Open-top chamber for the Experiment in the Central Amazon.
Photo Credit: © Dado Galdieri
Scientific Frontline: Extended "At a Glance" Summary: Amazon Understory Carbon Uptake Under Elevated \(CO_2\)
The Core Concept: Experimental exposure to elevated \(CO_2\) demonstrates that understory trees in the Amazon initially increase their carbon uptake and growth, though this long-term capacity is ultimately constrained by soil nutrient availability.
Key Distinction/Mechanism: To support increased growth from extra atmospheric \(CO_2\), Amazonian plants must rapidly redistribute their root systems into the fallen leaf litter layer and release enzymes to decompose organic matter. This aggressive extraction of scarce phosphorus intensifies competition with soil microbes and depletes organic reserves, distinguishing these nutrient-limited tropical responses from those in more fertile ecosystems.
Major Frameworks/Components:
- In Situ \(CO_2\) Simulation: The use of transparent, open-top chambers to simulate future atmospheric \(CO_2\) conditions directly within the forest understory without altering natural rainfall or temperature.
- Nutrient Acquisition Strategies: The study of root redistribution, enzymatic organic matter decomposition, and efficient internal nutrient cycling to secure phosphorus.
- Plant-Microbe Competition: The ecological trade-off where increased plant scavenging for nutrients intensifies competition with essential soil microbes.
- Free Air \(CO_2\) Enrichment (FACE): The foundational methodology for testing ecosystem responses to elevated carbon dioxide, being uniquely adapted here for highly diverse tropical lowland forests.
Branch of Science: Terrestrial Ecology, Biogeochemistry, Climate Science, and Environmental Geoscience.
Future Application: The methodologies and initial data establish the baseline for the large-scale AmazonFACE project, which will ultimately refine global climate models by providing accurate constraints on the carbon sequestration limits of tropical primary forests.
Why It Matters: The Amazon rainforest is a critical tipping element in the global climate system, often relied upon to absorb anthropogenic carbon emissions. Discovering that nutrient constraints place a hard limit on its ability to act as a continuous carbon sink highlights the severe vulnerability of these ecosystems under future climate conditions.
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| Open-top chamber for the Experiment in the Central Amazon. Photo Credit: © Maria Juliana de Melo Monte |
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-\(CO_2\) 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 Amazon forest is one of the tipping elements in the global water and climate system, storing and absorbing huge amounts of \(CO_2\). Still, it is not clear, to which extent trees can increase growth, with more \(CO_2\) 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 \(CO_2\) 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.
Future atmospheric CO₂ conditions simulated in an experiment
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 \(CO_2\) 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 \(CO_2\) 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.
Pilot study for bigger 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 \(CO_2\) concentrations. FACE is an abbreviation for Free Air CO₂ Enrichment. "FACE outdoor experiments have been done in many places already, but none in one 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.
Published in journal: Nature Communications
Title: Amazonian understory forests change phosphorus acquisition strategies under elevated \(CO_2\)
Authors: Nathielly P. Martins, Lucia Fuchslueger, Laynara F. Lugli, Oscar J. Valverde-Barrantes, Richard J. Norby, Iain P. Hartley, Izabela Aleixo, Fabricio B. Baccaro, Barbara N. S. Brum, Crisvaldo Cássio Silva de Souza, Carine M. Cola, Raffaello Di Ponzio, Amanda Damasceno, Tomas F. Domingues, Vanessa R. Ferrer, Katrin Fleischer, Sabrina Garcia, Alacimar Guedes, Florian Hofhansl, David M. Lapola, Juliane G. Menezes, Anna C. M. Moraes, Ana Caroline Miron, Leonardo Ramos de Oliveira, Cilene Palheta, Iokanam S. Pereira, Maria Pires Martins, Gyovanni Ribeiro, Jéssica Schmeisk-Rosa, Anja Rammig, Flavia D. Santana, Yago R. Santos, Lara Siebert Silva, Bruno Takeshi T. Portela, Gabriela Ushida, and Carlos A. Quesada
Source/Credit: Universität Wien
Reference Number: eco042826_01
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