Disturbing the carbon balance
The Amazon rainforest covers an area of more than 5 and a half million square kilometres in South America. Some describe it as the “lungs of the planet” because, each year, the forest soaks up more than 2 billion tonnes of carbon dioxide. But the area is under threat from logging, including the practice of “selective logging” although how this affects the forest - and the carbon budget - isn’t known. Chris Smith hears from one researcher who has been finding out...
Camille: My name is Camille Piponiot and I work at the University of French Guiana. We’re looking here at ‘selective logging’ which is a widespread practice in the Amazon. It consists in selectively harvesting a few trees and leaving the rest of the forest to regenerate after logging. We know that it has big impact on the forest – the trees that you harvest and the trees that you damage by harvesting some trees will emit carbon to the atmosphere which means greenhouse gases. But what we also know is that the rest of the forest that is left to regrowth will store carbon back and that’s what we are interested in – how it stores carbon.
Chris: This is quite a complicated question though, isn’t it? So how did you approach studying that and working out what the impact of that process is across the Amazon and is it the same all over the Amazon, because the Amazon is a big place?
Camille: Yes. So, what we have is data from satellites that can tell you how the forest is growing or you can have data that has been measured on the ground with for example, measurement of tree growth. Each tree is measured and that’s what we have. We have some plots all over the Amazon that have been logged and then that have been measured for several years as much as 30 years. By using that data, we have a lot of data on all the processes that go on in the forest after logging.
Chris: So you can basically work out how much carbon must be being emitted when you look at the scale of the logging and the removal of certain trees, and damage of trees when big trees are felled. And then you can look at the long term – what grows back and therefore, how much carbon do you think it’s drawing down as it grows back and you can sort of do the carbon equation.
Camille: Exactly. You can measure very easily how much carbon was emitted in the first year. It’s a lot harder to have data on the – all the processes that go on after logging because it lasts for many decades.
Chris: So, on what sort of scale is this happening? Tell us how much of the Amazon is being exploited in this way?
Camille: So, in the Amazon, 2 million hectares are exploited every year at least, and that means 1 per cent of the whole region every year.
Chris: Gosh! That’s a lot, isn’t it – 1%? And what do the numbers look like when you begin to do the sums for carbon? What does the equation look like?
Camille: We looked at the effect of climate and also soil properties, and we found that there were big differences between regions, and that the regions that have less rainfall and more seasonality of rainfall will have less carbon storage after logging. And so, the forest grows less after logging and stores less carbon than in the north of the Amazon where there is more rainfall and less seasonality of rainfall. And there, we saw that stored more carbon especially the big trees that were left after logging.
Chris: So what you're saying is that although you found this relationship, the Amazon isn’t made equal everywhere and there are some areas which are going to be more vulnerable to certain types of logging or agricultural practice than other places. And so, it’s not just a simple case of, we have one-size-fits-all as a policy for how to log the Amazon sustainably.
Camille: Yes, exactly. We saw that everywhere, the big trees were the ones that were storing more carbon. So, I guess that having logging practices that avoid damage on big trees will help the carbon recovery after logging, but not all regions are equal and the south of the Amazon is more vulnerable to logging or maybe other practices and grows slower after logging.
Chris: So, what is the take home message and how does this change our opinion of the Amazon is managed?
Camille: The Amazon is a very diverse region and when we think about planning logging, you have to be very careful of where we are because it might affect some forests more than others. Also, if we want to improve carbon models, taking into account climatic or other environmental variables is very important because you could improve a lot the accuracy of the models.