Forest ecosystems absorb about a quarter of humanity’s annual carbon emissions. They thus reduce the amount of greenhouse gases (GHGs) that end up in the atmosphere and contribute to global warming. However, trees’ appetite for carbon could be waning, even as this food is increasingly plentiful in the atmosphere, according to a study published today in the journal Science.
Through photosynthesis, trees absorb carbon from the air. If the atmospheric concentration of CO2 increases, this “carbon fertilization” stimulates their growth.
Until recently, the scientific community believed that forests would sequester more and more carbon as the amount of CO2 in the air increases. But for the past ten years, regional studies have called this hypothesis into question.
To see more clearly, researchers at the University of Utah undertook a large-scale analysis. These scientists compared the growth of trees located in dozens of forests around the world with the flow of carbon above their crowns. They found that the two phenomena did not go hand in hand.
“Rather than an increase in tree growth, we could perhaps have a decrease. The phenomenon remains to be quantified, but it could have significant consequences for the ability of forests to store carbon in the future”, says Antoine Cabon, a postdoctoral researcher who signed the study, in an interview at the To have to.
To assess tree growth, the researchers visited 31 forests on several continents to collect wood cores. By measuring tree-ring thickness, they calculated the carbon sequestration of representative specimens from the plot. They also drew on existing data.
At the same time, “flux towers” in service for around twenty years have made it possible to measure the gaseous exchanges of carbon between 78 forest ecosystems and the atmosphere above their canopy. One of these towers topped with electronic sensors is located in northern Quebec, 30 km from Chibougamau.
In large scale
“With all this data, we manage to show that, on average, the correlation between photosynthesis and tree growth is quite low,” explains Mr. Cabon.
Other factors limit tree growth and may outweigh carbon fertilization. For example, in cooler forests—where temperature restricts tree cell division—tree growth is less coupled to photosynthesis. In forests topped with a dense canopy of foliage, the same phenomenon is observed.
“It’s a fairly convincing study,” said Évelyne Thiffault, a forestry professor at Laval University who is not involved in the project. A few small-scale studies had previously suggested that forest growth is not strongly limited by carbon availability, but the new publication demonstrates this for the first time on a large scale, she points out.
If the carbon absorbed by photosynthesis is not sequestered in the wood of the tree, it does not disappear. It is found in the leaves or the soil. Carbon is also released into the atmosphere through the process of respiration. In any case, it is probably not stored long term.
The study by Mr. Cabon and his colleagues will have important consequences for forest planning, thinks Ms.me Thiffault. “The carbon that’s in the ground, it won’t have the same utility, from a forest management perspective, as the carbon that’s in the trees,” she says.
According to this professor, the new study would be sufficiently “robust” to convince forest modellers to adjust their parameters in order to avoid overestimating the production of wood in response to an increase in the concentration of CO2 in the air.
In an analysis accompanying the publication in Science, Julia Green and Trevor Keenan, researchers from the University of California at Berkeley, observe that the results of their Utah colleagues have implications for the effectiveness of tree planting in offsetting GHG emissions. “If less carbon than expected is stored in biomass in future years, the usefulness of this ecosystem service is greatly diminished,” they write.
Beyond CO concentration2 in the atmosphere, climate change itself will influence the evolution of forests. Higher temperatures will favor the development of some northern forests, but the lack of water caused by increased evaporation will limit the growth of trees in many ecosystems.