|The carbon cycle (Illustration: Lisa Larsson)|
Carbon is transformed in a complex cycle between living organisms, atmosphere, oceans and soil. Plants store carbon via photosynthesis, and carbon is released again through respiration and decay. Humans add extra carbon to the cycle by burning fossil fuels, leading to an increased carbon dioxide content in the atmosphere and, hence, the greenhouse effect. Our land usage has reduced the total amount of living plants and disturbs the soil, leading to reduced carbon storage on land, which instead moves to the atmosphere and the oceans. It takes a long time for carbon dioxide to be taken up from the atmosphere, so it is therefore vital to reduce burning of fossil fuels in order not to cross any "tipping points" after which changes can become practically irreversible. Carbon sequestration by forests only amounts to a small fraction of the fossil fuels burned globally.
A lot of carbon is stored in trees due to their size, but is important to make a distinction between carbon stock and carbon sequestration (the latter indicating the difference between carbon uptake through photosynthesis and carbon release through respiration). A young forest sequesters much carbon but does not have a large stock of it. Old-growth forests can continue to sequester carbon while simultaneously having a large carbon stock. If such a forest is harvested a lot of carbon is released from the trees as well as from the soil. In general it takes 10-20 years for a clear-cut to begin sequestering carbon again, and on peat lands it can take even longer, if at all. In order to regain the same carbon stock, the forest must grow to at least the same age it had before harvesting.
In forests, carbon is stored in trees and other organisms and in dead organic matter on and below the ground. Below the ground, plant roots and fungi are joined in large networks spanning entire forests, called mycorrhiza. A large fraction of the carbon in old forests is stored by myccorhiza networks. When a forest becomes really old, carbon sequestration levels off, but in modern forestry, harvesting is done long before this stage is reached. Despite this, European forests have worked as a carbon sink over the last 15 years, but in a longer perspective of of 250 years European forests have released a lot of carbon to the atmosphere due to forestry, in particular the conversion from mixed forests to pine forests.
A plantation has a smaller carbon stock than an old natural forest because trees are not allowed to grow really large and because the soil is disturbed in harvesting and other forestry operations. We have to preserve the large carbon stocks in old, never clear-cut forests. In the forests that we continue to use, methods must be optimized to maximize the amount of carbon in the ecosystem. Whether forestry methods exist that simultaneously store carbon, conserve biodiversity and allow for harvests is not clear.