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 (Es) is known to vary seasonally and vertically along tree stems. We measured Es 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 (Ts), daily stem diameter increment (Di), and difference in vapor pressure deficit between nighttime and daytime (VPDdiff) on seasonal variation in daily Es were examined by a generalized linear model (GLM). Daily Es exhibited a substantial seasonal variation, which was mostly affected by Ts and 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 Es from the combination of Ts and Di showed 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 Ts and Di, was closely related to the annual diameter increment. Furthermore, a strong relationship was found between annual Es and 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 CO2 efflux.
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