Let’s start with some good news: Tropical forests ravaged by agricultural conversion or heavy logging are healing themselves through the process of natural regeneration. Known as succession or secondary forests, these now account for about 28 percent of tropical and neotropical forests.
Allowing secondary forests to grow is one of the best things humans can do for the planet. At the UN Climate Change Conference (COP26) last November, delegates promised to end forest loss by 2030, pledging USD 12 billion in public funding to “support work to protect, restore and sustainably manage forests,” and allowing forests to regenerate is a key way to meet this goal. Indeed, succession forests bring back biodiversity, mitigate climate change, help provide clean water, and can re-introduce natural resources to people living in areas where they were lost. However, they require time to grow – and little disruption while they do.
A recent study, “Multidimensional Tropical Forest Recovery,” published in Science and spearheaded by researchers at the Netherlands’ Wageningen University now helps show how regrowth happens and how long it takes.
Looking at 77 different sites in 12 countries and evaluating 12 criteria – from soil and plant functioning to ecosystem structure to species diversity – the researchers conducted a type of computer modelling called chronosequencing to infer long-term trends in forest recovery. On average, they found that forest attributes in secondary forests attain 78 percent of their old-growth status after about 20 years.
The catch is that those degraded areas need to be relatively close to old-growth or even fairly mature secondary forests to manage this feat. “We see the potential in areas that have not been used too intensely or extensively,” says Lourens Poorter, professor in functional ecology at Wageningen University and lead author of the paper. “If it’s in the middle of a soybean field, it’s not going to regenerate.”
Poorter’s team looked at areas where the typical land use consisted of shifting agricultural systems, where farmers practice slash-and-burn, using fire to clear their land; cultivate for a few years; then leave the area fallow once the soil is depleted or invaded with weeds. A sufficient quantity of nearby seed trees, as well as birds and animals that come in and disperse the seeds, “can kickstart succession,” he says.
Soil recovers the fastest, according to the study, with carbon levels restored after about 10 years. “If you slash and burn, you affect the above-ground vegetation, but there is less impact on the below-ground soil system,” he explains. “There are nutrients in the soil, and it bounces back quickly.”
Plant and animal species take 60 years to fully recover, but it takes twice that time for above-ground biomass to recover.
What’s more, this new growth sucks in more carbon from the atmosphere than old-growth forests – about 11 times more, according to various studies. Poorter likes to compare the burgeoning new trees and plants to teenagers: “They consume lots of stuff,” he says, compared to their elders.
Forest recovery also has implications for water, with previous studies in Brazil’s Amazon biome showing how its devastation has had a severe effect on rainfall, not only there but in other parts of South America.
Indeed, a 2020 study by Celso Silva Junior, a professor at the University of Maranhão and researcher with Brazil’s National Institute for Space Research (INPE), found that about one-third of Brazil’s deforested landscapes have recovered naturally, offsetting 12 percent of the carbon emitted into the atmosphere from deforestation in the Amazon.
“Although our mapping has not explored the proximity of mature forests around secondary forests,” says Silva Junior, “we were able for the first time to map the forest areas that managed to regenerate throughout Brazil from 1986 onwards. In addition, we were able to calculate the age of these forests, which is important information for understanding the recovery of carbon stock and biodiversity in these areas.”
Silva Junior also calculated a period of 30 to 40 years for regeneration to take place. “Species richness and compositional similarity of secondary forests reach on average 88 percent and 85 percent, respectively, of values found in old-growth forests after 40 years,” according to the Brazilian study. “In Atlantic Forest fragments, secondary forest-growth recovered around 76 percent of taxonomic, 84 percent of phylogenetic and 96 percent of functional diversity over a period of 30 years after abandonment. Besides, the recovery of these fragments, when compared with primary forests, allowed the retrieval of 65 percent and 30 percent of threatened and endemic species, respectively.”
However, leaving those secondary forest pieces intact is key. “Our recommendation for policy-makers is: let natural forest regrow naturally when you can and plant where you need,” says Poorter. “It’s the ideal nature-based solution in the sense that it’s more effective than planting [trees] in terms of the amount of biodiversity, carbon mitigation and soil recovery. It’s a way more efficient system.” What’s more, he pointed out that reforestation by humans is expensive, and approximately 30 percent of planted seedlings don’t survive.
Silva Junior agrees that secondary forests need to be protected. “For example, in the Amazon region, secondary forests’ deforestation rates are higher than mature forests,” he says. They tend to be close to transportation links and settlements. As a result, he adds, “there is a need to ensure that these secondary forests remain standing long-term, thus ensuring that they provide their ecosystem services.”
Poorter also says that with global land restoration pledges totaling 550 million hectares by 2030, nations need to harness the potential of secondary forests. “There are people living in these areas,” he adds, “so you can’t just say ‘abandon your farm and we’ll solve the problems of the world.’ Many of these secondary forests are used again after anywhere from 7 to 20 years, so they can never live up to their potential. But if governments can use the money they save in planting trees to lease land or compensate local people for ecosystems services to make up for the loss of their livelihoods, it would be a better solution.”
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