Mitigation: Planting more trees

Trees store carbon. One of the practical ways to combat climate change is to lock up or sequester more carbon from the atmosphere through planting more trees - as long as the right trees are planted in the right place.

A headline statistic is that the carbon sequestered (or stored) by half a hectare of conifer woodland over one rotation can compensate for the carbon dioxide emissions associated with car fuel consumption during one average driver's lifetime.

However, with 30 million registered drivers in the UK, three quarters of the land area of the nation would have to be covered in forest to make car use alone carbon-neutral. Therefore planting more trees is an attractive part of mitigating climate change, but can clearly never be the whole solution.

Just how much carbon does one tree store?

A recent study carried out at Kielder Forest has calculated that the Forest’s 150 million trees lock up 82,000 tonnes of carbon* annually. This means that as a rough estimate each tree at Kielder is locking up 0.546 kg of carbon per year – equivalent to 2 kg of carbon dioxide.

Although this example does answer the apparently simple question ‘how much carbon does one tree store,’ in reality the answer is far from straight forward; it is dependent on species, growing conditions and how a tree is managed. For example, 2500 trees might be planted per hectare in a commercial plantation (broadleaf or conifer) but only 50–500 remain when the final crop is harvested as a result of natural mortality and thinning.

*One tonne of carbon is equivalent to 3.7 tonnes of carbon dioxide.

In addition, young trees absorb carbon dioxide quickly while they are growing, but as a tree ages a steady state is eventually reached. At this point the amount of carbon absorbed through photosynthesis is equal to that lost through respiration and decay.

It is, therefore, a much simpler concept to talk about how much carbon an area of woodland can sequester or store.

In the UK, forest soils contain around four times as much carbon as the trees. Maintaining the forest area will help ensure these stocks of carbon are protected. Soils can release carbon dioxide when they become aerated as a result of disturbance (such as planting and felling) or drainage. This effect is most marked in organic or peat soils. On most soils, long-term carbon gains through new woodland establishment will outweigh initial carbon losses due to soil disturbance. The continual input of organic materials from leaf litter and decomposing roots will gradually increase the soil carbon content.

Over a full rotation, including planting to felling, a conifer forest can sequester around 14 tonnes of carbon dioxide per

hectare per year. When UK woodlands are looked at as a whole, the average is around 5.4 tonnes of carbon dioxide per hectare per year (or 1.4 tonnes of carbon per hectare per year), including broadleaved and unproductive woodlands. In terms of total carbon

storage, a commercial conifer plantation grown over 50 years might sequester 50–100 tonnes of carbon per hectare. In contrast, an old growth forest may store up to 250 tonnes carbon per hectare but over a much longer period (300 years or more).

Definitions:

Sequestration This is the act of removing (literally seizing) carbon dioxide from the atmosphere and storing it in biological material.

Sink A forest is termed a carbon ‘sink’ if there is a net transfer of carbon from the atmosphere to the forest. A forest only remains a sink while its carbon stock continues to increase.

Store Wood products are a store of carbon, as they themselves do not capture carbon dioxide from the atmosphere, but keep it locked up throughout their lifetime.