New forest study hints at negative climate feedback

But fails to address all-important issues like droughts, forest fires, and invasive pests. Besides, the CO2 balance between increased soil oxidation and a nitrogen-induced tree growth spurt remains unclear.

Science has almost settled around the CO2 fertilisation theory (prevalent in the US during the 90s) – and proven it wrong. We know plants easily adapt their stromata and, what is more important, carbon is hardly ever -in any ecosystem- the growth-limiting nutrient.

That is not to be said of nitrogen, or phosphorous. Especially in colder ecosystems, with thick organic soils, consisting of peat – only very slowly decomposing and recycling its elements back to biology.

When we look at for instance the boreal forests or taiga in the northern parts of North America, Europe and Asia, the low bioavailability of nitrogen could well limit the growth with which the endless amounts of pine trees grow. A limitation on growth is a limitation on biomass production – so a nitrogen deficiency may well be an ecological restriction to the amount of carbon dioxide that is being sucked from the air.

Changing the typical climates of both the boreal forests and the temperate forests on the northern hemisphere may change this biochemical bottleneck, a recent publication in PNAS suggests.

A group of 15 researchers, most of whom connected to the Marine Biological Laboratory in Massachusetts, have artificially warmed the soil of a piece of deciduous forest in that same state – and closely monitored the carbon cycle around that plot for 7 years.

The temperature was increased by 5 degrees Celsius. That is a substantial warming – but within the range of IPCC scenarios for this century and one that climate scientists fear could well be reality if positive feedbacks to climate change pick up.

Both uptake and emissions increase

The researchers found two main effects: CO2 emissions of the organic soils rose, due to increased oxidation of the dead biomass. Meanwhile bioavailability of nitrogen within the soil also improved. This provoked increased tree growth and therefore increased CO2 absorption by the living biomass, although that did initially not outweigh the increased oxidation emissions.

Over the years this seemed to change though, with increased CO2 uptake of tree growth possibly becoming the dominant force with time passing by.

But that is what happens to any ecosystem that you give a constant environment and then leave undisturbed: it grows happy. Problem with climate change in the real world is that it is a grumpy player.

It does not manifest itself as a linear temperature rise as much as it shows itself unexpectedly, in different forms, ranging from record droughts, raging wildfires to pine beetle plagues. And then it’s just one hot, dry summer and we are talking net CO2 emissions that easily outweigh the Chinese coal consumption for a year – or even much, much more.

That’s worrying, because the taiga does not have its own delegation present at UNFCCC conferences. Nor does the Amazon [added 25% to global emissions 2010]. Or Borneo [added 40% to global emissions 1998], or the peat marshes of the Arctic tundra – all biotopes that represent many gigatonnes of instable carbon, close to the surface, ready to dry out, burn or indeed be eaten.

© Rolf Schuttenhelm |

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