Abstract:Measurements of sap flow were made in two 20-year-old Caragana korshinskii stands that largely differed in tree density for the period of 13-25 June 2006 in Liudaogou catchment in Loess Plateau of China. Soil water content, air temperature, relative humidity, solar radiation, wind speed, grass reference evapotranspiration (ET0), and water vapor pressure deficit (VPD) were also measured, and the actual canopy conductance was calculated from the inverted Penman-Monteith equation based on the actual data of stand transpiration. The results showed that the sap flows were closely coupled to the changing weather conditions except wind speed and could be tightly fitted to a simple linear equation of radiation. The transpiration from the two studied stands was effectively scaled by leaf area index. Canopy transpiration of the high density stand with LAI 2.3 reached in averaged of 3.83 mm d-1m-2 compared with 1.64 mm d-1m-2 for the stand with LAI 1.1 of the low density stand. The analysis of canopy conductance showed no meaningful responses to radiation, air temperature and ET0 but excellent fits to VPD and relative humidity in the case of soil drought. On the contrary, no correlativity of canopy conductance to VPD and relative humidity was observed in case of soil wet condition, though canopy conductance was tightly fitted to radiation, air temperature and ET0.