The amount of CO2 that trees absorb is overestimated

The role of trees in the fight against climate change is likely to be overestimated. It concluded an international group of researchers on Thursday in science† They show that trees sometimes grow less rapidly than climate models predict based on the amount of CO2 which absorbs the trees from the sky.

Trees absorb the greenhouse gas CO through their leaves2 up from the air. They convert it into sugar through photosynthesis. They grow on this energy source: they process the carbon (C) from the air into extra wood tissue.

Most climate models assume a linear relationship between CO2registration and tree growth. If the concentration of CO2 increases in the air, the idea is that tree growth will increase proportionally. Climate skeptics sometimes point to this so-called CO2-fertilization effect. More CO2 in the air provides extra plant growth. It is actually beneficial.

less flux

But the now-published study shows that this relationship does not always last. Sometimes tree growth lags behind. This is especially the case in dry and colder areas as well as older forests, the researchers say.

They collected data from 78 so-called skovflux studies in which the exchange of CO2 between atmosphere and forest over time. They combined it with data from tree growth in the same or nearby areas. They retrieved this data from tree databases – the growth rate can be read from the width of the growth rings.

“Current models are far too simple,” said Roel Brienen, a professor of forest ecology at the University of Leeds. “They can not simulate the complicated physiological processes in plants at all.”

In a commentary published at the same time, Julia Green and Trevor Keenan from the University of California at Berkeley write that tree growth of more than just CO22 depends on the. You also have the available amount of water, nutrients in the soil, temperature. All of them can affect growth.

This is not the first time that the linear relationship between CO2registration and tree growth are questioned. There was already a release in early May science from, among others, an Amsterdam and a Wageningen researcher who showed that CO2 The growth of tropical forests in West Africa over the past 500,000 years has been lower than previously thought – water and forest fires played a much greater role. The authors therefore suggest that the contribution of CO2fertilization in predictive climate models must be adjusted.

And two years ago, Australian researchers showed, for example, that eucalyptus trees are exposed to rising concentrations of CO2 not grow fasterNatureApril 8, 2020). The leaves absorbed more CO2 but the trees did not capture the extra carbon that was absorbed. Finally the carbon in the form of CO2through the bottom just as hard free again.

Tree rings do not say everything

The research that has now been published has a few shortcomings, Green and Keenan write. Trees not only store carbon above the ground in tree growth, but also, for example, in their leaves and root system. Research in wooden rings therefore does not paint the whole picture.

“In addition, trees can temporarily store carbon in reserves throughout the plant, sometimes using it for growth many years later,” says Brienen. The connection in time of CO2absorption and growth are therefore also complex.

Green and Keenan further point out that the researchers only used the trees Hospitalized CO2 has taken while some of the CO2 released again through the bottom. It is better, they write, to convert net CO2recording to go out.

Yet they also conclude that current climate models most likely overestimate the extent of carbon sequestration of trees.

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