Climate research: Reforestation is not an easy solution

Not all trees are the same and not all forests are the same. Reforestation can be a measure against climate change, but only if it is used correctly. This is the result of a new one Study, which recently appeared in the journal “Science” and dealt with various reforestation projects in Africa. Most of these are being carried out as part of the African Forest Landscape Restoration initiative, with which, according to various reports, 34 or 35 countries have committed to restoring forests on an area almost four times the size of Germany.

Based on satellite data, the authors conclude that in 52 percent of all cases examined, the trees are planted in savannahs and grasslands and thus adversely change the ecosystems there. These areas are often incorrectly declared as forest. In addition, 60 percent of the trees planted are non-native species. However, the extent of the problem on site is unclear, especially since the resolution of the satellite data is not very fine and the classification of the ecoregions is “not precise,” as Anja Ramming from the Technical University of Munich, who was not involved in the study, notes.

A project that started in Niger in the 1980s and involves local farmers takes a completely different approach than direct reforestation. The initiator Tony Rinaudo, who later won the so-called Alternative Nobel Prize, recognized at the time that conventional tree planting in the Sahel zone would not get you very far. He also noticed that the bushes that were often still visible grew from old tree roots. So he encouraged farmers to let the bushes grow back into trees, as the British newspaper “The Guardian” reports, among others. In addition, hollows were created around the trunks in which rainwater could collect. By using these and the shade of the trees, the farmers were able to noticeably increase their yields and thus rediscovered an old farming method. The European colonial rulers once suppressed it with tree taxes, so that it ultimately fell into oblivion. Today this practice is used on six million hectares in Niger and neighboring countries and has significantly increased the tree population.

Vegetation warms up more

Meanwhile, another study carried out using completely different methods also came to the conclusion that the positive effect of forestation measures is overestimated. The reason: Important climate effects of the new forests are not taken into account, as was also published in “Science” last week Study is called. For this purpose, the authors examined the effects of new forests using a so-called Earth system model. This is a whole system of mathematical equations that not only depicts the atmosphere, but also oceans, land surfaces, biosphere and ice and their respective interactions with one another.

A powerful large computer uses this model to calculate how various factors such as climate, ocean currents and others develop when boundary conditions are changed. This allows you to take into account, for example, that the vegetation is usually significantly darker than the exposed ground. If it increases, less sunlight is reflected and therefore more energy is stored in the Earth system. Reforestation removes CO from the atmosphere₂ and reduces the warming a little, but at the same time it also promotes it a little. With the calculations, these effects could now be weighed against each other.

Trees change the chemistry of the air

Another example of the influence of forests on the climate, to which particular attention was paid in this study, is the emissions from trees. These emit volatile organic compounds to ward off enemies, to communicate with each other, to stimulate growth and for other purposes. These change the air chemistry and create more methane and ozone, both very potent greenhouse gases. They also contribute to the formation of small floating solids, so-called aerosols, which on the one hand serve as condensation nuclei and thus promote precipitation and on the other hand reflect sunlight. If all of these sometimes opposing effects are not taken into account, the positive influence of reforestation on the climate is overestimated by around a third, the study concludes.

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