Keystone copse

How much carbon dioxide does a forest soak up? It's an important question because planting forests is one of the actions that countries can take to offset some of their carbon-dioxide emissions, and achieve their target set by the Kyoto Protocol - the agreement drawn up by developed countries, in 1997, to limit their greenhouse-gas emissions. And coniferous plantations, with their trees laid out in neat rows, are valuable commercially for their timber, as well as being an environmental asset by helping to soak up some of the carbon dioxide belched out by power stations, traffic and industries. It is no wonder then that these plantations are springing up all over the developed world, as governments race to get their green credits established.

But, unfortunately, coniferous plantations may not be as "green" as they might appear. Recent research from Scotland suggests that commercial plantations are not soaking up as much carbon dioxide as was first thought, and that sometimes they may even be contributing to the global-warming problem. Dave Reay, and his colleagues from the School of Geosciences at the University of Edinburgh, have discovered that the practice of thinning trees in plantations turns the forests from absorbers of carbon dioxide into producers of it.

Just like a gardener needs to thin out their baby lettuce, carrot and cabbage plants, forestry managers need to thin out their young trees. Thinning stops the plants and trees from overcrowding and gives them the space to grow. The result is tall, strong and sturdy trees, which are valuable commercially. But unlike the gardener, who thins out seedlings using a finger and thumb, the forestry manager has to thin the forest using heavy machinery, which makes a much bigger mess. Roughly every fifth row of trees is chopped down; the branches are stripped from each tree and the trunks taken away to the sawmill. Huge piles of branches, called brash, are left behind, to rot into the ground.

And this is where the problem lies. Bacteria that decompose plant matter gobble up the brash and react with oxygen to produce carbon dioxide, water and energy. Much of the carbon that was once stored in the brash is released back into the atmosphere as carbon dioxide.

The Edinburgh team has been studying Griffin forest, a privately owned sitka spruce plantation about 45km to the north west of Perth, Scotland. Over the past six years they have been measuring the flux of carbon dioxide entering and leaving the forest, using instruments perched on scaffolding just above the tops of the trees. Infra-red gas analysers suck the air in through a tube and then measure the levels of carbon dioxide and water vapour 20 times every second. By combining the measurements from different heights in the tree canopy, the scientists are able to calculate the overall amount of carbon dioxide going into and out of the forest at any one time.

Until April 2004 the results seemed to suggest that sitka spruce plantations are excellent absorbers of atmospheric carbon dioxide. Calculations revealed that the forest was drawing down around 24 tons of carbon dioxide per hectare per year, balancing out the emissions from four average British households. Small forests like Griffin (4,000 hectares, or around 4,000 football pitches) appeared to be capable of absorbing all the carbon-dioxide emissions from a town. But then in April 2004 the forest was thinned and everything changed.

Within a matter of weeks more than a third of the trees had been felled, and huge great piles of brash lay rotting into the soil. Reay and his colleagues realised that the decomposition of the brash would be releasing carbon dioxide into the air, but they didn't know how much, so they set about trying to measure it. They constructed enormous tunnels made from plastic sheets, like the poly-tunnels that strawberries are grown in, over some of the piles of brash. Around the outside of the tunnels sausage-shaped sandbags held the plastic down and infra-red gas analysers were connected up to the air inside the tunnel, via plastic tubes.

The emissions were huge. Reay cannot give the precise figures at the moment because the data has not been published, but he gave some idea of the magnitude by saying, "the process of thinning the trees completely reversed the forest from being a powerful carbon-dioxide sink to being a carbon-dioxide source".

Part of the reason for these extreme carbon-dioxide emissions was the time of year that the thinning was carried out. Summer is normally the peak time for carbon-dioxide uptake by the forest, with the warm weather and long days encouraging tree growth. Unfortunately summer is also the time of year when dead matter decomposes fastest. By carrying out the thinning in the spring, Griffin forest suffered a double blow, losing more than one-third of its trees at its most productive time and gaining large amounts of fast-rotting brash.

Trees only draw down carbon dioxide during the day, when they are photosynthesising, while decomposition keeps going day and night. In Griffin forest there was no way that the remaining trees could keep up with the huge amounts of carbon dioxide being pumped out by the rotting brash. Come winter, the rate of rotting is likely to slow down and eventually, when the brash has completely rotted, the forest will return to being a carbon-dioxide sink. Reay is continuing his monitoring to find out how long it takes before the switch back occurs.

Whatever the time of year that the forest is cut, Reay's results show that forest thinning has a large, albeit temporary, detrimental effect on the forest's overall ability to absorb carbon dioxide. Thinning forests also involves lots of heavy machinery and is an expensive process.

At the moment the timber market is not booming, and commercial forests in Europe are usually thinned just once, when the trees are around 25 years old. In the United States thinning sometimes happens a little more to create fire-breaks and reduce the risk of a blaze. However, if timber starts to become more valuable, it is likely that forestry managers will try to thin their forests more frequently to improve the quality of the trees.

Obviously thinning is an important part of the management process of the forest, but Reay has a clever idea to offset some of the damage done by thinning. "The brash is perfect to use as a bio-fuel," he explains. "I can imagine brash from forest-thinning keeping a good-sized power station running." Although this wouldn't stop the brash from emitting carbon dioxide, it would indirectly help decrease our carbon-dioxide emissions by reducing our need to burn fossil fuels to produce our power.

Bio-energy is beginning to be taken more seriously and it could help the Government achieve its target of supplying 10 per cent of the country's electricity from renewable energy by 2010. In October 2004 the Government set up a task force to encourage the production of bio-fuel crops.

Coniferous plantations make up nearly 60 per cent of all British woodland. In Scotland these commercial plantations are big business. As our desire for paper and cheap furniture grows, these plantations are springing up all over Europe, the United States and Canada. Even when thinning is taken into account it is likely that most of these plantations will still be overall absorbers of atmospheric carbon dioxide. But the thinning process means that they are not as good at sucking in carbon dioxide as we first thought, and governments may have to come up with other ideas to gain their carbon credits. Using the brash as a bio-fuel would be a good start.