Forests and grasslands: carbon sinks
11/20/09

If cultivated farmland areas and peat lands are net carbon emitters, forests and grasslands are ‘sinks.’ These genuine carbon ‘banks’, capable of accumulating it or releasing it to a quicker or slower extent, deserve to be analysed in minute, magnifying glass detail.

Oceans and vegetation: if these two gigantic carbon sinks did not exist, the planet Earth would be an unbearable fire ship for most animal and plant species. Just in themselves they manage to capture and recycle half of the carbon gas produced by humankind and emitted into the atmosphere, and which is responsible – along with other gases – for climate warming. But what will the situation be like tomorrow? How will these two gigantic natural ‘plugs’ be able to play their regulating role, whilst each passing day sees an additional 35 million tons of CO2 reach the atmosphere? (1)

Fundamental for the future of our societies, such a question is the object of intense curiosity in research bodies the world over. Europe is not being outdone. For around 15 years, the CarboEurope network, which gathers together around a hundred science institutions spread throughout the European Union (in other words around a fifty of the sites studied), has worked hammer and tong in order to better understand carbon fluxes, and through this to draw up a carbon balance sheet of continental Europe. Thanks to the efforts carried out we today know that the European terrestrial ecosystems (Russia included) sequester around 205 Teragrams of carbon a year (Tera = a million millions, in other words 1012). The term ‘sequester’ means that the ecosystems absorb more CO2 than they emit. To make these figures more concrete, we can compare them to the annual emissions in the Union: these 205 Tgs constitute 12% of the total CO2 emissions or 70% of the emissions linked solely to terrestrial forms of transport. This is far from being negligible.

Carbon cycle

Unsurprisingly, the forests carve out a lion’s share for themselves in this balance sheet. They sequester two to three times more carbon than the grasslands, in other words 220 Tg for the former and 85 Tg for the latter (cultivated land and peat lands are for their part net emitters of respectively 33 and 76 Tg, which explains that the European ecosystems as a whole sequester 220 + 85 – 33 – 67 = 205 Tg). But is the role the forests play really that unsurprising? Marc Aubinet, who has directed for close to ten years the Biosystems Physics Unit at Gembloux Agro-Bio Tech (University of Liège), as well as the measuring stations integrated within CarboEurope (at Vielsalm in the Ardennes and at Lonzée in Hesbaye), invites us to not underestimate the importance of such a discovery. ‘We obviously suspected that through photosynthesis and the fabrication of biomass the European forests fully carried out their role as carbon sinks, above all in the Spring and Summer periods. But we shouldn’t forget that a good part of this carbon is exuded by the plants themselves. All in all it is not so easy to put figures on the net sequestration which results from these two conflicting processes. We thus also asked ourselves if, for example, the oldest forests didn’t play an opposite role, releasing – through respiration – larger quantities of carbon than they absorbed. Thanks to CarboEurope, we today know that this is not at all the case. Even aged forests (hundred years and more) store carbon.’

 

(1)« Avis de tempêtes », Jean-Louis Fellous, Odile Jacob, 2003.

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