Anyone who harms the climate can have trees planted to compensate. Companies also advertise that they offset their emissions through reforestation. Trees store CO2 and thus help the climate – the obvious and charming idea at first glance.

But things are not that simple. Even if the planted forest benefits the climate, this often cannot be quantified, as scientists explain. In the worst case, such an action even has the opposite effect. The basic idea is not wrong, says Christopher Reyer from the Potsdam Institute for Climate Impact Research (PIK): “We need more forests.”

A team of researchers recently evaluated the sustainability reports of 100 of the world’s largest companies. 66 companies said they were implementing eco-measures, 44 of them were planting trees, as reported in the journal Science. But the study also showed that over 90 percent did not report any ecological result. In addition, none of the reports quantified the social or economic impacts on local stakeholders.

“There is very little transparency at the moment, so it is not easy for anyone to judge whether projects bring benefits to ecosystems or people,” says lead author Tim Lamont from Lancaster University. “If a company says it has planted thousands of trees to restore habitat and absorb carbon – how do we know if this has been done, if the trees will survive, and if it has created a functioning ecosystem that supports biodiversity and benefits people?” says Lamont. “In many cases, we have found that the evidence that major companies have provided to support their claims is not sufficient.”

Large international corporations could play a key role in restoring ecosystems, the researchers write in Science. However, more transparency and accountability is needed to ensure that their projects deliver good results.

Trees bind CO2 from the air and store carbon. How much depends on the tree species, site conditions and lifespan. For photosynthesis, the tree takes carbon dioxide (CO2) from the atmosphere. The carbon (C) from CO2 is used, among other things, to form roots, trunks and leaves: much of it is stored in the tree. The oxygen (O2) from the CO2 is released into the air.

The amount of storage depends on the tree species: the heavier and denser the wood, the more carbon is stored. The age of the trees also plays a role: very young forests store less than old ones. The geographical location should also be taken into account, writes the Forest Enterprise Foundation, which is committed to nature conservation and forest research: tropical forests grow faster than forests in Germany and therefore store more CO2 in the same period of time. As a rule of thumb, the foundation states: One hectare of forest stores around six tons of CO2 per year across all age groups.

In terms of the basic idea, the idea is “not wrong,” says PIK researcher Reyer, who researches the consequences of climate change for forests and possible countermeasures. “But in practice it’s often not done so well.” If companies plant trees, “that doesn’t mean those trees will survive.” Planting just one type of tree usually makes no sense: monocultures have little resistance to storms or drought and fall victim to pests more quickly.

Sometimes existing ecosystems, such as moors or steppes, are destroyed through reforestation projects. Elsewhere, illegal settlements would be cleared, with the result that people would build new homes elsewhere and in the process invade other, still intact ecosystems. A newly planted forest could also dry out the soil or have other side effects. “Ultimately, you have to keep an eye on the overall climate footprint,” says Reyer.

All of this is known in principle. And there are really great projects that take this to heart, emphasizes the scientist. Only the consumer who wants to calm his conscience with this “indulgence trade” cannot distinguish between them. Reyer is critical of the various certification systems on the market. There is no legal framework for tree planting campaigns, but there is “a lot of wild growth”. A tip from the expert: reforestation tends to be better than new planting, as this at least ensures that the location is suitable for forest.

Even if tree projects alone do not save the climate, forests can contribute a lot to climate protection, according to a report from the European Forest Institute from 2022 (“Forest-based climate protection and adaptation to climate change in Europe”). However, it also says: “Reforestation alone won’t get us anywhere. If we really want to limit the damage, we have to combine different activities: avoiding deforestation, reforestation, using wood differently and recycling it.”

According to the report’s authors, if this “holistic” approach were consistently pursued, “European forests and wood products could make a significant contribution to achieving climate neutrality by 2050.”