set targets, make better decisions and, depending on the situation, keep watch to see if a forest is recovering more slowly than expected or than required to achieve a particular goal.”

The Science article resulted from a study by an international team called the Secondary Forests Research Network (2ndFOR), involving more than 100 researchers from 18 countries, including Brazil, and coordinated by Lourens Poorter, first author of the article, jointly with Bongers and Masha van der Sande, all affiliated with Wageningen University; and Catarina Jakovac, a professor at the Federal University of Santa Catarina (UFSC) in Brazil.

The group analyzed 12 forest attributes of 77 sites and over 2,200 forest plots across Central and South America, including Amazonia, and in West Africa. The attributes were divided into four groups, relating to soil (apparent density, carbon, nitrogen), ecosystem functions (nitrogen-fixing tree species, wood density, specific leaf area), forest structure (above-ground biomass, maximum trunk diameter, structural heterogeneity); and biodiversity (richness, species composition).

The scientists concluded that tropical forests and their soil are highly resilient, as all attributes recover in up to 120 years. The speed of recovery differs strongly across forest attributes, however. Recovery to 90% of old-growth forest values is fastest for soil fertility (less than 10 years) and plant functioning (less than 25 years); intermediate for structure and species diversity (25-60 years); and slowest for above-ground biomass and species composition (more than 120 years). 

“An important point is that tropical forest species richness recovers, but not always with the same species composition. Not all species in old-growth forests recolonize regenerated forests. Some are more sensitive and may disappear,” Brancalion said.

Deforestation

In a statement released to publicize the findings, Poorter stressed the importance of actively protecting old-growth forest and stopping deforestation. “But we also found that tropical forests have the potential to regrow naturally in already deforested areas on abandoned lands. These regrowing forests cover vast areas and can contribute to local and global targets for ecosystem restoration. They provide global benefits for climate change mitigation and adaptation, and for biodiversity conservation, as well as many other services for local people, such as water, fuel, wood, and non-timber forest products,” he said.

According to a report issued on April 28, 2022, by Global Forest Watch based on data from the University of Maryland, the tropics lost 11.1 million hectares of tree cover in 2021. Brazil topped the list of countries with the most primary tropical forest loss, with over 40% of the total. The report highlights concern about the loss of 3.75 million hectares in tropical primary rainforests, given their critical importance for carbon storage and biodiversity.

It also notes that carbon dioxide emissions resulting from tropical primary forest loss were equivalent to India’s annual fossil emissions.

Over half the world’s tropical forests are not old-growth, but naturally regenerating forests of which a large part is secondary forest. In tropical Latin America, secondary forests cover as much as 28% of the land. Secondary forests regrow naturally after nearly complete removal of forest cover for anthropogenic use (usually for cultivation or cattle ranching). 

In an article published in the journal PNAS in November 2021, the same group of researchers addressed what they called the “functional recovery” of secondary tropical forests, meaning the restoration of characteristics that determine plant behavior, such as leaf thickness and wood density. A third of all Neotropical forests are secondary forests that regrow naturally after agricultural use through secondary succession (natural replacement of species), the authors note. Dry and wet forests differ in functional composition during initial stages of succession and follow different paths over time, but their functional characteristics become more similar as they grow older. 

The article “Multidimensional tropical forest recovery” is at: science.org/doi/10.1126/science.abh3629.