Tucked away down a quiet country lane in the UK’s West Midlands region lies one of the most studied patches of forest in the world.
This fragment of woodland, populated mainly by 180-year-old English oak trees (Quercus robur), is at the heart of a major global project to understand how the world’s forests will cope as levels of carbon dioxide rise in the atmosphere.
It is a crucial question. Globally, the world’s forests , once emissions from forest fires, deforestation and other disturbances are accounted for. Temperate forests, such as those in the UK, are responsible for almost half of that uptake.
Advertisement
But can we rely on this carbon sink as pollution increases? By 2050, atmospheric concentrations of CO2 will be 40 per cent higher than today’s levels if current trends continue, at roughly 570 parts per million (ppm). Many of today’s trees will still be standing. How will they respond?
At the forest in Staffordshire, scientists have been subjecting groups of mature oaks to a simulated future atmosphere. Since 2017, they have been pumping CO2 around these trees, elevating local concentrations to 570 ppm.
at the UK’s University of Birmingham, which runs the site, has been watching carefully to see how the trees respond. Basic biology tells us that with higher levels of CO2, the rate of photosynthesis increases. In other words, trees and plants will respond to more CO2 in the atmosphere by drawing down more of it. Experiments on young trees . But until recently, we knew little about how more mature trees would respond.
“The way that a young plant in a greenhouse or an agricultural field will respond over a short period, that’s quite well known,” says MacKenzie. “But how an older plant sitting in its soil for 180 years will respond to a similar stimulus is completely unknown.”
The answer is critical for ensuring climate models accurately represent the role of forests in absorbing CO2 in the future. If the photosynthesis rate changes and the trees absorb more carbon, the global carbon drawdown rate of temperate forests will increase. On the other hand, if the carbon uptake rate of the trees doesn’t change, the role of forests as a climate solution will be smaller than we thought. “We will influence how accurate the Earth system models are in terms of their treatment of the land carbon sink,” says MacKenzie.
Thankfully, the results so far are promising. After seven years under elevated CO2 conditions, the mature oaks have increased their photosynthesis rate and , compared with nearby trees under today’s atmospheric conditions.
“We’re pleased by the results because they don’t show a forest that is going to fall over, ecologically speaking, under this kind of stress. It does seem like there is some adaptive capability here,” says MacKenzie. “It might be the case that even as we go into a high-CO2 atmosphere, the land carbon sink maintains its current role in slightly bending the curve [of atmospheric CO2].”
Yet , which also began in 2017, has found no link between elevated atmospheric CO2 and extra tree growth. Why is this British forest different?
The answer lies in the availability of nitrogen and phosphorus, key nutrients that enable trees to make use of excess CO2. In Australia, the forest was limited by a lack of nutrients, but the Staffordshire site has plenty – thanks in part to fertiliser use on nearby farmland. “Everything about the results we have got so far is really down to the fact that the forest has sufficient nitrogen to utilise the carbon,” says MacKenzie.
There is also emerging evidence that the mature oaks are deploying new strategies to secure their supplies of nitrogen. They are growing new root networks at a rapid pace to mine for fresh nitrogen reserves in the soil and conserving their supplies by releasing less nitrogen through their roots and leaves, the team has found. “These strategies have enabled this forest not to exhibit any sign of nitrogen limitation,” says , also at the University of Birmingham.
It represents a remarkable shift in activity for middle-aged trees, says MacKenzie. “They have had 180 years to explore the soil,” he says. “You might well hypothesise that the soil is completely mapped from the plant’s perspective. But it turns out there are still things they can do if the resource changes.”
Other shifts are afoot in the forest. The trees in elevated CO2 conditions have more bitter chemicals in their leaves, which the research team suspects might be a sign they are investing more in their resistance to pests and diseases. There are also signs these trees might be recovering more quickly from short periods of heat stress, resuming photosynthesis activity before the control trees.
But sustaining the rapid pace of activity depends on nitrogen remaining plentiful. In western Europe, liberal use of agricultural fertilisers has left soils overloaded with nitrogen. The trees under elevated CO2 are drawing down this excess now, but it may not last forever. In previous elevated CO2 experiments performed on younger trees, nitrogen supplies have eventually dwindled, leading to a slump in the rate of photosynthesis. “If the trees are using more nitrogen, they are going to eventually deplete the soil of nitrogen,” says at the University of Birmingham. “It’s a positive story for us so far, but in the long term we are not sure.”
The Staffordshire experiment will run until 2030, at which point the researchers hope to have unpicked whether nitrogen limitation is a serious threat to temperate forests. Another elevated CO2 experiment is under construction in the Amazon rainforest, to test how tropical forests respond.
Yet tweaking CO2 concentrations can only tell us so much. While these trials can simulate future atmospheric CO2 levels, they can’t simulate future weather. Advancing climate change will bring more frequent and more intense heatwaves, droughts and floods. Researchers can study the trees’ response to these extreme events when they occur in the real world, but it won’t be an accurate reflection of the wilder, more extreme conditions the forests will face by 2050.
In any case, wood is also only ever a temporary carbon store. Even if trees survive the growing threats of droughts, heatwaves, floods, pests and diseases, they will eventually die. As the wood rots, the stored carbon is released back into the atmosphere.
It would be foolish, therefore, to rely too heavily on forests as a climate saviour, says MacKenzie, even if their increased photosynthesis rate is sustained. “It’s only a help; it’s not a solution,” he says.
