Relationship between stem CO2 efflux and stem temperature at different measuring depths in Pinus massoniana trees

The response of stem CO2 efflux to temperature is crucial in predicting the carbon cycle in forest ecosystems under future scenarios. However, the relationship between stem CO2 efflux and stem temperature at different measuring depths is not well understood. In this study we measured stem CO2 efflux and stem temperature at different depths in six Pinus massoniana trees for 2 days. We found a strong diel hysteresis between stem CO2 efflux and stem temperature. The diel hysteresis varied with increasing depth from a counterclockwise direction at a depth of 0 cm to a clockwise direction at depth of 7 cm. The effects of the high resistance to radial diffusion and sap flux on stem CO2 efflux may contribute to the diel hysteresis. The results also showed that the Q10 values of stem CO2 efflux increased from 1.51 at a depth of 0 cm to approximately 2.22 at a depth of 7 cm, indicating that the depth for stem temperature measurement plays a pivotal role in estimating the temperature sensitivity of stem CO2 efflux. Moreover, we found that 3 cm deep was the appropriate depth for stem temperature measurement, because of the highest R2 (0.96) for the efflux-temperature curve and nearly no hysteresis between stem CO2 efflux and temperature in our study. Therefore, consideration of the stem temperature measurement depth is necessary to understand the response of stem CO2 efflux to temperature and accurately fit the Q10 value of stem CO2 efflux. However, this generalization must be examined under other conditions such as different seasons or other tree species.