Abstract: By the method of pressure chamber we monitored the diurnal change of water potential (Wp) of small branch in 4 tree species (Caragana korshinskii, Populus hopeiensis, Populus beijingensis and Populus simonii) once a month. The correlation between Wp and temperature (T), light intensity (L), relative humidity (RH) and leaf natural water content (LWC) were analyzed respectively;the correlation between Wp in the morning and soil water content was studied. The results were as follows:The diurnal change curve of water potential in four species is parabolic shape. The average value of potential in the species in growing season was respectively:Caragana korshinski (-1.569 MPa), Populus hopeiensis (-1.030 MPa), Populus beijingensis (-0.993 MPa)and Populus simonii (-0.971 MPa). The order of fitness degree between Wp and single climatic factor was L, T then RH. The correlation between Wp in the species and the products (L*T) of light intensity and temperature, the quotient (L/RH) of L and RH was more significant than with that of single climatic factor (T, L, or RH), respectively. The correlation between Wp and LWC is not very significant. The model type of Wp in predawn with soil water content(X)was:Wp=Ae^-BX. Soil water moisture put more influence on predawn water potential 4 tree species.

Key words: water potential of small branch, environmental factor, relational model, tree species