The paper dealt with the characteristics of main tree species,which was in the extra
drought press condition,Caragana dorshinskii,Populus
hopeiensis,Populus simonii,Populus beijingensis,Robinia pseudoacaci and native tree Malus micromalus in loess plateau region in northern Shanxi.The results were as follows.The diurnal changes of water potential(Wp)in all 6 tree species was parabolic shape.The diurnal average value of Wp in 6 tree species was C.korshinskii,-2.028;P.hopeiensis,-1.289;P.s imonii,-0.946;P.beijingensis,-0.943;R.pseudoacacia,2.383and M.micromalus,-1.556(MPa),respectively.The affecting degree of environmental factors on Wp was temperature>relative humidity>light intensity.The relationships between Wp and environmental factors involved several types.The diurnal change of
transpiration rate(Tr)in C.Korshinskii was parabolic,while that of Tr in P.hopeiensis,P.simonii,and P.Beijingensis was flag type.The daily average value of tr in C.Korshinskii,P.hopeiensis,P.simonii,and P.Beijingensis was 0.7117g·g -1 LD·h -1 ,0.5234g·g -1 LD·h -1 ,0.4632g·g -1 LD·h -1 ,0.6060g·g -1 LD·h -1 ,respectively.The relationship between Tr and environmental factors was not significantly except for the relationship between Tr and light intensity in C.Korshinski.Soil water content had an effect on daily lowest water potential.Under the extra
drought press the water potential at certain period of the day was lower than osmotic potential at turgor loss point,which put on a negative effect on the growing of the trees species.