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 CO ₂ efflux. Stem CO₂efflux (Eₛ) is known to vary seasonally and vertically along tree stems. We measured Eₛat 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 (Tₛ), daily stem diameter increment (Dᵢ), and difference in vapor pressure deficit between nighttime and daytime (VPDdᵢff) on seasonal variation in daily Eₛwere examined by a generalized linear model (GLM). Daily Eₛexhibited a substantial seasonal variation, which was mostly affected by Tₛand Dᵢ. Any effect of VPDdᵢff, which was assumed to be an index of daily sap flow rate, was not detected. The GLM prediction of daily Eₛfrom the combination of Tₛand Dᵢshowed good agreement with the observed seasonal trend in daily Eₛ. 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 Eₛ. Additionally, the intercept of GLM, which indicates the basal daily efflux that is independent of Tₛand Dᵢ, was closely related to the annual diameter increment. Furthermore, a strong relationship was found between annual Eₛand annual diameter increment. These findings suggest that variation in stem diameter growth along stems is considerably responsible for the observed vertical variation in Eₛ. Therefore, stem radial growth can affect both seasonal and spatial variations in stem CO₂efflux.