A headline statistic is that the carbonsequestered (or stored) by half ahectare of conifer woodland over onerotation can compensate for the carbondioxide emissions associated with carfuel consumption during one averagedriver's lifetime.

However, with 30 million registereddrivers in the UK, three quarters of theland area of the nation would have tobe covered in forest to make car usealone carbon-neutral. Thereforeplanting more trees is an attractive partof mitigating climate change, but canclearly never be the whole solution.

Just how much carbon does onetree store?

A recent study carried out at Kielder Forest has calculated that the Forest’s150 million trees lock up 82,000 tonnesof carbon* annually. This means that asa rough estimate each tree at Kielder islocking up 0.546 kg of carbon per year– equivalent to 2 kg of carbon dioxide.

Although this example does answer theapparently simple question ‘how muchcarbon does one tree store,’ in realitythe answer is far from straight forward;it is dependent on species, growing conditions and how a tree is managed.For example, 2500 trees might beplanted per hectare in a commercialplantation (broadleaf or conifer) but only50–500 remain when the final crop isharvested as a result of natural mortalityand thinning.

In addition, young trees absorb carbondioxide quickly while they are growing,but as a tree ages a steady state iseventually reached. At this point theamount of carbon absorbed throughphotosynthesis is equal to that lostthrough respiration and decay.

Mitigation: Planting more treesTrees store carbon. One of the practical ways to combat climate change is to lockup or sequester more carbon from the atmosphere through planting more trees - as long as the right trees are planted in the right place.

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Forests and forestry can be an important and attractive part of the solution

Definition: The term mitigation refersto activities aimed at reducinggreenhouse gas emissions and/orremoval of carbon dioxide from theatmosphere.

Facts and figures• UK forests and woodlands contain

around 150 million tonnes ofcarbon in the biomass and 640million tonnes of carbon in the soil.

• UK forests and woodlands are acarbon sink, as they remove about10 million tonnes of carbon fromthe atmosphere every year.

• Current (2008) UK emissions ofcarbon dioxide are about 530million tonnes per year.

• Other greenhouse gas emissionsequate to an additional 100 milliontonnes of carbon dioxide per year.

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

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It is, therefore, a much simpler conceptto talk about how much carbon an areaof woodland can sequester or store.

In the UK, forest soils contain aroundfour times as much carbon as the trees.Maintaining the forest area will helpensure these stocks of carbon areprotected. Soils can release carbondioxide when they become aerated asa result of disturbance (such as plantingand felling) or drainage. This effect ismost marked in organic or peat soils.On most soils, long-term carbon gainsthrough new woodland establishmentwill outweigh initial carbon losses dueto soil disturbance. The continual inputof organic materials from leaf litter anddecomposing roots will graduallyincrease the soil carbon content.

Over a full rotation, including planting tofelling, a conifer forest can sequesteraround 14 tonnes of carbon dioxide per

hectare per year. When UK woodlandsare looked at as a whole, the average isaround 5.4 tonnes of carbon dioxideper hectare per year (or 1.4 tonnes ofcarbon per hectare per year), includingbroadleaved and unproductivewoodlands. In terms of total carbon

storage, a commercial coniferplantation grown over 50 years mightsequester 50–100 tonnes of carbon perhectare. In contrast, an old growthforest may store up to 250 tonnescarbon per hectare but over a muchlonger period (300 years or more).

• As long as the right trees are planted in the right place,planting more trees can be an attractive way of removingcarbon dioxide from the atmosphere

• However overall, tree planting can only ever play a verysmall part in climate change mitigation

Summary

Carbon offsetting

Trees and forests have a clear role toplay in helping to mitigate climatechange, and tree planting projectshave been proposed as valid ways tohelp ‘offset’ unavoidable carbonemissions – carbon offsetting.However, there has been resistance tosuch projects for a number of reasons,including the following:

• Is the activity ‘additional’ or wouldthe tree planting have occurredanyway?

• Will the woodland be permanent, orwill the carbon be re-emitted in thefuture?

• Will the carbon be counted onlyonce – or by a number ofindividuals/organisations?

• Are the emissions reductions real?

The most important point is thatoffsetting – whether through treeplanting or not – should not be the first

thought; reducing emissions shouldalways be the main objective.

Secondly, offsetting requires certaintyin the emissions reductions takingplace. This is a very difficult issue fortree planting projects, which generallyprovide funding for carbon uptake inthe future.

However, it is undeniable that plantingnew woodlands in appropriatelocations removes carbon dioxide fromthe atmosphere, and also provides anumber of other environmental andsocial benefits that many otheroffsetting options do not provide.

For these reasons the ForestryCommission has developed a ‘Code ofgood practice for forest carbon projects’which will be published in 2010. Thiswill provide answers to thesequestions and provide a ‘standard’which tree planting projects can signup to and be assessed against.

Definitions:

SequestrationThis is the act of removing (literallyseizing) carbon dioxide from theatmosphere and storing it in biologicalmaterial.

SinkA forest is termed a carbon ‘sink’ if thereis a net transfer of carbon from theatmosphere to the forest. A forest onlyremains a sink while its carbon stockcontinues to increase.

StoreWood products are a store of carbon, asthey themselves do not capture carbondioxide from the atmosphere, but keepit locked up throughout their lifetime.

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