Abstract: With the aid of TDP (thermal dissipation sapwood flow velocity probe) and autometeorological station, the spatial variation pattern of Platycladus orientalis stem sap flow in Miaofengshan Experimental Forest Farm of Beijing Forestry University was studied in 2005. The results showed that the sap flow velocity (SFV) increased with increasing trunk height, and the peak time of sap flow appeared earlier at upper trunk part than at lower. The sap flow curve was narrower and sharper, and the curve slope was greater at upper trunk part than at lower part. The average maximum SFV at the trunk heights of 6.6, 4.6, 2.6, and 0.6 m was 0.015, 0.009, 0.006, and 0.003 cm·s^-1, respectively, in July 2005. Both the SFV and the water consumption amount of different diametric single trees were increased with increasing diameter, but the change of SFV was not linear. Soil water content limited the water consumption capacity of the tree, and the sap flow flux had a distinct exponential correlation with soil water content, especially in 20-40 cm soil layer. Different meteorological factors had different effects on the stem sap flow. Radiation intensity, air temperature, and wind speed were positively correlated to the sap flow index belonged to the first principal component that affected the sap flow directly; while air relative humidity and soil temperature were negatively correlated to the sap flow index belonged to the second principal component that affected the sap flow slowly. By stepwise regression, a multiple linear equation describing the relationships between sap flow and meteorological factors was established, and it was feasible to use meteorological factors to forecast stem sap flow.

Key words: Photosynthesis-evapotranspiration coupled model, Water and heat fluxes, Diurnal variation