University of Birmingham

Measuring the impact of climate change on forests

Forests are known as the lungs of the planet because they take up carbon dioxide (CO₂) and return oxygen to the atmosphere. They are a critical tool in our fight against man-made climate change: the CO₂ which they capture through photosynthesis is converted to glucose, which underpins all the forest food chains, while trees and soils sequester huge reserves of carbon. Yet forests across the world are under threat from land clearance and changing climate patterns. The University of Birmingham's Institute of Forest Research (BIFoR) is shedding light on their uncertain future.

BIFoR’s researchers are tracking how forests are responding to both rising levels of CO₂, and extreme temperatures, drought and wildfire. To test the effects of the planet’s increasing CO₂ levels, they use a huge open-air experiment known as the BIFoR Free Air CO₂ Enrichment Facility, or BIFoR FACE—a mature area of Staffordshire oak woodland, where CO₂ is pumped into the air to simulate a future atmosphere. Elsewhere, BIFoR is aggregating huge pools of historical data on tree size and health to measure how forests, in the UK and around the world, are responding to changing climatic patterns.

Testing how forests respond to higher CO₂

The BIFoR FACE facility, launched in 2017, is one of only three worldwide, with counterparts in Brazil’s tropical Amazon and Australia’s eucalyptus forest. CO₂ levels at BIFoR FACE have been increased by almost 40% to simulate the conditions we’re on track to have by 2050.

“We are taking an experimental approach from the laboratory into the forest and looking at the whole forest response, not just the response of a single leaf or tree,” says Professor Rob MacKenzie, BIFoR’s inaugural director. “The question is whether the forest, at 180 years old, is able to adapt to the very rapid change we’ve imposed on it, or will it just not have that agility? And we’re seeing a lot of adaptability in our forest.”

Now in their eighth growing season, the trees living under higher CO₂ conditions are showing changes. The 40% increase in CO₂ levels has boosted their summer average photosynthesis by around 20%. In turn, that has resulted in faster growth—though by a lower margin of 10%, says Professor MacKenzie. “The trees are apparently more vigorous in the high CO₂ environment. That should not be a surprise. Plants absolutely love CO₂. They evolved in a higher CO₂ atmosphere than we are presently in. But they do need a balanced diet”, he warns.

As plants alter their nutritional balance to make use of the sugars produced by faster photosynthesis, changes are also evident below ground. “We’re seeing the oaks do really clever things, like conserving nutrients so they can reuse them to harvest even more carbon,” says Professor MacKenzie. “They change their nutritional balance partly by adapting themselves, and partly by adapting the microbial system around them. It’s brilliantly sophisticated.”

Extreme temperatures push forests to a tipping point

It is not only rising CO₂ levels that forests must adapt to. They are also being stressed by changing climate patterns, from rising temperatures to drought and wildfires. Research by BIFoR ecologist Dr Adriane Esquivel Muelbert suggests this is having a potentially devastating impact on tropical forests.

Her work draws on huge international datasets on forest health. Simple measurements of tree diameter, for instance, highlight which species are thriving and how much carbon trees are accumulating. By integrating these datasets, Dr Esquivel Muelbert gathers a picture of how forests are changing. “We can only do this because hundreds of people have been collecting data for 30 years across many regions,” she says.

The findings suggest that the combined stresses of climate change and deforestation could push the Amazon towards a tipping point. A new stable state for the Amazon would be characterised by trees storing less carbon and more open canopies. This would compromise key services Amazonia provides to the climate stability of the planet.

“Tree mortality is increasing in the tropics. Trees dying more often leads to more forest gaps, where pioneer trees thrive,” Dr Esquivel Muelbert explains. The proliferation of those pioneer species is a problem because they grow fast and die younger, storing less carbon. “We may lose the capacity of the forest to work as a carbon sink,” she says. “And that is where the changes in species across the Amazon could lead us to a tipping point, where the death of slow-growing species releases carbon into the atmosphere, the planet gets warmer, and that favours those pioneer species again. It’s a positive feedback loop.”

Similar trends are evident in tropical forests across the world. Another of Dr Esquivel Muelbert’s research projects assesses the mortality of giant trees in Cameroon, Brazil, Malaysia and Panama funded by NERC and NSF in partnership with the Cary Institute of Ecosystem Studies. Though these giants make up to half a tropical forest’s biomass, very little is known about them. Her research aims to find how long those ancient trees remain in the canopy, what causes their death, and how much carbon will be emitted if they are lost. This will help us understand the future of these giant trees and thus of tropical forests.

Informing restoration efforts

Findings from BIFoR’s research have real policy impacts. The FACE facility is informing climate modellers in the UK, who supply projections for the Intergovernmental Panel on Climate Change’s (IPCC) scientific assessment reports. These, in turn, feed into the Conference of the Parties (COP) policy process. “The most fundamental way we can change society’s response to climate change is through that IPCC process,” says Professor MacKenzie. Funding from the UK’s Foreign and Commonwealth Development Office (FCDO) is allowing BIFoR to share lessons with the younger AmazonFACE facility. Scientists from the three FACE facilities are working together to assess how their different forest biomes might contribute to the battle against rising emissions.

BIFoR research is also contributing to the stewardship of forests. Dr Esquivel Muelbert led a policy brief for the UN’s Scientific Panel for the Amazon in 2023. By establishing which species show resilience to extreme temperatures or drought, her work informs restoration efforts in Brazil’s ‘Amazon arc of deforestation’, where trees have been cleared for agriculture. “We are using the data on the ecology of the forest, and how species are distributed, to find which species are going to be best suited now and in the future, so they can be planted,” she says. The findings are feeding into a reforestation campaign sponsored by Brazil’s Development Bank, in partnership with the Serrapilheira Institute.

Yet tree-planting projects should not be seen as a solution to climate change or land clearance,

the researchers stress. No amount of reforestation can counter the effects of man-made climate change, cautions Professor MacKenzie: “Reforestation is definitely not a silver bullet for the amount of carbon dioxide we are putting into the atmosphere,” he says. “There is not enough forest in the world, and nor could there be, to relieve us of the need for drastic reductions in emissions from fossil fuel-burning.”

Yet forests do give hope, and the strides we are making to understand them support critical conservation efforts. “We should be clear that restoring an area of forest doesn’t have the same value as an intact forest—in terms of biodiversity, in terms of ecosystem services, in terms of the amount of carbon that is stored there,” Dr Esquivel Muelbert says. “Our research also provides evidence for the argument of preserving forests.” Doing so will benefit humans and biodiversity.