Such forests – also known as degraded and secondary forests – have had some degree of disturbance by human activities, such as deforestation or fires.
They currently cover about 10 per cent of the tropical forest area worldwide and are concentrated in the Amazon, Borneo and central Africa.
The new research, published in Nature, uses satellite data to assess how much carbon these forests accumulate in their aboveground vegetation and estimates their potential to store carbon in the future.
The researchers find that degraded and secondary forests in humid tropical regions have stored, on average, 107m tonnes of carbon (MtC) annually between 1984 and 2018 – enough to offset 26 per cent of the carbon emissions generated from forest loss in those regions during that period.
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Recovering forests are important for conservation and restoring the carbon stored within them. The research has shown that they also have other co-benefits or ecosystem services that they can continue to provide if you let them recover.
Dr Viola Heinrich, research associate, University of Exeter
Moreover, the research estimates that conserving such forests could lead to an annual carbon sink of 53MtC.
The study says that investing in conservation for secondary and degraded forests is essential, but warns that this should not come at the expense of conserving old-growth forests, which “remains the most cost-effective climate mitigation strategy in the land-use sector”.
Recovering forests
Researchers use the term “recovering forests” to collectively refer to degraded and secondary forests.
Degraded forests are those that have suffered any human-induced disturbance that has led to a partial loss of their tree cover or function. Secondary forests are those that are regrowing naturally in deforested areas.
These forests are primarily located in the Amazon, Borneo and central Africa – three regions that together accounted for 29 per cent of global emissions from forest loss during 2001-19.
But these regions are not just significant for their forest loss, says Dr Viola Heinrich, the lead author of the study and a research associate at the University of Exeter. She tells Carbon Brief:
“Recovering forests are important for conservation and restoring the carbon stored within them. The research has shown that they also have other co-benefits or ecosystem services that they can continue to provide if you let them recover.”
For example, the study says, in Malaysian Borneo, degraded forests have been found to “provide access to clean water, clean air and regulate temperature”, while “older secondary forests can increase biodiversity in both species richness and diversity”.
However, recovering forests are also impacted by logging, fires and climate change.
Monitoring of those forests is crucial for financing schemes, such as the Reducing Emissions from Deforestation and Forest Degradation (REDD+) framework. It is also a key part of the Global Stocktake, a global review of international progress towards fulfilling the Paris Agreement goals. In order to be effective, the stocktake requires accurate reporting of all carbon sources and sinks.
The study uses satellite images to quantify the growth of recovering forests across three vast regions: the Amazon, Borneo and central Africa. Heinrich explains that this method yields a better understanding of the forests’ spatial patterns and changes over time than previous studies, which have generally focused on data collected in the field.
Dr Ricardo Dalagnol, co-author of the study and postdoctoral researcher at the University of California, Los Angeles, says that this is the “first time” that researchers have taken such a large-scale look at recovering forests. He tells Carbon Brief:
“We show that [both forests] may be still growing, and we can quantify how much they can contribute in terms of carbon [sink].”
Carbon sinks
The researchers combine two satellite datasets. The tropical moist forests dataset tracks forest degradation considering changes in land use, such as deforestation, logging or other disturbances; they use this to map degradation from 1984 to 2018.
Then, using a dataset of aboveground biomass from 2018 – which takes into account trees, leaves, grass and all other vegetation that grows above the soil of tropical forests – they determine how much carbon is sequestered across the humid tropical forests.
They also apply growth models to determine how these forests might sequester carbon in the future. They estimate the carbon stock of recovering forests in 2018 and model their potential carbon stock by 2030, if forests remain conserved.
Dalagnol says this modelling is “one of the innovations” of their approach, allowing them to “create the trajectory of recovery” for the forests.
The study finds that degraded and secondary forests in the Amazon, Borneo and central Africa stored 107MtC annually across the analysis period, counterbalancing 26 per cent of carbon emissions from tropical forest degradation during that time.
The charts below show the accumulation of aboveground carbon in degraded forests and secondary forests following disturbances in the Amazon (light blue), Borneo (green) and central Africa (grey). The map illustrates the three regions focused on in the study and their breakdown into old-growth (dark green), degraded (medium green) and secondary forests (light green).
Accumulation of aboveground carbon as a function of time since last disturbance in degraded forests (top left) and secondary forests (top right) for three different regions: the Amazon (light blue), Borneo (green) and central Africa (grey). The map (bottom) shows the spatial extent of each region. The three pie charts show the percentage of each region occupied by different forest types: old-growth forest (dark green), degraded forest (medium green), secondary forest (light green) and other (yellow). Source: Heinrich et al. (2023).
Heinrich and her team calculate that there are 60m hectares of recovering secondary and degraded forests across the three regions – about 1.5 per cent of the world’s forested area. But she points out that they play an outsized role in carbon sequestration, absorbing 5 per cent of all carbon absorbed by forests.
The study also looks at regrowth rates in the three regions. These rates can vary due to climate variables, such as temperature or water availability. They find that in Borneo, regrowth rates were 45 per cent and 58 per cent higher than in central Africa and the Amazon, respectively.
Heinrich points out that these high rates of recovery in Borneo make sense because the island is “very equatorial and generally has more rainfall than the other broader regions”.
A possible limitation of the study is the exclusion of other tropical areas such as Central America, western Africa and south-east Asia. However, that would have added more complexity to an already difficult analytical process, says Dr Emilio Vilanova, a Venezuelan environmental and forest sciences researcher who is part of the Amazon Forest Inventory Network (RAINFOR).
Although other studies have analysed the carbon dynamics in recovering forests, most of them have been conducted at a small scale, says Vilanova, who was not involved in the study. Even when this type of research is carried out at a larger scale, it generally has limited coverage, making it difficult to build global predictive maps, he adds:
“The combination of [methods] indeed puts this study in a unique and innovative position to be a key resource regarding data on the carbon sink of recovering tropical forests.”
Conserving degraded forests
Modelling the carbon stock of recovering forests by 2030, the study finds that conserving such forests has a carbon sink potential of 53MtC annually across the tropical regions analysed.
However, this projection does not necessarily account for what will happen in the future if the planet continues to experience more extreme weather and climate conditions, warns Heinrich.
For example, fires significantly reduce the ability of forests to recover.
“This is a big issue in the Amazon, where very large fires affect the forest. They lose carbon and the ability to recover it,” says Dalagnol.
Water deficiency, a drought indicator, also affects the carbon recovery of forests, Heinrich tells Carbon Brief:
“That was evident across the regions, especially in the Amazon and in Borneo, which we found surprising because there is a much less extreme water deficit or drought [there] compared to the Amazon.”
The study stresses the relevance of investing in conservation of recovering forests as they are a “sizeable carbon sink”, despite their slow rates of recovery.
The results also show the importance of preserving old-growth forests, says Dalagnol, as recovering forests will not help slow climate change alone.
No official global mechanism specifically addresses the state of recovering forests. However, Vilanova points out the importance of forest degradation in the international agenda:
“Forest degradation is one of the key elements of the REDD+ strategies that have been put in place to protect tropical forests…Although there are many successful experiences where tropical forests at risk of deforestation have been protected, these mechanisms have had limited effect on the ground”.
Heinrich hopes that the new insights on degraded forests provided by the study can “hopefully…[be] the first stepping stone to conserving them as well”.
This story was published with permission from Carbon Brief.
