Rainforests can only absorb CO2 as long as nutrient stores last

Investigations by an international team of researchers – including the Technical University of Munich (TUM) and the University of Exeter – indicate that the absorption capacity is severely limited by the phosphorus content of the soil.

Trees are seen as saviours in an era of climate change. Via their leaves, they absorb carbon dioxide and transform the 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.

To date, 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,” said Dr Katrin Fleischer.

“It is entirely possible that the absorption capacity will even decrease.”

Dr Fleischer, an ecologist from the Professorship for Land Surface-Atmosphere Interactions at TUM, worked together with ecologists and ecosystem modellers from 10 countries to investigate the extent to which the nutrient supply in the Amazon region limits the production of biomass.

In doing so, the team did pioneering scientific work: To date, nobody has investigated this connection in depth.

“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,” Dr Fleischer said.

“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.”

In order to find out how the rainforest will react to an increase in atmospheric carbon dioxide concentration, the researchers selected 14 different ecosystem models.

The Joint UK Land Environment Simulator (JULES), which is developed by the Met Office, Centre for Ecology and Hydrology and UK universities including Exeter, was among the models investigated for this research.

All models were then used to simulate biomass production for the next 15 years: first for the current carbon dioxide concentration of 400 part per million (PPM) and in a second scenario for an increased concentration of 600 PPM.

The result: Additional carbon dioxide can be absorbed by the trees and transformed into biomass – but only if sufficient phosphorous is available.

If it becomes too scarce, the CO2 fertilization effect once again decreases.

The various models, which take into account different factors, predict a decrease in the theoretically possible additional CO2 absorption in the second scenario of 50 percent on average – whereby some even predict a 100 percent 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 human kind,” Dr Fleischer said.

“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 needs to be researched in greater detail.

Dr Fleischer added: “What's certain is that the tropical rainforests are not infinitely resilient CO2 sinks.”

The paper, published in Nature Geoscience, is entitled: “Amazon forest response to CO2 fertilization dependent on plant phosphorus acquisition.”