Abstract: Analysis on the relations of four coniferous young trees' photosynthetic and transpiration rates to soil moisture content in semi--arid region shows that these two rates tend to decrease with decreasing soil moisture content, which can be expressed by regression equation y=A+Be^-kx.Calculation results show that when net photosynthetic rate(y)equals to zero,the values of soil moisture content(x) are diffferent for each coniferous species.Tree species with smaller x values have a stronger drought resistance,which is in order of Plalycladus orientalis>Pinus sylvestris var.mongolica>p.tabulaeformis>Picea meyeri.

Key words: Net photosynthetic rate, Transpiration rate, Soil moisture content, Drought resistance