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.