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Trees absorb CO₂ through their leaves and turn greenhouse gases into oxygen and biomass. According to estimates by the International Panel on Climate Change (IPCC), the Amazon rainforest in particular, absorbs around a quarter of the CO₂ that is released each year due to fossil fuel combustion. Today’s global climate models assume that this capacity will be maintained in the future due to the so-called CO₂ fertilization effect, which proposes that rising levels of CO₂ promote vegetation growth by accelerating the rate of photosynthesis that plants use to produce biomass. Deforestation, agricultural expansion, and rising temperatures are however pushing the storage capacity of the Amazon forest to its limit, and according to researchers, it is unclear how long it will remain a carbon sink. An international team including researchers from IIASA explored this question using data from the first tropical Free Air CO₂ Enrichment (FACE) experiment conducted in the middle of the Amazon rainforest.

The AmazonFACE project includes a unique experimental technical set-up at a study site located approximately 70 kilometers north of Manaus, Brazil, in which the ambient CO₂ concentration is artificially increased, thus enabling a realistic investigation of how future CO₂ concentrations will affect the ecosystem. Researchers monitor how the trees are growing and the leaves are developing aboveground, while also tracking root growth and what is happening in the soil below ground. In their paper published in Nature Geoscience, the authors applied a number of ecosystem models to investigate the extent to which soil nutrient supply might limit the production of biomass in tropical forests.

“To date, nobody has investigated this connection in depth,” explains Katrin Fleischer, a researcher based at the Technical University of Munich (TUM) and lead author of the study. “Most ecosystem models that simulate the future development of ecosystems have been developed for temperate latitudes, where there is generally enough phosphorus. In many parts of the Amazon, however, this element is scarce – the ecosystem is several million years old, and the soil is correspondingly leached of nutrients.”

The researchers used 14 different ecosystem models to explore how the rainforest responds to an increase in CO₂ concentrations in the atmosphere. These models were then used to simulate biomass production for the next 15 years – first, for the current CO₂ concentration of 400 ppm, and in a second scenario, for an increased concentration of 600 ppm.

The results indicate that trees indeed absorb additional CO₂ and convert it to plant biomass, but only if plenty of phosphorus is available. If the supply is insufficient, the CO₂ fertilization effect decreases substantially. The different models, which take into account various factors, predict a possible reduction of additional CO₂ uptake by an average of 50%, while some predict a reduction of up to 100% depending on the scenario.

According to the researchers, this would indicate that the rainforest could already be at its limit to absorb manmade CO₂ emissions. If this scenario proves true, the Earth’s climate would heat up much faster than previously assumed.

“Most models try to reduce the complexity of the system and therefore could be missing important ecosystem processes, which might include crucial feedbacks and thus result in an overestimation of the CO₂-fertilization effect,” says Florian Hofhansl, a postdoctoral researcher at IIASA and one of the study co-authors. “The AmazonFACE experiment will provide us with new insights for model development, which should enable us to make more reliable predictions and thus improve future projections.”

The authors note that how the ecosystem reacts – whether trees will be capable of getting additional phosphorus from the soil through enzymatic processes or by forming more roots and symbiotic interactions that can provide scarce nutrients – must be further investigated. One thing is however certain, tropical rainforests are not an infinite CO₂ sink and the Amazon forest reservoir must be preserved.

Click on the video link below for a closer look at The Amazon FACE project: Exploring the impact of climate change on the Amazon forest