Forests and grasslands: carbon sinks
2003, a black year
But for how long? How will the ecosystems – notably forestry ones – behave over the course of the 21st century, when the Intergovernmental Panel on Climate Change (IPCC) projections are expecting a temperature rise of two to six degrees from now until the year 2100? At this stage nobody can provide an answer with any certainty. Nevertheless, various measurements carried out on the occasion of extreme events, such as the drought which struck Europe in 2003, provide some elements of a response. And they are not necessarily reassuring. ‘That particular Summer the behaviour of the ecosystems in a way went into a reverse process,’ observes Marc Aubinet. ‘Instead of sequestering carbon, the terrestrial ecosystems released 500 Tgs of carbon into the atmosphere, in other terms a quantity which is as good as the equivalent of what is sequestered over two and a half years!’ The increase in the frequency of such extreme events thus risks significantly diminishing the sequestration potential of terrestrial ecosystems.’
What exactly happened in 2003? It appears that two phenomena played in opposite directions. On the one hand, the drought slowed down micro-organism activity, leading to a reduction of respiration on the part of forest ecosystems and, through that, a reduction of CO2 emissions. But on the other hand it also slowed down carbon assimilation by the plants. The latter phenomenon is thought to have had greater weight than the former, leading finally to a net carbon emission. This phenomenon was not experienced everywhere: very paradoxically 2003 resulted in one of the largest sequestrations of carbon out of the thirteen years studied at Vielsalm. Why? The exact causes of this unusual behaviour remain to be discovered.