Oak trees accumulate more wood when there is more carbon dioxide (CO₂) in the atmosphere. That’s the key finding of our new study, conducted in an ancient forest in Staffordshire, England, which we turned into a giant field experiment by adding extra CO₂. After we raised the CO₂ levels to the levels the planet will reach in the 2050s, the trees absorbed more of it from the atmosphere and their wood production increased by 10%.

In some ways, this result is reassuring. We know that more CO₂ in the atmosphere often helps plants grow bigger and faster, because photosynthesis fixes the carbon that plants are mostly made of. But so far, the only comparable study in an older, mature forest (an Australian eucalyptus forest) has failed to find a link between extra CO₂ and tree growth. Our work shows that this link does indeed exist – at least in some common broadleaf forests.

But woodier trees are no panacea for climate change. While carbon is certainly better stored in trees than in the atmosphere, where it causes global warming, this is not a long-term solution. Over decades or centuries, wood decays and carbon dioxide is released back into the atmosphere. So trees are not nearly as valuable as carbon stores as they are in coal seams and oil deposits deep underground.

To investigate the impact of even more CO₂ in the atmosphere on trees in the future, we used a technique called Free-Air CO₂ Enrichment (face). This involves installing eight-story-high pipes on the windward side of various forest patches and then carefully releasing air enriched with additional CO₂. We then observe the trees to see if the CO₂ has had any effect.

Our new study uses patches of 180-year-old pedunculate oaks (Quercus robur), much older than those previously studied, to see how they will behave in the future atmosphere.

Some forests cannot use additional carbon

Why did our studies on oak trees produce different results than studies on Australian ecalyptus trees, which found no connection between CO₂ and wood growth?

The difference is almost certainly in the nutrient content of the two forests. Trees in forests require a balance of nitrogen (N) and phosphorus (P), as well as a variety of “micronutrients” to grow well. The Australian forest grows in ancient, “highly weathered” soils where additional phosphorus is very scarce, so even if the trees receive extra carbon “for free”, they cannot use it to form more plant matter.

Our oak forest does not have such a severe nutrient shortage, so the experiment is a more direct test of whether the trees can use the new “free” carbon resource. And it seems they can, especially to form wood.

Although each forest is unique in its own way, our findings are likely to be relevant to many of the world’s temperate broadleaf forests – those forests that are neither exceptionally hot nor cold and that lose their leaves every year.

The Australian results may provide insight into how trees will respond in other phosphorus-poor environments, such as tropical rainforests. Scientists in Brazil are currently beginning a similar experiment at the Amazon Face facility, which will be operational later this year. This should add another important piece to the global forest carbon puzzle, although other Face facilities are needed elsewhere.

We looked for changes in other tissues – leaves, fine roots and seeds – in trees with increased CO₂, but the extra growth was minimal and indistinguishable from measurement uncertainty. In any case, these other tissues convert back to atmospheric CO₂ more quickly than woody tissues. So it’s fortunate that the extra carbon is stored in “woodier trees,” because the world’s hope of avoiding catastrophic climate change rests on forests compensating for the use of vital fossil fuels.

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