a hand holding brazil nut husks on the left and another holding biochar on the right

To effectively combat climate change, humanity needs to both stop putting carbon dioxide into the atmosphere and to lock away at least some that we’ve already unleashed, a process called carbon sequestration. Reforestation — replanting trees at large scale — is often touted as a cheap and easy way to sequester carbon. But, as new research by scientists at CEES Center for Amazonian Scientific Innovation (CINCIA) shows, the reality behind this approach to combating climate change is much more complex than it seems.

Reforestation brings benefits for people and wildlife beyond just carbon sequestration, and so if properly implemented, reforestation is a worthy endeavor for the environmental and conservation groups that do it. However, often the driving forces behind large-scale reforestation projects are the amount of potential carbon sequestration they could achieve. It is important that the carbon accounting of these projects be as accurate as possible.

That has generally not been the case: trees do sequester carbon, but, like virtually any other activity we do, tree planting has a carbon footprint. Growing seedlings in a nursery, moving them to their new homes, and even transporting people to plant the trees all take energy and fuel, and these carbon costs of reforestation have generally been ignored by conservationists. “I could not help but wonder what the carbon footprint of setting up a reforestation plot was,” says David Lefebvre, a CINCIA researcher based at the UK’s Cranfield University. Lefebvre is uniquely qualified to answer this question, as his PhD research assessed the life cycle impact of greenhouse gas removal technologies.

Now, thanks to his expertise, we now have an idea of the total carbon impact of reforestation.  Lefebvre is the lead author on a new study, published October 7th in Scientific Reports, that demonstrates a better way to measure the carbon capture of reforestation projects. Using data from CINCIA’s reforestation program, Lefebvre and his research team performed a life cycle assessment of the carbon footprint of establishing a tropical forest reforestation plot and managing it for one year. CINCIA has been planting trees in Peru’s Madre de Dios region, with the goal of re-vegetating degraded mining lands, for several years. To date, they have reforested 42.5 hectares of land, the equivalent of about 106 football fields. By now, they’ve got a pretty good recipe for success. That recipe enabled Lefebvre to answer his question about reforestation’s carbon footprint.

a satellite view of a mosaic of deforested patches in the Amazon

A drone image of the landscape where CINCIA works

A key ingredient in CINCIA’s approach to reforestation is biochar, a substance similar to charcoal that is made by burning agricultural waste. Using an approach developed in collaboration with Wake Forest scientists, including CEES affiliate and chemistry professor Abdou Lachgar, CINCIA makes their own biochar from Brazil nut husks through pyrolysis. “Pyrolysis is combustion of biomass at high temperature with little to no oxygen input, which avoids the biomass turning into ashes,” explains Lefebvre. This process emits greenhouse gases — in fact, for CINCIA it is the most carbon-emitting step of setting up a reforestation plot. However, “biochar is resistant to degradation and can stay in the soil for a very long time. For this reason, it is considered a carbon removal technology,” explains Lefebvre. During planting, CINCIA technicians add about a kilogram of biochar (the weight of a small watermelon) to each of the approximately 1,111 seedlings in each of their typical reforestation plots.

Although biochar production produces carbon dioxide, the energy investment that goes into it pays off: Lefebvre found that, although producing enough biochar for a reforestation plot emits about 0.3 metric tons of carbon, planting it with the seedlings absorbs triple that amount. That is enough to tip the carbon balance of reforestation into the negative numbers, meaning that more carbon is being sequestered than emitted throughout the process. Without the biochar, Lefebvre calculated, replanting a hectare of forest is a net carbon-emitting process. The growing seedlings only cover their “carbon debt” and begin to sequester carbon about two months after they are planted.

The goal of a typical reforestation project is for each tree planted to capture 100 kg of carbon. According to these findings, CINCIA’s plots achieve this in a little over four years. But that target is only achieved if the trees that were initially planted stay alive and in the ground; the carbon sequestration “payoff” is by no means instantaneous just because they were planted. In their study, Lefebvre and his colleagues point out that this fact is often ignored by companies eager to prove how eco-friendly they are with the number of trees they have planted to date. Planting means nothing (and actually emits more carbon dioxide into the atmosphere) if the newly revegetated forest is not protected from future destruction. In other words, reforestation is a great tool, but it doesn’t absolve us from addressing the reasons that the forest disappeared in the first place.

“The most surprising thing I learned through the process is that even though trees do capture carbon, the questions of how much, for how long, at which rate, and how to represent and monetize it are far from having a clear and simple answer. It was astonishing to realize how little we [scientists] agree on, despite massive investments into reforestation/afforestation and claims about its importance in us reaching carbon net-zero,” Lefebrve says. His painstaking accounting gets us a little closer to answering these questions.