Biophysical controls on canopy transpiration in a black locust ( Robinia pseudoacacia ) plantation on the semi-arid Loess Plateau, China
In the semi-arid Loess Plateau of China, black locust (Robinia pseudoacacia) was widely planted for soil conservation and afforestation purposes during the past three decades. Investigating biophysical controls on canopy transpiration (Ec) of the plantations is essential to understanding the effects of afforestation on watershed hydrology and regional water resources. In addition to monitoring of micrometeorology and soil water content, sap flux densities (Fd) of six representative trees in a 27-year stand were continuously measured using thermal dissipation probes during the growing seasons in 2013 and 2014. Ec was derived by multiplying stand total sapwood area (AST) with Fd. The daily mean Ec in the growing season was 0.14 and 0.23mmday1 in 2013 and 2014, respectively. The responses of daily Ec to Rs and vapour pressure deficit were explained with an exponential threshold model. The variability of monthly Ec was mainly explained by leaf area index (LAI) (R2 = 0.92). The inter-annual variability of Ec was influenced by LAI that fluctuated dramatically during 2013 and 2014. We found that the status of soil water content at the beginning of the growing season had large impacts on LAI and Ec during the growing season. Contrary to common beliefs that the plantation uses a large amount of water, we found that the black locust plantation had rather low transpiration rates (5.3% of precipitation and 4.6% of ET0). This study suggests that the black locust plantation has adapted to local soil water condition by reducing transpiration, and the major water loss from the plantation was not transpiration.