Key message Besides stem temperature, seasonality and vertical gradient in stem diameter growth strongly affect both seasonal and vertical variations in stem CO2 efflux.
Abstract Stem CO2 efflux (E-s) is known to vary seasonally and vertically along tree stems. We measured E-s at various heights in a 50-year-old hinoki cypress [Chamaecyparis obtusa (Sieb. et Zucc) Endl.] stand over 2 consecutive years in Central Japan. Effects of stem temperature (T-s), daily stem diameter increment (D-i), and difference in vapor pressure deficit between nighttime and daytime (VPDdiff) on seasonal variation in daily E-s were examined by a generalized linear model (GLM). Daily E-s exhibited a substantial seasonal variation, which was mostly affected by T-s and D-i. Any effect of VPDdiff, which was assumed to be an index of daily sap flow rate, was not detected. The GLM prediction of daily E-s from the combination of T-s and D-i showed good agreement with the observed seasonal trend in daily E-s. These results suggest that, in addition to stem temperature as the important environmental factor, stem radial growth is a significant phenological factor influencing seasonal variation in E-s. Additionally, the intercept of GLM, which indicates the basal daily efflux that is independent of T-s and D-i, was closely related to the annual diameter increment. Furthermore, a strong relationship was found between annual E-s and annual diameter increment. These findings suggest that variation in stem diameter growth along stems is considerably responsible for the observed vertical variation in E-s. Therefore, stem radial growth can affect both seasonal and spatial variations in stem CO2 efflux.