Research by biologists at West Virginia University, in the US state of the same name, has established that trees around the globe are consuming greater amounts of carbon dioxide (CO₂) than previously believed. This means that forests are even more important as CO₂ ‘sinks’ play a much greater role in regulating the Earth’s atmosphere than has hitherto been recognised.

Professor Richard Thomas and Dr Justin Mathias published the results of their research in the Proceedings of the National Academy of Sciences. They analysed carbon and oxygen isotopes in tree rings covering the period from 1901 to 2015. The samples were taken from 36 tree species at 84 sites from across the globe.

The tree species selected represented just about every forest or wooded ecosystem in the world. Tundra, taiga, temperate, subtropical and tropical forests; grasslands, shrublands; conifer forests, broadleaf forests and mixed forests; all were represented.

“This study really highlights the role of forests and their ecosystems in climate change,” pointed out Thomas. “Our research shows that forests consume large amounts of carbon dioxide globally. Without that, more carbon dioxide would go into the air and build up in the atmosphere even more than it already is, which could exacerbate climate change. Our work shows yet another important reason to preserve and maintain our forests and keep them healthy.”

The research was focused on the phenomenon in trees known as ‘water use efficiency’. This is the ratio of CO₂ absorbed by a tree (because it is used by the tree for photosynthesis) to the water vapour emitted by that same tree (a process called transpiration). It has been known for some time that the water use efficiency of trees worldwide has increased by some 40% since 1901, with the amount of CO₂ in the atmosphere increasing by about 34% over the same period. Moreover, both these developments have accelerated by some four times since the 1960s, in relation to the previous sixty years.

Until now, it was believed that this increase in tree water use efficiency was the result of the increased levels of atmospheric CO₂ causing the pores on tree leaves to close slightly. In other words, they were using the same amount of CO₂ but ‘breathing out’ less water vapour. (The term covering the amount of water vapour a tree releases through its leaf pores is ‘stomatal conductance’.)

Thomas’ and Mathias’ research established that this was not so. In 83% of cases, the increase in water use efficiency was because the trees were engaging in greater photosynthesis and so were processing more CO₂. Only in 17% of cases was the cause owing to stomatal conductance. The research also showed that these results were not uniform around the world. They varied depending on precipitation, temperature and the dryness of the atmosphere.

“We’ve shown that over the past century, photosynthesis is actually the overwhelming driver to increases in tree water use efficiency, which is a surprising result because it contradicts many earlier studies,” highlighted Mathias. “On a global scale, this will have large implications potentially for the carbon cycle if more carbon is being transferred from the atmosphere into trees.”

The data from the study will assist in the refining of models predicting the impact of climate change on global water and carbon cycles. “Having an accurate representation of these processes is critical in making sound predictions about what may happen in the future,” he added. “This helps us get a little closer to making those predictions less uncertain.”