Trees do not grow faster in a CO2 -rich atmosphere

Whether rising atmospheric carbon dioxide (CO2) concentrations will cause forests to grow faster and store more carbon is an open question and one that scientists at the Paul Scherrer Institute (PSI), together with partners at the University of Basel, have been investigating for several years.

In a mature forest just outside Basel, researchers developed a new system to distribute CO2 to the treetops. The use of stable isotopes for studying the carbon balance under elevated CO2 is a speciality of PSI. With these tools the PSI researchers determined the amount of carbon(C) that was assimilated by the tree crowns and the proportions of how much C was invested in leaves, wood, roots and soil, or lost by respiration.

The data, published in the latest issue of Science, (Vol. 309/Nr. 5739) evidences that the optimistic prediction of the CO2 storage capacity of forests probably needs to be qualified. Although the trees in the study took up more carbon in a CO2 rich atmosphere, there was no sustainable increase in biomass carbon. The data suggests that they instead “pump” more carbon through their body.

Isotopes are atoms of the same element that are differentiated by their mass and stable isotopes such as the rare 13C and the abundant 12C atom (which forms 99% of carbon reserves) are invaluable tools in environmental research. The artificially distributed CO2 contained less 13C than the CO2 usually present in the atmosphere. Based on the different 13C / 12C isotope ratio in the organic matter the amount of new carbohydrates in leaves, wood and roots and consequently the amount of carbon released by the tree back into the atmosphere could be determined. The relationship of the isotopes was measured at PSI with a mass-spectrometer. In this instrument CO2 gas is ionised and accelerated in a high voltage field. The ion beam is deflected by a magnet and due to their differing mass the isotopes are separated out and their number can thus be determined.