- Stem CO2 concentrations (stem [CO2]) undergo large temporal variations that need to be understood to better link tree physiological processes to biosphere–atmosphere CO2 exchange.
- During 19 months, stem [CO2] was continuously measured in mature subalpine Norway spruce trees (Picea abies) and jointly analysed with stem, soil and air temperatures, sap flow rates, stem radius changes and CO2 efflux rates from stem and soil on different time scales.
- Stem [CO2] exhibited a strong seasonality, of which over 80% could be explained with stem and soil temperatures. Both physical equilibrium processes of CO2 between water and air according to Henry's law as well as physiological effects, including sap flow and local respiration, concurrently contributed to these temporal variations.
- Moreover, the explanatory power of potential biological drivers (stem radius changes, sap flow and soil respiration) varied strongly with season and temporal resolution. We conclude that seasonal and daily courses of stem [CO2] in spruce trees are a combined effect of physical equilibrium and tree physiological processes. Furthermore, we emphasize the relevance of axial diffusion of CO2 along air-filled spaces in the wood, and potential wound response processes owing to sensor installation.