The toco toucan, Brazil. Photo: agustavop / Getty Images.

Hosting much of the Earth’s carbon and biodiversity, tropical forests are, arguably, the planet’s most important ecosystems. Yet they continue to be destroyed by humans at an alarming rate, with devastating implications for climate change and the world’s species.

An international team of scientists from Europe, Brazil and Australia, including SEI’s Toby Gardner, measured carbon and surveyed more than 1600 plant, bird and dung beetle species in 59 naturally regenerating secondary forests and 30 undisturbed primary forests in the eastern Amazon.

Their study, published in the journal Global Change Biology, shows that primary forests contain more biodiversity and carbon than even relatively mature regenerating forests. This underlines that protecting primary forests must remain a conservation priority.

However, the study also provides strong evidence that secondary forests can also play a critical ecological role.

“We found that the carbon and biodiversity of secondary forests recovered to more than 80% of the levels found in undisturbed primary forests,” says the paper’s lead author Gareth Lennox of Lancaster University. “This is undoubtedly good news for climate change mitigation and biodiversity conservation.”

The expansion of secondary forests

The world’s remaining primary tropical forests are still under threat from deforestation – an area of which equal to the size of Austria continues to be felled each year. However, a large and growing area of tropical farmland is being left to regenerate. Part of the reason is migration of former small-scale farmers from rural to the cities as economies grow. Their abandoned landholdings gradually return to forest.

In Brazil, secondary regenerated forest in the area of the Brazilian Amazon has grown from less than 30 000 km2 to over 170 000 km2 in the past 30 years.

“A win-win scenario” for biodiversity and climate

With international funding to meet biodiversity and climate targets far short of what is needed, discovering areas where biodiversity and carbon can be protected and enhanced simultaneously, and at relatively low cost, is a key conservation goal.

“We found that biodiversity recovered at least as quickly as carbon in the study areas, which is not the case in all tropical forests. Secondary forests in the eastern Amazon may therefore present the win-win scenario in which conservation investments can alleviate climate change and protect tropical species at the same time,” says Gardner.

Furthermore, co-author Joice Ferreira of the Brazilian Agricultural Research Corporation (EMBRAPA) points out that not only can secondary forests provide high-value ecosystem services, but these services are available through natural regeneration, without active (and potentially costly) intervention.

Forest regeneration is already part of the strategies of international initiatives like the Convention on Biological Diversity and REDD+ to protect and enhance tropical biodiversity and carbon stocks. Brazil has pledged to restore an additional 120 000 km2 of forest by 2030.

Protecting the gains

Secondary forests are frequently re-cleared, meaning that any conservation gains are lost – and potential gains often never achieved. While the team found species richness had recovered to on average, nearly 90% of the levels found in equivalent undisturbed primary forests, this happened gradually. It sometimes took decades for biomass to regenerate to levels where species of high conservation value returned. In the first 20 years of regeneration, biomass recovered at 1.2% per year, equivalent to a carbon uptake rate of 2.25 Mg/ha per year. Forest species of high conservation value tended to return only once carbon uptake had reached 75 Mg/ha.

According to Jos Barlow, a study co-author from Lancaster University, “In the Brazilian Amazon, the average time before secondary forests are deforested again is just five years. Moreover, across the tropics, secondary forest management regimes are beset by legal uncertainties, inconsistent decision-making, and the chronic undervaluation of these important ecosystems.

“For secondary forests to achieve the socio-ecological potential we uncover … they must be incorporated as key elements of landscape management and conservation planning.”

This story is adapted from a text by the Environment Centre, Lancaster University.