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

Tropical forests are one of the planet’s most important carbon sinks – often also called “the lungs of our Earth”. But their future in a high-CO₂ world remains uncertain. New research from the Central Amazon with researchers from the University of Vienna, the Technical University of Munich, and the National Institute for Amazon Research, Manaus, suggests even small understory trees may initially buffer climate change more strongly. But their long-term capacity to store carbon could be restricted by nutrient availability – highlighting the vulnerability of these ecosystems under future climate conditions. The results have been published in Nature Communications. 

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. 

The new study is co-led by Lucia Fuchslueger (CeMESS, University of Vienna) and Nathielly Martins (Technical University of Munich, Germany; INPA Manaus, Brazil), together with a team of Brazilian and international collaborators. 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”, 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 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”, 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.

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 CO₂concentrations. FACE is an abbreviation for Free Air CO₂ Enrichment. “FACE outdoor experiments have been done in many places already, but none in a highly diverse, tropical forest system”, says Fuchslueger. AmazonFACE, 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.

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 AmazonFACE project.

Nathielly Martins is a Research Associate and Postdoctoral Researcher at the Professorship of Land-Surface Interactions at the Technical University of Munich (TUM)