Is it true that big trees give off more C02 by breathing then they use by photosythesizing?

Dear Laska,

There was a badly-reported story on this in the press a little while ago, and unfortunately this myth was the result. All normal plants, in the process of photosynthesis, take up carbon dioxide by day and release oxygen. This switches over in the dark, and they then produce carbon dioxide through respiration in the same way as animals. While they are growing, they take up more carbon dioxide overall than they give out, which gets turned into plant tissues (especially cellulose and, in the case of trees, lignin).

When a plant reaches full size, the difference between the carbon it takes up by day and emits by night drops. Trees still take up more carbon than they give off, because they continually produce leaves, repair tissues, increase their root network and add to their trunks. A lot of this carbon ends up back in the air though – when leaves and branches fall and rot, the carbon is simply released again. The forest as a whole can be approximately in balance, or can even lose carbon due to other processes (which is where the bad science reporting comes in).

There may be some occasions when a tree raises its respiration rate above photosynthesis – this might occur when producing lots of flowers or leaves, fighting a pathogen or creating chemical defences. This will, however, only be a short-term effect. Also, many deciduous trees in Europe rot in the middle of their trunk – you may see some old British oaks like this. It’s usually not harmful to the tree, but it does mean that they ‘give off’ carbon dioxide, albeit not because the tree intends to! When the tree is dying the same will happen.

So in answer to your question – overall no, large trees don’t give off more carbon dioxide than they take up, but the difference might only be very small.

Last edited by Markus Eichhorn (20th Feb 2007 09:29:06)