Effect of stem radial growth on seasonal and spatial variations in stem CO₂ efflux of Chamaecyparis obtusa

Key message: Besides stem temperature, seasonality and vertical gradient in stem diameter growth strongly affect both seasonal and vertical variations in stem CO₂efflux. 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 (VPD_diff) 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 VPD_diff, 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.