Spatial variation in sap flow velocity in semiarid region trees: Its impact on stand-scale transpiration estimates

In this study, we aimed to clarify spatial variations in xylem sap flow, and to determine the impacts of these variations on stand-scale transpiration (E) estimates. We examined circumferential and radial variations in sap flow velocity (F _d) measured at several directions and depths in tree trunks of black locust (Robinia pseudoacacia) and native oak (Quercus liaotungensis), both of which have ring-porous wood anatomy, in forest stands on the Loess Plateau, China. We evaluated the impacts of circumferential variations in F _d on stand-scale transpiration estimates using a simple scaling exercise. We found significant circumferential variations in F _d in the outermost xylem in both species (coefficients of variation = 20-45%). For both species, F _d measured at the inner xylem was smaller than that of the outermost xylem and the F _d at the depth of > 10 mm was almost zero. The simple exercises showed that omitting circumferential variations in F _d affected the E estimate by 16-21%, which was less than the effects of omitting within-tree radial and tree-to-tree variations in F _d in both species. These results suggest that circumferential variations in F _d can be a minor source of error for E estimates compared with within-tree radial and tree-to-tree variations in F _d, regardless of the significant circumferential variations.