Trees: Carbon Sinks or Sources?

by Joanna Wolstenholme

With a warming planet, you wouldn’t be laughed at for thinking that trees may come to our rescue: we release more CO2, the greenhouse effect leads to global warming, and this increase in temperature leads to plants being able to photosynthesise faster. Indeed, future climate warming is expected by many experts to increase plant growth in temperate ecosystems, and increase carbon sequestration. So is our planet coming to the rescue, is Gaia saving us from certain doom? Well, unfortunately, no. Heatwaves lead to droughts, and droughts put plants, trees in particular, under a lot of stress. Without sufficient water, plants struggle to transpire, and so are unable to take up enough nutrients and water for growth.

Amazon_Manaus_forest

Ciais et al (2005) found that during the heatwave and drought of 2003, plants in Europe managed to undo four years of their own carbon sequestration, by reverting to sources rather than sinks of carbon. Growth primary production was severely reduced, and respiration in plants and soil microbes fell dramatically. They suggested that increased extremes of temperature, as have been predicted if we fail to reign-in climate change, may counteract the effects of the mean warming and lengthened growing season.

This work was put into stark contrast by the 2005 and 2010 droughts in the Amazon. Usually, the Amazon acts as a vast carbon sink, absorbing 25% of atmospheric carbon, making it an important buffer against climate change. However, increased occurrence of droughts could lead to it becoming a net carbon source – a catastrophic positive feedback system which would cause a vast acceleration of climate change. The 2005 drought led to the release of approximately 1.6 billion tonnes of carbon to the atmosphere, and as much as 2.2 billion tonnes of carbon could have been released from the Amazon during 2010. That is about one-quarter of global emissions from fossil fuel use. The Amazon is such a vast forest (25 times the area of the UK, to put it into perspective) that even low level drought damage can have a large overall effect, and this is likely to be impounded in the coming years by more frequent drought stresses, even at a low level.

400px-DroughtNonetheless, fear not – a report published last year by University of Exeter and Colorado State University cast a more positive slant on the situation. They believed that previous models had failed to take into account the amount of water that the forest itself is able to recycle during droughts. As moisture cycling is normally a source of a third of the water rainforest plants use, it has the potential to act as a buffer during times of drought. However, moisture cycling is severely impacted in disturbed forest, so in order for the Amazon to still withstand periods of drought, forest conservation measures must be strongly enforced.

Lets hope the academics from Exeter and Colorado are right, and that the Brazilian government are able to protect their amazing forest. In the mean time, do your own little bit – plant a tree!

For more information, see: 

Ciais et al, 2005. ‘Europe-wide reduction in primary productivity caused by the heat and drought in 2003’. Nature 437 doi:10.1038/nature03972

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