Many missing sinks still remain in the global carbon cycling system. It is important to find the missing sinks in temperate-forest ecosystems as well as arctic-tundra ecosystems, since these forests cover a large part of the global ground surface. In general, a forest consists of several stories of plants, but few studies have focused on the forest floor vegetation. We have noticed the importance of understory plants in the carbon budget of a forest ecosystem. Therefore, we studied the carbon balance of forest floor vegetation composed of a Sasa senanensis community in a cool-temperate forest, in central Japan. The Sasa senanensis community showed seasonal and annual changes in the component organic matter with different age variations. All the leaves were viable for 3 years after spreading out and almost all of the culms were viable for about 5 years after sprouting. The annual averages of above-ground biomass were about 6.8 t d.w. h-1 in 1993 and 6.6 t d.w. ha-1 in 1994. The annual averages of underground biomass were about 5.9 t d.w. ha-1 and 3.9 t d.w. ha-1, respectively. The rate of monthly net production of the Sasa community was high in spring and autumn when the deciduous canopy trees were leafless. On the other hand, net production was minus during summer because of the shortage of solar radiation penetrating to the forest floor. The rate was also minus during the snowbound season. Net production and gross production were estimated to be 1.1〜1.2t C ha-1 yr-1 and 5.2〜6.3t C ha-1 yr-1, respectively. The annual respiration of the Sasa community was about 4.1〜5.1 t C ha-1 yr-1. We evaluated that this deciduous forest absorbed carbon dioxide in spring before greening of the deciduous trees, and in autumn after leaf shedding. This suggests that the active carbon assimilation of the floor vegetation contributes to the total carbon absorption of the whole forest during the leafless season.
