Stem CO2 sensors measure the carbon dioxide concentration inside a tree stem. The continuous measurements allow us to draw conclusions about the respiration and metabolism of the trees.
A thumb-thick sensor head is drilled 5 cm into the water-conducting wood, sealed airtight from the outside air and protected against direct sunlight with reflective bubble wrap. The sensor is thus located in an air-filled cavity in the stem measuring CO2 concentration of the enclosed air [in volume %]. Since the exchanges between the CO2 in the cavity and the CO2 in the stem water take place very quickly, these measurements can also be used to deduce the CO2 concentration in the transported water.
Increased CO2 levels occur especially during the wood growth phase in early summer. The values are lower in the dormancy in winter and in the second half of the summer. The CO2 concentrations measured in the stem can reach a two-digit percent by volume and are thus surprisingly high, up to several hundred times higher than in the ambient air.