Today’s paradox: CO2 uptake of ageing forests does not decrease

Could ageing forests be CO2 sinks?Somehow forest climate studies never make simple sense to us. All the more intriguing we find them of course.

The influence of forests on the world’s climate is very complex. All we really know is having more of them is ‘good for the climate,’ having less is bad – simply because it is still better to have your carbon in instable reservoirs on the Earth’s surface, than to have it in the air [in a much more stable state: CO2].

The forests’ imbalances

As you may have noticed we like to keep things clear and simple. When it comes to quantities, things can be plenty, rare, too much, enough, dying out – for all we understand. When it comes to qualities, things can be black – or they can be white. [We’re even unashamed to call it journalism.]

It is why we feel quite at ease to acknowledge forests are good – and would hope to leave it like that. However when researchers instead tell us forests are better – we get an uncomfortable feeling. Something isn’t right.

Last month there first was a publication in the Journal of Applied Ecology that told us cities are a net carbon sink – because they grow trees. That one was easy to place in perspective we thought. There’s population growth, urbanisation and city greening. There must be more young trees than old trees hence the total tree biomass in cities must (still) be increasing allowing net CO2 absorption.

But just a couple of days later there was a publication in Science that complicated things a lot further. Not just trees in cities, forests worldwide are a carbon sink, it said – ignoring the emissions of deforestation.

It got us to theorise about all sorts of possible forest climate feedbacks, like rainfall increases in tropical rainforests [even though we see the opposite in the Amazon], better nutrient availability in forest soils due to climate change [even though we found a positive climate feedback operating through soils too], an earlier start of the taiga growth season due to warming [ignoring that this same warming may negate all forest growth].

If you like such climate-forest feedback speculations: we had another one three days ago, about microbe plagues, a warning from a very distant past.

Tree species change

Today the news is good. Again there are forests that fail to be in balance. This time we look at some 65,000 square kilometres of aspen-dominated woodland in the northern part of the US Midwest – a forest type that has been researched and reported on by ecologists of Ohio State University.

The area had been deforested between 1880 and 1920 and subsequently abandoned. The aspen soon recolonised and are now close to their maximum age of around a century, meaning the forest is old – and many trees are dying.

Still however, the researchers say, these forests too are a net carbon sink, and that is for a special reason. While the aspen die, the undergrowth of new trees is much more divers, and allows other, slower growing tree species like oak and pine, which may reach ages of several hundreds of years.

At the researchers’ test forest at the University of Michigan Biological Station somehow the die-off of old aspen and the growth spurt from other trees from the forest floor allows the carbon balance to remain positive, with CO2 absorption outweighing natural emissions.

Ah, ‘emissions-offsetting forests’

Earlier the research group had calculated how the Midwest forests offset about 350,000 tonnes of carbon per year – and how this compensates for a portion of the local population.

Well then isn’t it convenient it seems nothing much will have to change about that good news story. The report ends as follows:

“The concept of using forests to store carbon has steadily gained attention among policymakers, especially since the Kyoto Protocol was adopted in 1997 as a global program to reduce greenhouse gas emissions. [The research] group has received $1 million in additional funding from the U.S. Department of Energy to continue evaluating forests’ role in storing carbon.”

Getting down to Earth

We still believe in a simple forest message: forests are good because of the carbon in the trees themselves, not because of any carbon we emit when burning fossil fuels – for the very simple reason [in the long run, globally] the two carbon equations do not correspond: an equal amount of biomass on land [the forests], cannot absorb an additional amount of carbon from burned coal layers too.

And if really you think that’s too uncomplicated for reality, than please do not forget that on a sunny autumn day our climate may in fact miss the old golden aspen trees, replaced as they may be by dominant dark pines.

© Rolf Schuttenhelm |

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