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The limits of rainforest growth
Trees are seen as saviours in an era of climate change. Via their leaves, they absorb carbon dioxide and transform greenhouse gas into oxygen and biomass. According to estimates by the International Panel on Climate Change (IPCC), the Amazon rainforests absorb a quarter of the carbon dioxide that is released each year from the combustion of fossil fuels. As yet, global climate models have assumed that this absorption capacity will also remain constant in the future.
"But there has been no proof of this to date", emphasizes Dr Katrin Fleischer. "It is entirely possible that the absorption capacity will even decrease." The ecologist from the Professorship for Land Surface-Atmosphere Interactions at the Technical University of Munich worked together with ecologists and ecosystem modelers from ten countries to investigate the extent to which the nutrient supply in the Amazon region limits the production of biomass.
Fourteen ecosystem models compared – Trees reaching their limits
In doing so, the team did pioneering scientific work: Up to now, nobody has investigated this connection in depth, says Fleischer: "Most ecosystem models which allow the future development of ecosystems to be simulated were developed for the temperate latitudes, where there is generally sufficient phosphorus. However, in many areas of the Amazon region it is in short supply – the ecosystem is many million years old, and the soil is leached of nutrients."
To find out how the rainforest will react to an increase in atmospheric carbon dioxide concentration, the researchers selected 14 different ecosystem models. All models were then used to simulate biomass production for the next 15 years: first for the current carbon dioxide concentration of 400 ppm and in a second scenario for an increased concentration of 600 ppm.
The result: Additional carbon dioxide can be absorbed by trees and transformed into biomass – but only if sufficient phosphorous is available. If it becomes too scarce, the CO2 fertilisation effect once again decreases. The various models, which take into account different factors, predict a decrease of 50 per cent on average in the theoretically possible additional CO2 absorption in the second scenario– whereby some even predict a 100 per cent decrease in absorption.
"This would mean that the rainforest has already reached its limit and would be unable to absorb any more carbon dioxide emissions caused by humankind", explains Fleischer. "If this scenario turns out to be true, the Earth's climate would heat up significantly faster than assumed to date."
How exactly the ecosystem would react, and whether the trees would succeed in absorbing additional phosphorous from the soil via enzymatic processes or by forming more roots which could bind and absorb the scarce nutrients, need to be researched in greater detail, summarises the ecologist: "What is certain is that the tropical rainforests are not infinitely resilient CO2 sinks."
The findings of the study will now be included in the project AmazonFACE. During this field experiment in the Amazon rainforest, trees will be "fertilised" with CO2 in order to research the reaction of the plants and the entire ecosystem. The aim is to use the data collected to improve the ecosystem models in the future.
Katrin Fleischer, et al. (2019). Amazon forest response to CO2 fertilization dependent on plant phosphorus acquisition. In Nature Geoscience. DOI: 10.1038/s41561-019-0404-9ID
Link to AmazonFACE on youtube