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AbstractStand transpiration (E) estimated using the sap flux methods is affected by the azimuthal, radial, and tree-to-tree variations of sap flux. Although several studies have examined the relative importance of the three variations in estimating E, the seasonality of the three variations remains unknown. In the current study, we attempted to clarify whether the relative importance of these three variations could show seasonal changes. Using sap flux data measured in a subtropical cloud forest from August 2010 to July 2011, we calculated the differences resulting from omitting the three variations in estimating E. The effects of the three variations in estimating Eshowed seasonality. The azimuthal and tree-to-tree variations were more pronounced during winter, whereas the radial variation was more pronounced during summer. However, the effect of tree-to-tree variation was consistently much larger than the other two variations throughout the study period. The tree-to-tree variation is more important in estimating Emonthly, seasonally and annually than both the azimuthal and radial variations, although all three variations have shown seasonality. In addition, the sensor allocation for summer would be acceptable for the practical estimation of Eif aiming at the long time scale.

AbstractSoil CO2levels reflect CO2production and transport in soil and provide valuable information about soil CO2dynamics. However, extracting information from soil CO2profiles is often difficult because of the complexity of these profiles. In this study, we constructed a simple numerical model that simulated soil CO2dynamics and performed sensitivity analyses for CO2production rates, soil water content and temperature, and gas diffusivity at the soil surface to clarify the relationships among these parameters. Increased soil surface CO2flux did not always coincide with higher soil CO2concentrations; increased CO2production at shallow depths had little effect on soil CO2concentrations, while the opposite may be true for high levels of soil water content. Higher soil CO2concentration did not always coincide with greater soil surface CO2flux; under high soil water conditions, soil surface CO2flux sometimes decreased despite increased soil CO2concentration. Increases in soil water content did not always enhance both soil surface CO2flux and soil CO2concentration. Under high soil water conditions, increases in soil water content could lower soil surface CO2flux and increase soil CO2concentration. Increases in soil temperature resulted in greater soil surface CO2flux and higher soil CO2concentration in our simulation (extremely high temperatures were not assumed in this study). Gas diffusivity in very shallow layers did affect, albeit weakly, soil CO2concentration. The findings of this study may help direct future observations and aid in the interpretation of their results.