Seasonal courses of leaf CO2 gas exchange in a growing season were examined in saplings of Thujopsis dolabrata var. bondai and Quercus mongolica var. grosseserrata in a cool temperature deciduous forest. Between the two tree species there were no large differences in the light compensation point of leaf photosynthesis, except for the season of new leaf expansion. However, light-saturated rates of net photosynthesis were obviously high in T. dolabrata var. bondai. Evergreen T. dolabrata var. bondai saplings had large photosynthetic production in two seasons, before the emergence of new foliage and after foliage fall of the overstory deciduous trees, because of the significantly high solar radiant energy penetrating under the forest canopy during the seasons. Saplings of deciduous Q. mongolica var. grosseserrata were heavily shaded throughout the growing season by foliage of the overstory trees, which resulted in a low daily surplus production. The annual surplus production of leaves in the growing season was estimated to be 2300 mmol CO2 m-2 in T. dolabrata var. bondai and - 100 mmol CO2 m-2, slightly negative, in Q. mongolica var. grosseserrata. These results supported the high survivability of T. dolabrata var. hondai saplings and the high mortality of Q. mongolica var. grosseserrata in the deciduous forest.