Determining the relationship between tree-stem respiration and CO ₂ efflux by ÎO₂/Ar measurements

Respiration in forest tree stems is an important component of the global carbon cycle. This respiration is traditionally estimated by measurements of the CO₂ efflux from the stem. However, recent studies have suggested that movement of CO₂ in the transpiration stream causes large errors in the respiration estimated by the CO₂ efflux. Here we demonstrate a new approach for determining the ratio of respiration to CO₂ efflux, which is based on specially designed chambers, and combined CO₂ and O₂ measurements. The high accuracy O₂ measurement is performed by mass spectrometric measurement of the O₂/Ar ratio. Testing the method gave repeatable results which point that in some conditions up to 40% of the respired CO₂ can be carried away from the site of respiration.