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. Copyright © 2011 John Wiley & Sons, Ltd.