Research shows tree planting may not be effective climate solution

Tree planting has been touted as an effective way of tackling the climate crisis, but researchers say it’s not always a productive solution.

Scientists from the University of Gothenburg and elsewhere are warning that forests planted on nutrient-poor land won’t act as an effective carbon sink in the long term. 

There is also evidence that when forests age the level of CO2 they can absorb declines and that additional carbon can be released from the soil each time a new forest is planted. 

Through several long-term field experiments, researchers measured the amount of biomass growing under air with elevated CO2 concentrations and found growth was poor or non-existent when plants were based in poor quality soil.

Louise Andresen, a researcher at the University of Gothenburg, said: ‘The total biomass that binds carbon was not stimulated more by the elevated CO2 levels in our experiments over time. Exactly when growth slowed down depended on various factors, but one important one was how much nitrogen the soil contained.’ 

The study also provides a more nuanced picture, as it takes into account the fact that when trees die they stop binding CO2 and release carbon prematurely. 

Previously it was thought that elevated levels of carbon dioxide would lead to an increase in growth of biomass, but outdoor experiments in areas such as the US, Switzerland, Denmark have shown otherwise. 

Forests, grasses and other vegetation were exposed to high levels of CO2 in the air and researchers noted that when soil had a low concentration of nitrogen, biomass stopped increasing after a few years of growth. 

‘That’s not great news, given that carbon dioxide levels in the Earth’s atmosphere have risen from 380 ppm to over 410 ppm in the last 20 years alone,’ explained Louise. ‘After these very long experiments, we know more about how vegetation responds to elevated carbon dioxide levels. When you take the whole ecosystem, the soil’s fertility and the whole life cycle of the plant into account, many ecosystems do not increase the amount of carbon they bind.’

Planting trees can also have unintended negative effects, the team has warned, as if heath or tundra-like land – an effective carbon sink on its own – is used for tree planting it risks releasing carbon into the atmosphere. 

Lousie continued: ‘We now know that we humans can’t just silence our consciences by planting forests; in the long term it doesn’t actually help. The only thing that will help is for humanity to reduce its greenhouse gas emissions.’ 

Photo by Geran de Klerk