In the tropics, trees are cleared to make way for farmland that is abandoned once the soil is no longer productive – but these sites can become forest again surprisingly quickly
9 December 2021
Tropical forests can re-establish themselves quickly on land that was originally deforested for agriculture and then abandoned. The finding suggests that so-called recovered forests, also known as secondary forests, could play a key role in restoring ecosystems and tackling climate change.
The destruction of tropical forests is happening at an alarming rate to make way for crops and pastures for livestock. After these lands are depleted of nutrients, they are typically abandoned, which can lead to the natural regrowth of the forest that was once there.
To better understand this process, Lourens Poorter at Wageningen University and Research in the Netherlands and his colleagues analysed 77 secondary forest sites across the tropical regions of central and south America, and in west Africa.
Most of the land at the sites was only subject to low to medium intensity farming, for which soil degradation wasn’t extreme. The sites were all at various stages of regrowth – for example, some were 20 years old, and others were 120 years old – which allowed the team to reconstruct what forest recovery looks like over time.
“These forests are a bit like apples and pears – you have really tall, jungle-like rainforests and you also have rather small, dry tropical forests that [are] maybe 15 metres tall,” says Poorter.
So, to compare recovery across sites, the team contrasted each secondary forest with nearby old-growth forests – forests that haven’t had major disturbance. The more similar the secondary forests were to their neighbouring old-growth forests, reasoned the researchers, the more they had recovered.
The team found that after 20 years, the average secondary forest that had grown from farmland that was used with low to medium intensity had recovered 78 per cent of old-growth forest attributes. “It goes way faster than we thought,” says Poorter.
But the researchers found significant variation between the recovery time for the different forest attributes. Soils were the quickest to bounce back, with most recovery happening within 10 years. It took between 25 to 60 years for plant species diversity to recover, and they projected it would take over a century for the forest biomass to mostly recover.
Despite the enormous amount of deforestation that has and continues to occur, there is hope that these forests can bounce back naturally, says Poorter. Secondary forests currently make up over 28 per cent of tropical forests in central and south America, and are important for locking up carbon which is crucial to tackle climate change. In addition, they attract mammals, birds and insects back to the area, which is important for ecosystem restoration. They can also be vital for the livelihoods of people who live close by.
“These results are promising and highlight that natural regeneration and assisted natural regeneration are excellent restoration strategies in many cases,” says J. Leighton Reid at Virginia Tech. “However, one limitation is that the sites in this study do not represent regeneration on the average piece of deforested land [which might have been used more intensively], and many deforested sites will recover much more slowly than the ones studied here.”
Journal reference: Science, DOI: 10.1126/science.abh3629
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