PRIMARY PRODUCTION AND CHEMICAL COMPOSITION OF EMERGENT AQUATIC MACROPHYTES, SCHOENOPLECTUS MUCRONATUS SSP ROBUSTUS AND SPARGANIUM FALLAX, IN LAKE YUAN-YANG, TAIWAN

摘要

The climate in the Yuan-Yang Lake Area is temperate with high humidity due to rainfall and fog and characterized by low solar radiation and low air temperature. The aquatic macrophyte Schoenoplectus mucronatus ssp. robustus grows at the edge of the lake. For most of the population, only below-ground tissue is in the water. The plants cover an area of about 0.6 ha. During this study the annual production of above-ground tissue ranged from 400 to 900 gdw m(-2) y(-1). The below-ground tissue, which could not be distinguished from dead tissue, showed no significant change. Growth of the plant started in March, and the biomass reached a maximum in July, followed by a decline in above-ground biomass, stem number, and plant height. Starting in July, the soluble carbohydrate content of the below-ground tissues increased significantly relative to that of the above-ground tissues, indicating an allocation of energy to below-ground tissues. Close to Schoenoplectus mucronatus ssp. robustus, Sparganium fallax was growing in water no deeper than 2m and covered an area of about 1.5 ha. Annual net productivity of Sparganium fallax ranged from 600 to 1500 gdw m(-2) y(-1) during the study, half of which was below-ground tissue. Growth started in February; the biomass reached a maximum in July and then decreased, along with the plant height. However, the number of seedlings under water increased in August, and the soluble carbohydrate content of above-ground and below-ground tissues did not change significantly. Potassium and nitrogen were the major mineral nutrients in the tissues of both plants, reaching a maximum concentration in the above-ground tissues in March and April; possibly due to the onset of growth. The variation of annual primary productivity of both aquatic macrophytes was correlated with the magnitude of the maximum potassium content in the above-ground tissues of both plants, which indicated that the environment could be potassium limited. However, different patterns of seedling growth and metabolite translocation between these two aquatic macrophytes revealed differences of adaptation strategy in different habitats.