Mature forests play key role in fight against climate change

Mature forests have a key role to play in the fight against climate change – extracting carbon dioxide (CO₂) from the atmosphere and locking it into new wood, a new study reveals.

Researchers from the University of Birmingham’s Institute of Forest Research (BIFoR), working alongside colleagues from Bangor University, Swansea University and the University of Exeter, discovered that older trees responded to increased atmospheric levels of CO₂ by increasing production of woody biomass – countering existing theories that mature woodland has no capacity to respond to elevated CO₂ levels.

The experts found exposure to elevated levels of the greenhouse gas (ambient atmosphere CO₂) increased wood production by an average of 9.8% over a seven-year period. There was no corresponding increase in production of material such as leaves or fine roots which release CO₂ into the atmosphere relatively quickly.

Their findings, published in Nature Climate Change, support the role of mature forests as long-term carbon stores and natural climate solutions – thanks to data from the long-running free-air CO₂ enrichment (FACE) experiment at the University of Birmingham’s Institute of Forest Research (BIFoR), in central England.

Researchers at BIFoR established a FACE experiment in a 180-year-old deciduous woodland dominated by 26-m tall Quercus robur trees – six 30 metre diameter plots, three exposed to elevated CO₂ with the other three plots acting as a control.

Ecologists from Bangor University studied fine root production of the mature oak trees at BIFoR FACE to determine a belowground component of net primary productivity. Ephemeral fine root production, mortality and decomposition are fundamental to the carbon dynamics in forest ecosystems.

Lead author Professor Richard Norby, from the University of Birmingham, commented: “Our findings refute the notion that mature forests cannot respond to atmospheric CO₂, leaving the natural capture of carbon to young, growing forests.

“Evidence from BIFoR FACE of a significant increase in woody biomass production supports the role of mature forests as natural climate solutions in the coming decades while society strives to reduce its dependency on carbon.”

FACE experiments mimic future atmospheric conditions and provided valuable data on interaction between biosphere and atmosphere. Previous experiments found that forest productivity can increase under elevated CO₂ but were conducted in young tree plantations – raising questions about whether older trees would respond in the same way.

Co-author and BIFoR Director Professor Rob McKenzie, from the University of Birmingham, commented: “We believe the results of our seven-year experiment at BIFoR will prove invaluable for policy makers around the globe as they grapple with the complexities of climate change in the coming months and years.

“FACE experiments such as ours are hugely important for predicting future atmospheric CO₂ concentrations and driving policy decisions. But even if the increase in tree growth translates to longer-term increase in carbon storage in the ecosystem, this cannot be seen as reason to delay reductions in fossil fuel consumption.”