Isao Ikusima

Life cycle of Egeria densa Planch., an aquatic plant naturalized in Japan

Abstract The ecophysiological life cycle of Egeria densa Planch., in a lotic irrigation ditch was evaluated. Phenological and quantitative measurements were made from August 1984 to July 1985. Lateral shoots with roots developed and elongated to the surface of the water when the bottom-water temperature increased above c. 15°C. Root crown developed simultaneously as the plant biomass increased. Dense shoot crown was formed under the water surface after the shoot reached the water surface. Plant biomass had two maxima in August and December-January. The bimodal curve in plant biomass was caused by the difference of growth attributed to the winter-type and summer-type plants. Relative growth rate, in length, of summer-type plants occurred at an optimum temperature of 20.7°C in culture. The seasonal activity of photosynthesis and respiration was measured in March, August and December. The optimum temperature of net photosynthesis of the summer-type plants reached a high 35°C similar to that of the C 4 plant. The compensation for light intensity at 35°C was 340 lux. Each photosynthesis-temperature curve suggested that Egeria had the ability to adapt to the seasonal changes in temperature in the natural habitat. The maximum starch concentrations reached 25.4% in the leaf and 22.6% in the stem in December. The shortage in the balance of organic matter for overwintering was found to be maintained by stored starch in the leaf and the stem.

Population dynamics, productivity and biomass allocation of Zizania latifolia in an aquatic-terrestrial ecotone

The population and production ecology of a Zizania latifolia stand at a sheltered shore of the Hitachi-Tone River were investigated. Shoot emergence was observed twice a year; the fist was a synchronized shoot emergence in April and the second was from August to October. Aboveground biomass was mostly occupied by leaves and peaked at 1500 g dry weight m−2 in August. The belowground biomass also reached its peak, 750 g dry weight m−2, in August. The secondary shoots were small in spite of their high density. Leaves were produced continuously throughout the season. The leaf life span was as short as 55.6 days for cohorts that emerged from May through to September. Total annual net production of Z. latifolia could be more than 3400 g dry weight m−2. Shoot clusters of several centimeters were observed in April. The following self-thinning caused a regular distribution of the remaining shoots in August. Most shoots produced in August to October were found near a shoot persisting since April. They showed more concentrated distribution than shoots in April. A large biomass allocation to leaves and the ability to produce many clump shoots during the late growing period may facilitate dominance of Z. latifolia in relatively sheltered sites.

Production and population ecology ofPhyllospadix iwatensis Makino. I. Leaf growth and biomass in an intertidal zone

In an intertidal zone on Choshi coast, Japan,Phyllospadix iwatensis Makino emerges at daytime in spring and summer, while at night time in winter. The plants therefore experience seasonally different stresses caused by emergence, for example, intense light, ultraviolet rays, extreme temperature and desiccation, all of which the plants are unable to avoid during daytime emergence. Seasonal changes in the biomass and LAI suggest that the optimum periods for growth ofP. iwatensis would be in March when the emergent period is short or nil and light availability is high while water temperature is not too low. Dense foliage and low canopy height ofP. iwatensis in the intertidal zone relieve the plant from the stresses in emergent periods and from the disturbance caused by strong water movement in some coastal areas with active wave action.

Ecological aspects of bamboo flowering ecological studies of bamboo forests in Japan, XIII

Since 1953, we have conduced continuous field studies on the bamboo (Phyllostachys bambusoides) stands of Chiba. A part of these stands began flowering in 1960. The emergence of season-insensitive, short slender bamboo (similar to dwarf bamboo) precedes the bamboo flowering. The regenerative process is short slender bamboo with small leaves as undergrowth foretelling flowering (Rs)→ regenerated, short slender bamboo with small leaves (Rs)→a new-generation of short, slender bamboo with large leaves (Rl)→a new generation of short, slender bamboo with medium-sized leaves (Rm)→a tall, regular growth of bamboo (T). Some of the short slender bamboo flower (Rf), but some of them do not flower (Rnf). The productive structure develops from the grassy type to the herbacious type after flowering (Rs→T). Leaf characteristics and dry matter distribution are measured according to the regenerating stages. The most characteristic stage, Rl, appears to play an important role in bamboo regeneration because of its large amount of photosynthetic biomass and the high photosynthetic rates. Rm is a miniature version of normal tall bamboo.

Production and population ecology ofPhyllospadix iwatensis Makino. II. Comparative studies on leaf characteristics, foliage structure and biomass change in an intertidal and subtidal zone

A seagrass in Japan,Phyllospadix iwatensis Makino, is distributed in the lower intertidal zone and upper subtidal zone making a dense population on the Choshi coast, Japan. IntertidalP. iwatensis is able to receive sufficient light for photosynthesis but experienced severe exposure to the air, which decreased a large amount of aboveground biomass in April to June (i.e. the daytime exposure season). SubtidalP. iwatensis was never exposed throughout the year and the aboveground biomass increased gradually over the daytime exposure season. However, the maximum aboveground biomass and shoot density of the subtidal plant never exceeded that of the intertidal plant. The dense foliage, large aboveground biomass and high shoot density of both intertidal and subtidal plants is likely to be an adaptation to heavy water movement, but the subtidal plants always received insufficient light for photosynthesis as a result of having dense foliage, particularly in turbid water. In choppy and swell sea,P. iwatensis did not seem to be adapted to growing in the subtidal zone where there was shortage of light.

Primary production and population ecology of the aquatic sedge Lepironia articulata in a tropical swamp, Tasek Bera, Malaysia

Abstract An attempt was made to estimate the amount of photosynthetic production and of the dead organic matter supply of an emergent sedge Lepironia articulata (Retz.) Domin, one of the most important primary producers in a large tropical swamp in West Malaysia. The values for plant density, standing crop, growth and production in an unburnt area were compared with those in an area burnt one year before the field survey. The leafless culms grow up to 2 m in height and the maximum standing crop of living parts, including well-developed rhizomes, reaches 850 g dry weight m−2. The spatial distribution pattern of the apex of rhizomes tends to approach a random pattern. It takes about 7 months for culms to mature and the maximum mean dry weight of a culm reaches 3.40 g in the unburnt area and 2.25 g in the burnt area. The mean lifetime of culms is estimated at 227 days (unburnt area) and 309 days (burnt area). The mortality of young culms is not as high as in seedlings of many other plants. The production of Lepironia was estimated from the growth curve, life table, the interval of emergence of new stems, respiration rate, etc. The daily gross production in the unburnt and the burnt areas is estimated at 4.89 g m−2 day−1 and 2.11 g m−2 day−1 in a dry season (net production is 2.24 and 0.73 g m−2 day−1, respectively). The maximum efficiency of solar energy conversion of gross production is less than 0.5%. A large amount of peat accumulates under the Lepironia stand. The dead organic matter supplied to the bottom mud amounts to 2.24 g m−2 day−1 (unburnt area) and 0.73 g m−2 day−1 (burnt area).

Evaluation of the faster initial decomposition of tropical floating leaves of Nymphaea elegans Hook

Contributions of abiotic and biotic processes to the decomposition of floating leaves of Nymphaea elegans were separately evaluated by comparing the rate obtained from an in situ experiment of submerging dry leaf material in a lake, and that from a laboratory experiment of submerging dry leaf material in lake water with a bio-fixing reagent. It took 8 days to decompose 79.4% of the initial dry weight of the floating leaf of N. elegans in a tropical lake. Of the dry weight loss, 32.9% and 67.1% were atributed to abiotic and biotic decomposition, respectively. The relationship between decomposition rate and the mesh size of the leaf litter bags was examined by the application of a mathematical model. A reasonable value of decomposition loss at an early stage could be obtained using a bag with a mesh opening of 9.9 mm2. The decomposition rate of floating leaves is faster than that of other aquatic plants. Rapid decomposition of N. elegans leaves may be attributed to the fact that the plant has a low carbon to nitrogen ratio.