Science&Technology: Keystone Copse ; Coniferous Plantations Were Billed as the Ideal Way to Soak Up Greenhouse Gases. but the Way We Manage Our Woodlands May Actually Be Contributing to Global Warming Rather Than Mitigating Its Effects, Says Kate Ravilious
Ravilious, Kate, The Independent (London, England)
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. …