On the other hand, Yan et al. (2006) found that seasonalWUE was positively correlated with seasonal precipitation in analpine meadow during the robust growth season. Using the eddycovariance technique, we found that diurnal variation in WUE atthe orchard-scale was significantly influenced by VPD in the rapidgrowth stage, but there was no such relationship in the floweringstage. This may be related to the characteristics of the orchardecosystem, the extent of ground vegetation cover at differenttimes of the year and meteorological factors such as differences invapour pressure. Unlike most field crops, the orchard ecosystemhas a two-tier structure comprising surface vegetation and the treecanopy. The understorey vegetation does not fully cover the groundsurface over the whole growth period. LAI values and understoreyvegetation cover during the flowering and early fruit enlargementstages were relatively low, and soil evaporation was significantlyelevated. However, during the fruit enlargement and ripeningstages, physiological water demand and water consumption werehighest. In the post-harvest stage, air temperature was relativelylow, and LAI and net CO2flux at the orchard-scale were relativelyhigh. Thus, WUE differs between pre- and post-harvest stages.