Tropical forests are home to more than 53,000 tree species, accounting for 96% of global tree diversity. These hyperdiverse forests are threatened by high levels of deforestation, mostly driven by agricultural expansion. Once agricultural fields are abandoned, they can be rapidly colonized by naturally regrowing forests. Could these secondary forests help reverse species loss and bring native species back?

An international team of ecologists from Latin America, the USA and Europe, led by researchers from Wageningen University & Research, published this week a study in Science Advances elucidating the role of regrowth on diversity conservation of tropical trees. They inventoried trees in 1800 tropical forest plots located in 56 sites across ten countries in Latin America. They used plot data from secondary forests of different ages and compared it to neighbouring, well-conserved, old-growth forests.

Quick recovery of species richness

Results show that species richness of these small forest patches recovers quickly. Within a few decades, but that it may take centuries before their species composition becomes similar to that of old-growth forests again.

Dr. Danaë Rozendaal, researcher Plant Production Systems at Wageningen University & Research and lead author of the study, says that the number of tree species quickly increases in regrowing, secondary forests. “On average, it takes only five decades to recover the total number of species found in well-conserved old-growth forests, and that within only 20 years already 80% of the number of species is present. This emphasizes the importance of secondary forests for biodiversity conservation in human-modified tropical landscapes.”

Recovery of species composition may take centuries

Although the number of species may recover relatively fast, the study also shows that the tree species found in regrowing forests are usually different from those in neighbouring old-growth forest. After 20 years of regrowth, only 34% of the original species composition has recovered. That forest looks completely different. It may take centuries before these regrowing forests harbour the same tree species as the original forest again, and maybe they never attain the original species composition. Although young secondary forests contribute importantly to biodiversity conservation in these modified landscapes, they do not contain many of the species found in well-conserved forests, says prof. Lourens Poorter, leader of the 2ndFOR network. “Therefore both secondary and old-growth forests must be preserved to guarantee biodiversity conservation in human-modified landscapes.”

Planting tree species from old-growth forest

This study has direct implications for forest restoration policies and practice. Natural forest regeneration has been seen as an ecologically sound way to restore large areas of forest at lower costs compared to active tree plantings. Natural forest regeneration may therefore be the ideal method to meet the goal to restore 350 million hectares of forest in 2030, as set under the Bonn Challenge. Prof. Frans Bongers: “It is great news that natural regeneration can restore tree biodiversity relatively fast. However, targeted restoration actions for the introduction of typical old-growth species as well as the conservation of old-growth forests may be necessary to guarantee long-term conservation of tropical tree species.”

Secondary tropical forests

Secondary forests are forests that regrow naturally after nearly complete removal of forest cover for anthropogenic use (usually for shifting cultivation; either agriculture or cattle ranching). Currently over half of 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 area.