The lush vegetation of tropical forests is a large and globally significant store of carbon. Because tropical forests contain more carbon per unit area than any alternative land cover, cutting them down releases carbon into the atmosphere. For the same reason, growing forests take up carbon from the atmosphere. Of course, trees cannot grow for ever, and neither can forests: in the absence of disturbances that kill trees en masse - such as fires, hurricanes or logging - every forest will eventually reach a point at which tree growth and death are in equilibrium, and at which the average change in tree carbon stocks is zero. It is thus surprising that undisturbed tropical forests currently do not seem to be at equilibrium. If you measure the size of trees in a given area, calculate their carbon stocks, and then repeat the process some years later, you will on average find that the forest holds more carbon than it did before. This was first reported for Amazonian tropical forests, and on page 1003 of this issue Lewis et al. show that African forests also have increasing stocks of tree carbon.
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