To regenerate successfully, it is essential for seedlings in the forest understory to maintain positive carbon balance. In addition, it is also needed to have tough tissues and to survive. For cool-temperate deciduous tree seedlings, which is more important at a certain light-availability? To address the question, we conducted growth analysis and model simulations as follows. We planted seedlings of a later-successional deciduous tree species, Fagus crenata, and a rather pioneer one, Acer rufinerve at five sites differing in canopy conditions. Then we measured their relative growth rate and survival rate, and associating traits such as net assimilation rate, leaf mass per area, allocation rates to leaf, stem and root, wood density, etc. In parallel, by the model simulation, we tried to clarify the effect of these traits on whole-plant carbon balance and tree architecture at various light-availabilities. From the result, we will discuss the various strategies of deciduous tree species in the forest understory.