Planting trees not always effective at binding CO2

Tree-planting has been widely seen as an effective way of binding carbon as carbon dioxide levels rise in the atmosphere — but now international researchers are warning that forests on nutrient-poor land won’t work as an additional carbon sink in the long term. While there has been debate about the size of our forests’ capacity to mitigate the greenhouse effect in the past, the new study provides a more nuanced picture.

The capacity of plants to bind carbon is a key factor in calculating the effects of climate change as carbon dioxide levels rise in the atmosphere. Previously, the prevailing theory was that elevated carbon dioxide leads to an increase in the growth of biomass. In long-term field experiments in the US, Switzerland, Denmark and elsewhere, forests, grasses and other vegetation were exposed to a manipulated level of carbon dioxide in the air; the researchers then measured the growth of the biomass, with their results published in the journal Global Change Biology.

“The total biomass that binds carbon was not stimulated more by the elevated CO₂ levels in our experiments over time,” said Louise Andresen, a researcher at the University of Gothenburg. Indeed, the researchers found that growth stimulation was poor or missing when the plants lived in poor soil, in some cases after only 10 years.

“Exactly when growth slowed down depended on various factors, but one important one was how much nitrogen the soil contained,” Andresen said. Where the soil had a low concentration of nitrogen, the researchers could see that the biomass stopped increasing after a few years of growth.

Other factors not previously taken into account are that some trees die and thus stop binding carbon dioxide; instead, their carbon is released prematurely. Furthermore, the actual planting of trees can have major negative effects, as a heath or tundra-like land is a good carbon sink as it is. If machines roll in to prepare the soil for planting, there is a high risk that the carbon bound in the soil will be released into the atmosphere.

“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,” Andresen said. “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.

“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.”

Image caption: Test site where extra carbon dioxide is emitted to the air and the biomass growth is measured in the plants. Image credit: Louise C Andresen.

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