Feedback from Climate Change: increased tropical forest growth could release carbon from the soil

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Measuring CO2 efflux from the soil in subplots where the forest floor has been replaced with litter with a distinct isotopic signature. A wire mesh tent excludes forest litter from the subplots. © Dr. Emma Sayer

More evidence has emerged of the possible feedback effects of climate change as the world warms. A new study shows that as climate change enhances tree growth in tropical forests, the resulting increase in litterfall could stimulate soil micro-organisms leading to a release of stored soil carbon.

A team of scientists from the Centre for Ecology & Hydrology and the University of Cambridge 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. Their results show that extra litterfall triggers an effect called '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. The conclusion results from a six-year experiment in a rainforest at the Smithsonian Tropical Research Institute in Panama, Central America.

Lead author Dr Emma Sayer from the UK's Centre for Ecology & Hydrology said, "Most estimates of the carbon sequestration capacity of tropical forests are based on measurements of tree growth. Our study demonstrates that interactions between plants and soil can have a massive impact on carbon cycling. Models of climate change must take these feedbacks into account to predict future atmospheric carbon dioxide levels."

The study concludes that a large proportion of the carbon sequestered by greater tree growth in tropical forests could be lost from the soil. The researchers estimate that a 30% increase in litterfall could release about 0.6 tonnes 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, this could affect the global carbon balance.

Understanding the complexities of Climate Change
Feedback effects are an important part in understanding the climate change process. As the world warms (or cools) the follow-on or side-effects of primary changes such as forest die back, habitat change or ice melt can amplify or diminish the primary change. A primary change is known as climate forcing. The follow on effects are called feedback.

While there is an overwhelming consensus in the scientific and research community that the world is warming, a clear and unambiguous understanding of what might happen to world as the climate changes is yet to be agreed. More and more studies, however, are showing that climate change is happening faster and that the changes are more complex and inter-related than first thought.

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. However, enhanced tree growth 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.

Dr Sayer added, "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."

Co-author Dr Edmund Tanner, from the University of Cambridge, said, "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."

‘Soil carbon release enhanced by increased tropical forest litterfall’ was published online in the journal Nature Climate Change on 14 August 2011 (DOI: 10.1038/nclimate1190).

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