Sap flow and tree conductance of shelter-belt in arid region of China

Variation in sap flow and transpiration was investigated in a 21-year-old Gansu Poplar (Populus gansuensis C. Wang and H.L. Yang) shelter-belt in the middle reaches of Heihe River Basin of northwest China during the growing season of 2003. Daily sap flow was measured by the heat-pulse velocity method on eight trees during the growing season. During the experimental period, May to September, the mean sap flow velocity on a sapwood area basis varied between 101.4 ± 49.2 and 232.8 ± 81.6 kg m−2 h−1. The transpiration rate of the Gansu Poplar shelter-belt ranged from 1.7 to 5.6 mm d−1. On a mean daytime basis, tree conductance for water vapour transfer was between 2.1 ± 0.7 and 13.1 ± 4.4 mm s−1 during the observation period. An exponential decay function, including the vapour pressure deficit, accounted for 75% of the variation in mean daytime tree conductance.