水生植物群落の生産力についての生態学的研究 II

Abstract

1. The profile structure, seasonal growth and the distribution of productivity at different horizons of a pure stand of Vallisneria denseserrulata were investigated in still water of an irrigation ditch in the northeastern suburb of Tokyo from late April to early November.2. This submerged plant makes a dense community in July-September with the maximum biomass of 212g dry wt./m2 in August. The greatest amount of chlorophyll and leaf area index, 1.7g/m2 and 9.3m2/m2 respectively, were reached in July. The profile structure of the community was similar to the so-called grass-type of terrestrial communities in having greater amounts of leaves in the lower strata.3. The light-saturated rate of photosynthesis in the upper part of foliage ranged from 12 to 18 mgO2/g dry wt./hr. (2.1-3.6mg CO2/100cm2 leaf area/hr.) and was 2.8-5.6 times as large as the corresponding rate in the lower part of leaves. The respiration rate of the former (0.97-2.Omg 02/g/hr, or 0.18-0.41mg O2/100 m2/hr.) was also 1.4-2.8 times as high as that of the latter.4. The vertical distributions of photosynthesis and respiration within the community were estimated from the photosynthesis-light intensity curve and the daily courses of light intensity at different horizons in the community under different weather conditions. Since the light is absorbed both by water and by plant leaves, the photosynthetic activity seldom reached the potential rate at light-saturation even in the upper part of leaves at fine midday.5. The contribution to the community's total production by the uppermost stratum was the greatest, while the respiration loss seemed more or less constant at any horizons in the community. The optimum leaf amount for the highest surplus production (Fops) varied with weather conditions and growth stages. The maximum daily surplus production reached 8g dry matter/m2/day at Fopt=6 on a fine day and 4g/m2/day at Fops=4 on a rainy day in August.6. During the period of maximum growth, the monthly mean of daily net production under normal weather conditions ranged from 1.7 in June to 3.9g dry matter/m2/day in August. The monthly mean ratio of net production to gross production was 22-49%.7. In spite of the fairly large amounts of existing leaves, the maximum daily net production in the submerged plant community of 'grass-type' was around 6g dry matter/m2/day or about 50% or less of the maximum values observed in terrestrial grasslands and crop communities. This inferior productivity may be mainly attributable to the deficiency of light within the community and partly to the lower photosynthetic rate per unit area of submerged leaves.