Tropical forest growth could increase carbon levels

Scientists from the University of Cambridge have concluded that enhanced tree growth in tropical forests could stimulate micro-organisms and lead to a release of stored soil carbon.

The above-mentioned hypothesis is based on a six-year experiment in a central American rainforest that studied how increases in litterfall – dead plant material such as leaves, bark and twigs which fall to the ground – might affect carbon storage in the soil.

Results indicate that extra litterfall triggers an effect known as “priming” where fresh carbon from plant litter provides much-needed energy to micro-organisms, which then stimulates the decomposition of carbon stored in the soil.

According to lead author Dr. Emma Sayer, most current estimates of the carbon sequestration capacity of tropical forests are based on tree growth measurements.  

“[However], our study demonstrates that interactions between plants and soil can have a massive impact on carbon cycling,” said Sayer.  

“Models of climate change must take these feedbacks into account to predict future atmospheric carbon dioxide levels.”

Indeed, Sayer believes that a large proportion of the carbon sequestered by greater tree growth in tropical forests could be lost from the soil – with a 30% increase in litterfall projected to release about 0.6 tons of carbon per hectare from lowland tropical forest soils each year.

This amount of carbon is greater than estimates of the climate-induced increase in forest biomass carbon in Amazonia over recent decades. Given the vast land surface area covered by tropical forests and the large amount of carbon stored in the soil, the significant increase is likely to affect the global carbon balance.

It should be noted that tropical forests play an essential role in regulating the global carbon balance. Human activities have caused carbon dioxide levels to rise but it was thought that trees would respond to this by increasing their growth and taking up larger amounts of carbon.

Yet, enhanced tree growth actually leads to more dead plant matter, especially leaf litter, returning to the forest floor – and it is unclear what effect this has on the carbon cycle.

“Soils are thought to be a long-term store for carbon but we have shown that these stores could be diminished if elevated carbon dioxide levels and nitrogen deposition boost plant growth.

“This priming effect essentially means that older, relatively stable soil carbon is being replaced by fresh carbon from dead plant matter, which is easily decomposed. We still don’t know what consequences this will have for carbon cycling in the long term,” the researchers added.