How will forests respond to climate change?

New research highlights just how important they are for storing greenhouse gasses

Scientists have known for decades that forests play a critical role in the fight against climate change. Trees capture carbon dioxide (CO₂) through photosynthesis and release life-giving oxygen into the atmosphere. But it has been harder to judge how big those benefits are, or how long they might last as humans pump more emissions into the atmosphere. Will trees absorb more greenhouse gasses? Or will the changes overwhelm them?

Scientists at the University of Birmingham are shedding light on those important questions – and their findings suggest forests provide even greater climate benefits than previously thought. Their research shows that mature trees can respond to rising levels of CO₂, locking more of it away as wood in their trunk and branches. It has demonstrated, for the first time at a global scale, that forests can absorb another potent greenhouse gas – methane.

Photo by: Vincent Gauci

An Untapped Methane Sink

Methane is responsible for about a third of the global warming that has occurred since the industrial revolution. Over 20 years, it has over 80 times the planet-warming power of CO₂. So it is a big problem that concentrations in the atmosphere have rocketed this century. But because methane is short-lived, staying in the atmosphere for only around 10 years, efforts to remove it can have quick results. They will be critical to meeting global climate targets.

Photo by: Vincent Gauci

Photo by: Vincent Gauci

Photo by: Vincent Gauci

Professor Vincent Gauci’s team set out to change that by measuring methane exchange in forests stretching from the Amazon to Sweden. After wrapping trunks in a plastic chamber, connected to a methane-measuring machine, they made a surprising discovery: while some trees emitted a small amount of methane from their base, they absorbed the gas higher up their trunks, acting, overall, as a sink.

To investigate whether this process was globally important, the scientists developed a new metric to calculate the surface area of the world’s tree bark, from trunk to twig. “It turns out there’s as much surface area of tree bark as there is on the entire global land surface. So it’s a huge area that’s in contact with the atmosphere, but it’s relatively understudied,” Professor Gauci says.

The results suggest that the climate benefits of forests could be up to 12% bigger than thought – a discovery which has implications for tackling climate change. “Now we know that trees are a methane sink, that presents an opportunity. We could have a very rapid climate win if we reforest quickly,” says Professor Gauci.

Because trees absorb methane through their bark, their surface area influences how much they can draw down, Professor Gauci explains. This means that dense young forests, which store less carbon than older trees, are likely big methane sinks and will offer faster climate benefits than thought.

Since different species absorb different amounts of methane, reforestation projects could be designed to maximize methane uptake, he says. Likewise, commercial forests could be planted with methane in mind. “We have the potential to make reforestation programs that were unaffordable, affordable, because the benefits are that much bigger,” he says.

Converting extra carbon to wood

A second University of Birmingham study shows that trees offer other previously unrecognized climate benefits. It finds that as CO₂ levels rise, mature forests can take more of the gas from the atmosphere and store it as wood, which can slow the rate of increase of CO₂ in the atmosphere. “The findings will challenge the notion that old trees won’t respond to increasing CO₂,” says lead author Professor Richard Norby.

It is no surprise that trees are a carbon sink: they absorb CO₂ through photosynthesis and convert it to glucose to grow. But less is known about whether they can take on more carbon as the atmosphere changes through an increased rate of photosynthesis – an effect known as “carbon fertilization”. Testing this at scale has been difficult, requiring researchers to raise CO₂ levels over huge areas of living forest.

To do so, the scientists used what is called a Free-Air Carbon Dioxide Enrichment (FACE) facility – a patch of ancient oak woodland in central England, fitted with huge towers that pump carbon into the surrounding atmosphere. Since 2017, CO₂ levels there have been raised by 40% to simulate mid-century conditions. The trees have responded by increasing wood production by 10%.

“The FACE experiment is giving credence to carbon fertilization as a real effect, and moving the system forward,” says Professor Norby. Birmingham’s facility is one of only three current experiments in forests globally, with partners in Australia and Brazil. So far, their findings have differed. When Australian scientists conducted a similar study of carbon fertilization in 2020, they found that its eucalyptus forest did not grow any faster as CO₂levels rose.

The difference probably lies in the nutritional condition of the two forests, according to Birmingham’s researchers. Plants have to draw more nutrients from the soil to keep up with the new growth spurred by carbon fertilization. Australia’s relatively nutrient-deficient soil may limit the eucalyptus trees’ capacity to do so. By comparison, Britain’s richer soils help its trees make use of the extra CO₂. If its trees can turn extra carbon to wood, it is likely that other temperate forests can too, the scientists argue. Once stored in wood, CO₂stays there for decades, released back into the atmosphere only when trees die or are cut down.

Photo by: Tom Downes

The limits of tree planting

That is spurring governments, companies and charities to plant millions of trees in a bid to curb global warming. But reforestation is not a panacea for climate change, the scientists warn. Environmentalists have long been concerned that authorities are resorting to “offsets”, in the form of planting trees, instead of cutting their real emissions. There is no amount of tree-planting that could absorb the greenhouse gasses emitted from burning coal and oil.

But the new research also strengthens the case against deforestation. “It highlights the importance of preserving existing forests,” says Professor Norby. “Forests are critical to carbon management, and it’s good news if they can take up a bit more carbon than had previously been predicted.”

Professor Gauci agrees: “The value of the forests we are removing in deforestation is much greater than we thought,” he says of his discovery on methane. “It means we need to protect those forests and strengthen legislation around them.”

Photo by: Tom Downes

The world’s forests are moving

Even without deforestation, the future of those forests is uncertain. The fires, floods and higher temperatures triggered by climate change threaten to critically degrade them. Dr Adriane Esquivel-Muelbert, a University of Birmingham ecologist, uses huge datasets to study how they are changing. Her work shows that climate change is driving the Amazon to a tipping point, and forcing a mass migration of trees across continents.

In the Northern Hemisphere, rising temperatures are driving species away from their traditional heartlands, in search of less extreme conditions, according to a study co-authored by Dr Esquivel-Muelbert. It drew together data from across Europe and North America, and found that trees are becoming denser in colder, wetter regions. That represents the first quantitative evidence that climate change is forcing forests to move at a continental scale, according to the researchers.

Similar patterns are evident in Brazil. Some tree species in its mountainous Atlantic Forest are migrating uphill to escape the heat, according to another study led by Birmingham postdoctoral researcher Dr Rodrigo Bergamin. It found that trees which need a colder climate are at risk of dying as temperatures rise.

These studies represent a major international collaboration, bringing together hundreds of scientists and data on millions of trees. “We work together in big networks because with enough data we can see these changing patterns, over continents and over time,” explains Dr. Esquivel-Muelbert. “Many researchers are studying the same thing in different places. Each individual site is small, but when you put everything together you get the bigger picture.”

Shedding light on how different trees respond to climate change will be critical for conservation as forests come under threat, she notes: “We are finding out about the vulnerability of different species, and about which species are suitable to different climates. That will allow us to plan for restoration programs, pinpointing what species we should invest in and where.”

In some areas, that could be the difference between preserving some forest, and none. But governments will also need to plan for change, Dr Esquivel-Muelbert argues. “It’s going to be important to facilitate migration by making sure there are corridors for trees to move through,” she says. “It’s very sad to think that some of these species will only exist in a restricted refuge. But we need to allow for that to happen if we want to preserve the millions of years of evolution that are in these very rich systems.”

This content was paid for and created by The University of Birmingham. The editorial staff of The Chronicle had no role in its preparation. Find out more about paid content.