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Trees are generous with their gifts, cleaning the air we breathe and slowing the ravages of global warming by absorbing about a quarter of all CO₂ emissions produced by humans. It is however likely that this generosity will not last forever in the face of unabated fossil fuel consumption and deforestation. Scientists have long wondered whether trees and plants could become saturated and no longer store additional CO₂ from the atmosphere.

In their study published in Nature Climate Change, an international team including researchers from IIASA, Stanford University, the University of Vienna, and the Autonomous University of Barcelona finds reason to hope that trees will continue to take up CO₂ at generous rates until at least the end of the century. The study however warns that trees can only absorb a fraction of the CO₂ emissions, and their ability to do so beyond 2100 is unclear.

“Keeping fossil fuels in the ground is the best way to limit further warming,” says study lead author César Terrer, an IIASA alumnus and winner of the institute’s 2017 Peccei Award, who is now a postdoctoral scholar in Earth system science in Stanford’s School of Earth, Energy, and Environmental Sciences. “Stopping deforestation and preserving forests so they can grow more and absorb CO₂ is our next-best solution.”

CO₂ – which is the dominant greenhouse gas warming the earth – is food for trees and other plants. Combined with nutrients like nitrogen and phosphorus, it helps plants grow and thrive. As CO₂ concentrations rise, plants will need extra nitrogen and phosphorus from the soil to balance their diet. The question of how much extra CO₂ trees can absorb given the limitations of these other nutrients, is a critical uncertainty in predicting global warming.

“Planting or restoring trees is like putting money in the bank,” explains co-author Rob Jackson, the Michelle and Kevin Douglas Provostial Professor in Earth System Science at Stanford. “Extra growth from CO₂ is the interest we gain on our balance. We need to know how high the interest rate will be on our carbon investment.”

Several individual experiments, such as fumigating forests with elevated levels of CO₂ and growing plants in high CO₂ chambers, have provided critical data but no definitive answer globally. To more accurately predict the capacity of vegetation to sequester CO₂ in the future, the researchers synthesized data from all elevated CO₂ experiments conducted so far – in grassland, shrubland, cropland, and forest systems – including ones they directed themselves.

The study results show that CO₂ levels expected by the end of the century should increase plant biomass by 12%, enabling plants and trees to store more CO₂ – an amount equivalent to six years of current fossil fuel emissions. The study highlights important partnerships trees forge with mycorrhizal fungi to help them take up the extra nitrogen and phosphorus they need to balance their additional CO₂ intake.

“The ability of trees to benefit from rising CO₂ levels critically depends on this trading relationship”, says co-author Oskar Franklin at IIASA. “The trees spend carbon to buy scarce nutrients from the mycorrhizal fungi that can extract them from the soil much more efficiently than roots alone can do.”

The authors emphasize the critical role of tropical forests, such as those in the Amazon, Congo, and Indonesia, as regions with the greatest potential to store additional carbon, and advocate for the protection of this invaluable resource to help curb global warming into the future.

“We have already witnessed indiscriminate logging in pristine tropical forests, which are the largest reservoirs of biomass on the planet. We stand to lose a tremendously important tool to limit global warming if this is allowed to continue,” concludes Terrer.

Reference:

Terrer C, Jackson R, Prentice I, Keenan T, Kaiser C, Vicca S, Fisher J, Reich P, et al. (2019). Nitrogen and phosphorus constrain the CO2 fertilization of global plant biomass. Nature Climate Change DOI: 10.1038/s41558-019-0545-2

Adapted from a press release originally sent out by Stanford University.