Forests are considered the green lungs of the earth, drawing climate-damaging CO2 from the atmosphere. Their ongoing deforestation, especially in the tropics, is bad for the climate and planet. But even if tropical forests that have been cut down slowly grow back again, this will initially do little to help the climate, as a study now suggests. According to this, tropical forests that recover from deforestation release more CO2 than they absorb for at least ten years. The CO2 comes mainly from the soil and from rotting wood.
The tropical rainforests of the world bind a large proportion of man-made greenhouse gases by absorbing them in the course of photosynthesis and using them to build up woody biomass. They are therefore considered carbon sinks. However, the increasing deforestation for wood production or to convert the areas into pasture and farmland reduces this climate-friendly effect. Losing even more tropical forests could further exacerbate global climate change.
Measurements on the ground and from the tower
After all, tropical forests grow back relatively quickly after deforestation. For a long time it was therefore thought that even recovering tropical forests already absorb more CO2 than they release. However, this assumption apparently needs to be revised, as a study by researchers led by Maria Mills from the University of Leicester now suggests. In order to find out whether regrowing tropical forests are actually already carbon sinks, the team examined eleven different areas in Malaysian Borneo that were in different degrees of recovery after deforestation.
The scientists erected a 52-meter-high tower above the forest canopy and continuously measured how much carbon was flowing in and out of the forest. In additional soil measurements, the researchers also investigated how much carbon is released from the soil and deadwood. Mills’ colleague Rob Ewers explains: “The measurements from the tower tell us whether the forest cover is a carbon source or sink, and the ground monitoring tells us why that is.” The measurements took place over a seven-year period between 2011 and held in 2017.
Regrown forests are net sources of carbon
The result: Heavily deforested and then reforested areas emit about 5.23 tons of carbon per hectare per year for at least ten years after deforestation, Mills and her colleagues report. According to the measurements, even moderately cleared forest areas still emit around 1.75 tons of carbon per hectare and year. In terms of their net balance, these areas are therefore carbon sources instead of carbon sinks, as previously assumed. “This means we need to re-evaluate their role in the global carbon budget – we can no longer make blanket assumptions that they are carbon sinks,” explains Mills.
But where does all that CO2 come from? Shouldn’t the regrowing trees absorb this from the atmosphere? The researchers report that they do, because in terms of their net primary production, the afforested forest areas do not differ significantly from untouched forests. But on the area that was once deforested, this is not enough to offset the carbon released from other sources. “A large part of the carbon released in recovering forests comes from collateral damage – trees that have died and rotted due to damage from deforestation, and from disturbed soil,” explains co-author Terhi Riutta.
However, the findings of the study are not intended to demonize the deforested forests as climate killers, as she emphasizes: “Formerly deforested forests are still valuable – we know that they have a unique biological diversity – and if we ensure that they are not damaged by better deforestation practices release additional carbon, this will promote their sustainability.” Whether the findings on the regrown forests gained in Borneo also apply to other tropical and non-tropical forests must now be examined in further studies.
Source: Imperial College London; Specialist article: Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2214462120)