The research reveals that older trees significantly increase their woody biomass production in response to higher atmospheric CO2 levels, challenging the existing belief that mature forests cannot adapt to elevated CO2 conditions.
The study found that over a seven-year period, exposure to increased CO2 levels (ambient atmosphere + 150 parts per million CO2; approximately a 40% increase) led to an average increase of 9.8% in wood production. Importantly, there was no corresponding rise in the production of leaves or fine roots, which release CO2 back into the atmosphere relatively quickly.
These findings, published on August 12 in 'Nature Climate Change', underscore the potential of mature forests as medium-term (decades-long) carbon reservoirs and natural climate solutions. The data was gathered from the long-running free-air CO2 enrichment (FACE) experiment at the University of Birmingham's Institute of Forest Research (BIFoR), located in central England.
BIFoR researchers conducted the FACE experiment in a 180-year-old deciduous woodland dominated by 26-meter tall English oak trees. The experiment involved six 30-meter diameter plots, three of which were exposed to elevated CO2, while the other three served as controls.
Professor Richard Norby, the study's lead author from the University of Birmingham, stated, "Our findings refute the notion that older, mature forests cannot respond to rising levels of atmospheric CO2, but how they respond will likely depend on the supply of nutrients from the soil."
He added, "Evidence from BIFoR FACE of a significant increase in woody biomass production supports the role of mature, long-established, forests as natural climate solutions in the coming decades while society strives to reduce its dependency on carbon."
FACE experiments simulate future atmospheric conditions and provide valuable insights into the interactions between forests, the atmosphere, and climate. While previous studies showed increased forest productivity under elevated CO2, they were conducted in younger tree plantations, leaving uncertainty about the response of older trees.
Co-author and BIFoR Director Professor Rob MacKenzie, also from the University of Birmingham, remarked, "We believe these results, at about the halfway point of our fifteen-year experiment at BIFoR FACE, will prove invaluable for policymakers around the globe as they grapple with the complexities of climate change."
He further emphasized, "FACE experiments such as ours provide foundations for predictions of future atmospheric CO2 concentrations and so greatly improve confidence in policy decisions. But even if the increase in tree growth translates to a medium-term increase in carbon storage in forests, this in no way offers a reason to delay reductions in fossil fuel consumption."
Since its inception in 2017, the BIFoR FACE experiment has been altering the atmosphere around the forest and measuring the impact of elevated CO2 on wood production using laser scanning to convert tree diameter measurements into wood mass.
Researchers calculated the overall forest growth (referred to as net primary productivity, NPP) by combining the wood production of the oaks and understory trees with the production of leaves, fine roots, flowers, and seeds, as well as the amount of biologically active compounds released from roots.
In 2021 and 2022, NPP was 9.7% and 11.5% higher, respectively, in elevated CO2 conditions compared to ambient levels-an increase equivalent to approximately 1.7 tonnes of dry matter per hectare per year. The majority of this increase resulted from wood production, with no significant change in fine-root or leaf mass production.
To contextualize this additional forest carbon storage, it equates to about 1% of the CO2 emitted by a single commercial passenger aircraft on a one-way flight from London to New York, per hectare per year. The total carbon uptake by the long-established forest per hectare per year is ten times greater, highlighting the scale of forest protection and management needed to offset even essential fossil fuel emissions.
The BIFoR FACE experiment will continue into the 2030s to explore long-term responses and the interactions between forest carbon, other plant nutrients, and the forest food web.
Research Report:Enhanced woody biomass production in a mature temperate forest under elevated CO2
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