Black locust (Robinia pseudoacacia) is a major reforestation species in the semiarid region in the Loess Plateau of China. There has been increasing concern about the sustainability of the plantations because of their possible high water-use. This study was, accordingly, undertaken to quantify the stand-scale water use of a middle-aged black locust plantation in the region. The thermal dissipation probe method was applied to 27 trees to measure sap flux densities in an experimental plot during the growing season of 2008. The monoculture stand has a basal area of 23.3 m2 ha-1 and a maximum plant area index (PAI) of 2.89. Sapwood areas were estimated by use of a regressive relationship with the diameter at breast height (DBH) for scaling up of stand transpiration. The results showed that DBH could be a good predictor of sapwood area of individual trees. The diurnal cycles of average sap flux densities differed among DBH classes. Daily transpiration can be predicted from mean daily daytime vapor pressure deficit (VPDm) using a fitted exponential saturation model. Model variables were different among seasons, probably owing to different soil water conditions and leaf phenology. By using the derived model for each month, stand canopy transpiration over the growing season was estimated to be 73.8 mm, with an average daily value of 0.41 mm day-1 and a maximum of 0.89 mm day-1. The relatively small estimates of stand transpiration might be attributed to low PAI and sap wood area of the middle-aged stand.
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