Tropical forest CO2 fertilisation: self-mitigation of emissions possibly around 15 percent

In which case increased tropical forest density would sort of average out emissions of tropical deforestation. [Is it just us or do you share the feeling something is uncomfortably unsustainable about that comparison?]

A couple of days ago we reported on a new Nature study, which states there could be extra release of CO2 from forest soils, due to an expected increase in litterfall – of around 30 percent for a 150 ppm rise in CO2 concentrations, citing an other Nature study, from 2009. We promised to take a better look.

Today we find out that the cited 2009 study [summary | PDF] does not actually look into the 21^st century, but back into the 20^th. That’s the wrong direction for making real forecasts, but it is of course the way to go if you want to take a look at trends.

If that trend should continue and if indeed that would be due to a CO2 fertilisation effect, here we do obtain a sense of numbers – or rather the utmost limits of optimism, as these numbers are within reach of being swallowed should the forests’ carbon cycle at any point start misbehaving, and throw around with some more of those rainforest droughts or boreal forest wildfires.

African tropical forests

Following findings that in the Amazon in old growth forests the forest density had been increasing a large international group of researchers led by Simon Lewis of the University of Leeds set out to find if they could show similar results for the tropical forests of Africa.

That’s what they studied, that’s what they found. But there are of course all sorts of extrapolations to be made.

Annual carbon increase

Sifting through existing data, the team discovered that between 1968 and 2007 across 79 plots (163 ha) in African tropical forests the living above ground biomass had increased with 0.63 tonnes of carbon per hectare per year, which means annual CO2 sequestration lay at around 2.3 tonnes per hectare of forest.

Presuming above ground biomass could be associated with an increase in roots [hot topic that is] and an increase in dead biomass carbon sequestration [which the new Nature study challenges, remember?] and extrapolating over the size of the African continent, the researchers conclude that over the last couple of decades the African tropical forests each year sequestered some 0.34 gigatonnes of carbon, equivalent to 1.26 Gt of CO2.

Comparable data from tropical America and tropical Asia indicate a similar yet somewhat lower forest density increase there, of around 0.49 tonnes of carbon per hectare per year.

This means worldwide, the tropical forests, over the last couple of decades, would have sequestered around 1.3 Gt of carbon, or 4.8* gigatonnes of CO2, per year.

[*) A month ago we looked at a Science study that placed the figure as high as 9 Gt CO2 per year for all the world's forests - between the years 1990-2007. A smaller portion of sequestration would be taking place outside the tropics.]

Option 1: placing in CO2 perspective

And here is where we can chose to safely land the CO2 fertilisation effect, and compare annual sequestration to emissions – and be done with it.

If CO2 is responsible for the forest density increase that means – comparing to last year’s fossil fuel CO2 emissions of 30.6 Gt – the world’s fossil emissions would mitigate 16 percent of themselves. [The percentage should actually be lowered by some 2 or 3 percent if we want to compare to all anthropogenic emissions, including land use change.]

Option 2: challenging the fertilisation hypothesis

There are however a lot of howevers. We can’t simply cut off one sixth of CO2 emissions and still get to the same rapid [last year +2.6 ppm] increase of atmospheric CO2 concentrations, scientists are measuring. We’d have to go look for a missing source.

And wouldn’t it be a little ironic if that missing source – despite CO2 fertilisation and beside continued large-scale deforestation which is of – turned out to be the same old growth tropical forests, as gigatonnes of emissions from recent drought records suggest?

There are also a couple of fundamental problems with the CO2 fertilisation hypothesis. The whole concept is built around the idea that CO2 is a limiting growth factor to plant growth. But on land much more often that’s nitrogen or phosphorous – or, speaking of climate change, plain old water. On the ocean’s too, apart from acidification concerns, it is unlikely extra CO2 leads to extra phytoplankton – as here it’s classically iron that is deficient.

Only under ideal growth circumstances, like in an agricultural greenhouse, may carbon be the factor plants would be craving for. And if of course ideal growth circumstances would be found in any real ecosystem, pointing to the tropical rainforests, wet & warm as they are, recycling nutrients like nitrogen in rapid biomass growth and break-down, does make better sense then trying to apply the same hypothesis to the taiga or to temperate forests.

But then there’s another thing. Recent research has found plants are closing down stromata under 150 years of rising CO2 levels. Why would they do that if they could use the extra CO2? And if stromata size is that adaptable, why didn’t the rainforests trees in preindustrial times just make them a little bigger instead?

© Rolf Schuttenhelm | www.bitsofscience.org