How do trees work as carbon sinks

A. Carbon is stored in many forms on Earth, mainly in the oceans and fossil fuels. However, trees also store large amounts of the Earth's carbon. These natural carbon storage systems are known as carbon 'sinks'.

The opposite of a carbon sink is a carbon 'flow', together these two processes make up the global carbon cycle. Hundreds of tons of carbon flow through the atmosphere via natural processes such as photosynthesis, breathing, and decomposition.

By acting as carbon 'sinks, forest ecosystems can store significant amounts of carbon, the main greenhouse gas and culprit of global warming.

Q. Where do trees store carbon

A. Trees absorb carbon from the atmosphere in the form of carbon dioxide to manufacture the nutrients they need for growth. It is stored in the cells of the tree's wood and leaves.

Q. Why are some governments proposing to plant extra trees to increase the amount of sinks

A. For two reasons. First, deforestation has resulted in an unwanted increase in carbon released from the trees and disturbed soil into the atmosphere. Replanting, it is hoped, will redress this imbalance.

Secondly, it is believed that planting extra trees will absorb and retain large amounts of the unwanted carbon currently in the atmosphere and put in check the effects of global warming. In the carbon enriched atmosphere trees are expected to grow faster than normal.

Q. Does this mean that no one has to bother reducing their carbon emissions in the first place

A. Some countries, notably the USA, were hoping to use trees as carbon sinks, rather than attempt to reduce their carbon emissions, a plan criticised by environmentalists.

However recent experiments suggest that increasing the number of trees will not result in a reduction of unwanted carbon in the atmosphere.

Q. Why arent trees as effective at removing and retaining carbon as previously thought

A. Although in carbon enriched atmospheres, trees do grow quicker, they only do so for three years after which they resume their normal growth rates. It seems that trees need more than just carbon to accelerate their growth rate, shortages of nitrogen and water appear to be significant factors also.

Additionally most of the carbon that the trees absorb is retained in their leaves, rather than the forest soils. The leaves are short lived, decomposing, and so re-releasing carbon dioxide into the atmosphere much quicker than if it was retained by forest soils.

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by Lisa Cardy