Abstract
The CO2 gas exchange by photosynthesis and respiration was investigated for 4-year-old seedlings of Quercus serrata growing under canopy in a secondary forest, and internal and external factors which lead them to death were examined. The initial slope of photosynthetic light response curve was kept at high levels throughout most of growth period. The gross photosynthesis, respiration of leaves, and respiration of non-photosynthetic organs in growth period were estimated to be29.7, 17.8, and12.5×103μmol CO2 seedling-1 in average of 1993 and 1994. Estimates of the net production per seedling in the two growth periods were 28 and -65×10-3g seedling-1 respectively, where the photosynthetic production was almost balanced with the respiratory consumption. The daily net production of seedling was kept definitely plus at initial stage of growth period, but it then declined to minus by development of the canopy foliage. The carbon balance was extremely deteriorated in late summer due to high temperatures. It was clarified through the relationship equation between net production rate of leaves and C/F ratio that even increases of 10 or 20% in C/F ratio would reduce net production rate to less than half. Critical C/F ratios concerning survivorship of seedlings, which were given by the relationship equation, were around 5, being useful in arguing death of seedlings in the forest floor.
