Tropical forests are among the world's most important carbon sinks. A study by the Technical University of Munich (TUM), the University of Vienna, and Brazil's National Institute for Amazonian Research suggests that even small understory trees can temporarily store more CO₂ as atmospheric CO₂ levels rise. However, their long-term capacity to sequester carbon may be constrained by nutrient limitations, potentially reducing this ability.
The Amazon forest is one of the tipping elements in the global water and climate system, storing and absorbing huge amounts of CO2. Still, it is not clear to which extent trees can increase growth with more CO2 in the atmosphere. "Around 60 percent of the Amazon forest grows on old and highly weathered soils, which are already quite depleted in mineral nutrients, such as phosphorus," says Lucia Fuchslueger, researcher at CeMESS, University of Vienna, and co-lead-author of the new study.
"Low levels of phosphorus could make it difficult for the forest to grow even more and make use of the extra CO2 in the atmosphere", she adds. However, Amazonian trees have developed highly efficient internal nutrient cycles that could allow them to gain access to even more nutrients. For example, they are withdrawing nutrients from their leaves before they drop them. Also, rapid organic matter decomposition on the ground provides additional nutrients, but it is not clear if this system can get any more efficient. So far, there has been no experimental evidence from in situ forest experiments.
About the study: Future atmospheric CO₂ conditions simulated in an experiment
Nathielly Martins, a research associate at the Professorship of Land-Surface Interactions at the Technical University of Munich (TUM), and Lucia Fuchslueger conducted an experiment together with a team of Brazilian and international researchers to close this research gap. The researchers used open-top chambers to simulate future atmospheric CO2 conditions directly in the forest understory.
These chambers are made of transparent plexiglass, are 2.5 m in diameter and 3 m high, and open at the top, so that plants do not overheat and receive natural rainfall. "After one to two years, trees indeed increased their carbon uptake and growth when exposed to higher CO2 levels - at least in the short term", says Martins. The researchers found the mechanisms behind this increased growth: plants redistribute their root systems to extract more nutrients, particularly phosphorus.
"The litter layer is a key nutrient resource for plants in these forests", highlights Martins. Roots increase their growth through fallen leaves, release enzymes that decompose organic matter, and get access to phosphorus before it is transferred into the soil and may become resorbed.
"However, this strategy intensifies competition with soil microbes and may deplete organic phosphorus reserves", adds Lucia Fuchslueger. Over time, nutrient constraints could limit the forest's ability to continue absorbing additional carbon. The findings reveal a critical trade-off: while tropical forests may initially buffer climate change more strongly, their long-term capacity to store carbon could be restricted by nutrient availability - highlighting the vulnerability of these ecosystems under future climate conditions.
Martins, N. P.; Fuchslueger, L.; Lugli, Laynara F. et al: Amazonian understory forests change phosphorus acquisition strategies under elevates CO2. Nature Communications: DOI: https://doi.org/10.1038/s41467-026-72098-0
- Nathielly P. Martins is a post-doctoral researcher at the TUM School of Life Sciences .
- Lucia Fuchslueger is a Junior Research Group Leader at the Division of Terrestrial Ecosystem Research at the Centre for Microbiology and Environmental Systems Science (CeMESS) and coordinates the Nutrient Research Area in the Amazon FACE project.
The study serves as a pilot study for the larger-scale, multi year AmazonFACE project which starts later this year. AmazonFACE aims to understand the role of tropical primary forests, specifically of the Amazon forest, under increasing atmospheric CO2 concentrations. FACE is an abbreviation for Free Air CO2 Enrichment.
