By the power of sunlight, forests turn huge amounts of carbon in the air into food: sugars for themselves and leaves, bark and roots that feed animals and microbes. Respiration, which happens in the cells of all living things in the forest, releases energy from that food and carbon dioxide (CO2) back into the air. As the amount of carbon in the atmosphere rises, this eat-and-be-eaten cycle increases to keep up. Metabolically, trees are running just to stand still.
In the course of all this cycling, forests are locking up the major part of the 33 per cent of human-caused emissions removed from the atmosphere into the land each year. In an experiment started in 2017, Global CO2 levels were around 280 parts per million (ppm) when oak trees in a forest were seedlings.
Carbon current accounts
To find out, researchers at the University of Birmingham's Institute of Forest Research use a free-air CO2 enrichment facility. Imagine a dinosaur-free Jurassic park with 102, 25 metre-tall towers treating forest patches with CO2-enriched air that replicates the mid-century atmosphere: 565ppm - 150ppm above present levels. Since 2017, forest patches exposed to higher CO2 appear healthy and productive. That may seem unsurprising. Well-managed forests can yield timber and fuel while reducing carbon in the atmosphere.
People and trees
Scientific models have estimated how much tree planting or reforestation is needed to offset rising CO2 in the atmosphere. As with most efforts to translate theory into action, the real-world experiences of actually doing this are often very messy indeed. How reforestation campaigns are funded and tree planting incentivised will determine where and which kinds of trees are planted.
Trees and climate crisis
Rather than asking if trees can help tackle the climate crisis, perhaps we should ask how much the world should really rely on trees as a climate solution. There is a lot to be learned from efforts that have increased tree cover and offered local people benefits, like new income sources. Often, these initiatives are successful because they take local needs and values seriously. Local and indigenous people are leaders in this process, not afterthoughts. And ultimately, reforestation will succeed if it benefits people, as well as the planet.
Trees need care too
How the land is ultimately governed will also decide how long new trees survive. International efforts to grow the planet's tree cover show how difficult these barriers can be to overcome. A recent study in northern India found that decades of expensive tree planting programmes had not increased total canopy cover. And the planted areas did not offer any major benefit to local people, like new food or firewood. This was because new trees couldn't be planted on nearby farmland and so were instead added to areas that already had some tree cover, reducing the potential carbon savings of the whole endeavour.
