Abstract: The seven trees species——Pinus sylvestris var. mongolica etc. were measured by using the Preesure-Volume Technique for two years (1986—1987) in semi-arid region, Huhhot, Inner Mongolia. The measured trees planred in the same site types were middle-aged. The results showed that the whole Osmotic Potential (π_P) and Relative Water Content (RWC) of measured branches in the early period when the protoplasm separated from the cell wall could be used as physiological index for comparing the drought-endurance of trees. The higher value of π_P (absolute value) and lower value of RWC are physiological features of stronger drought-endurance. The indexes changed with seasons and phenological phase during the annual growch period. During growing period of branch the value of π_P is least and the value of RWC is the greatest, but the value of π_P is the greatest, the value of RWC is the least when the branches all lignified. By means of using the minimum value of π_P and the annual variation value of π_P express the drought-endurance, the results are satisfactory.

Key words: Drought-endurance of trees, Pressure-Valume Technique, Osmotic potential, Relative water content, Relative osmotic water content