Amazon understory forests show short-term boost in CO₂ uptake – but this comes at a cost

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.

The study focuses on the Central Amazon and was co-led by Lucia Fuchslueger, a researcher at the Centre for Microbiology and Environmental Systems Science (CeMESS) at the University of Vienna and Nathielly Martins from the Technical University of Munich, Germany, and the National Institute of Amazonian Research (INPA) Manaus, Brazil, together with a team of Brazilian and international collaborators. The study has been published in Nature Communications.

"Around 60% of the Amazon forest grows on old and highly weathered soils, which are already quite depleted in mineral nutrients, such as phosphorus," says Fuchslueger. "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.

The researchers used a pioneering open-top chamber experiment to simulate future atmospheric CO₂ conditions directly within 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 (see picture).

"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," explains Martins. “We 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”.

Roots increase their travel 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. 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.

These findings build on earlier research by IIASA scientists, which combined models and experimental evidence to explore how tropical forests respond to higher CO₂ levels and limited nutrients. The earlier work suggested that while increased CO₂ can boost plant growth, low phosphorus availability may constrain this response, requiring plants to invest more in their root systems.

“Elevated CO₂ will increase carbon uptake and growth, but low nutrient availability – especially phosphorus – can limit this response, potentially shifting how carbon is stored in tropical forests,” notes IIASA researcher Florian Hofhansl, who was also a coauthor of the new study and led the previous work at INPA before joining IIASA. “This new study now provides direct experimental evidence supporting these ideas, showing that Amazonian understory plants adjust their nutrient acquisition strategies by increasing root activity in the litter layer to meet higher nutrient demands under elevated CO₂ conditions”.

Implications for the future: Pilot study for bigger project

The study serves as a pilot study for the larger-scale, multi-year Amazon Free-Air CO2 Enrichment

(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 outdoor experiments have been done in many places already, but none in one in a highly diverse, tropical forest system," says Fuchslueger.

AmazonFACE is located about 80 km north of Manaus in the middle of a typical terra firme, lowland forest, and will be the first in this large scale in the tropics. It is run by a team of Brazilian and international researchers and combines about 130 scientists, students, technicians, administrators, journalists and artists from about 40 institutions. A truly international, transdisciplinary effort.

Adapted from a press release prepared by the University of Vienna. Read the original article.

Reference
Martins, N.P., Fuchslueger, L., Lugli, L.F., Valverde-Barrantes, O.J., Norby, R.J., Hartley, I.P., Aleixo, I., Baccaro, F.B., Brum, B.N.S., Cássio Silva de Souza, C., Cola, C.M., Di Ponzio, R., Damasceno, A., Domingues, T.F., Ferrer, V.R., Fleischer, K., Garcia, S., Guedes, A., Hofhansl, F., et al. (2026). Amazonian understory forests change phosphorus acquisition strategies under elevated CO2. Nature Communications DOI: 10.1038/s41467-026-72098-0