Trees do not grow faster with higher CO2 uptake

The amount of CO2 that trees can catch is overestimated in current climate models. Especially in cold and dry forests, the models assume too high CO2intake.

To limit climate change, we need to emit far fewer greenhouse gases worldwide. Trees can help us with that. The sat you CO2 from the air in their stem, branches, leaves and root system. Forests therefore play an important role in the fight against climate change. But current climate models probably overestimate this contribution, says an international research group. Their research shows that trees do not necessarily grow faster if the amount of CO2 rises in the air – something most models assume.

CO2swallowers

During their lifetime, trees absorb CO2 up from the sky through their leaves. Through photosynthesis, they convert this – using sunlight and water – into sugar and oxygen. The sugars are used by the tree to grow. For example, a greenhouse gas is converted into wood, leaves and roots.

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If one looks solely at photosynthesis, it seems logical that more CO2 in the air causes trees to grow more. An increase in this greenhouse gas would therefore lead to more plants; as long as you provide enough space for them to grow.

Climate models usually assume that the ratio of CO2uptake and growth are approximately linear: twice as much CO2uptake means twice as much growth. But that’s not the whole story. More and more studies show that other environmental factors also play a role. Think about the temperature, the amount of water and the nutrients in the soil. If you want to predict how trees will react to a changing climate, it is important to know what factors can inhibit growth.

wood rings

In their new publication, an international research group shows that more CO2 does not equal more tree growth. To do this, the researchers studied trees in 78 forests in North America, Europe, Japan and Australia. There they measured the amount of CO2 that can accommodate the existing trees. They did so with measuring towers that measure CO2concentration and record the wind currents in the air. They combined these measurements with data on tree growth obtained from the International Tree Training Database. Seeds were drilled from trees for the tree ring measurements. The width of each ring tells you how fast a tree grew.

Also read: Climate models paint a too favorable picture of carbon uptake in tropical forests

In their analysis of these measurements, the researchers did not find the linear relationship between CO2registration and tree growth on which the climate models are based. If CO2the uptake of trees increased, there was no simultaneous increase or decrease in tree growth. This was especially true for cold and dry areas, the researchers write. This is in line with a previous study of the impact of CO2 on the growth of tropical forests over the past 500,000 years. This research, led by the University of Amsterdam, was published in early May in science† In it, the researchers also wrote that the impact of CO2 in the air on tree growth is overrated.

Wooden cores for measuring the wooden rings.
Photo: Antoine Cabon

Breathing

“These results emphasize that the relationship between photosynthesis and growth is not as simplified as the current representation in models,” US researchers Julia Green and Trevor Keenan write in a commentary published simultaneously. But, they continue, the study has some shortcomings. Such ‘spirits’ plant part of CO2 absorb them back into the air during photosynthesis. Therefore, not all absorbed carbon is available for growth. Researchers do not take this into account.

Green and Keenan further point out that only wooden rings were considered. But trees do not only store carbon in the form of wood. They also use it for the growth of their root system and leaves. Therefore, wood growth does not exactly correspond to the total amount of carbon captured.

Despite these limitations, research fits into a trend with publications showing that current climate models overestimate the contribution of trees. This indicates that researchers do not yet understand well enough how trees grow.

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