• KOREAN JOURNAL OF CROP SCIENCE
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• v.27 no.4
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• pp.361-370
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• 1982

The effects of climatic factors on organisms lire variable and complex, and it, however, can be interpreted in terms of those on the distribution and those on the population densities. The distribution of an organism may largely be determined by the temperatures, except some temporal organisms which are depended on the air mass movements. Population density of an organism is determined by various climatic factors, such as previous winter temperature, temperature of growing season and rainfall. The start of growing season of the rice plants has been shifted to earlier since last decade in Korea. This may mean that the overall climatic condition during the growing season might be considerably different from those in past years, and such a difference in climatic conditions might have close relation with the recent status of the diseases and insect pests through direct effects on the physiology and population dynamics of the organisms, as well as through on the biotic associations of the pest organisms. The white back planthopper and brown planthopper have become the key insect pests in Korea in recent years. They are migratory and have high reproductive pontentials and more generations than average residential insects. The synchronization of the migrants and physiological condition of the rice plants seems to be the important factors in relation to the recent outbreaks of these insects; the high reproductive rate can be obtained with the growth stage of rice being 30-50 days after transplanting. The modication of the microclimate associated with high plant density and some other introduced new cultural techniques also have some relation with the outbreak. The key diseases of the rice are the blast disease, sheath blight and the bacterial leaf blight. For the rice blast, the seedling blast and leaf blast during the early growing season and the neck blast, have become more serious, the former may be related to hotbed nursery and the later may be related to the high humidity in early August, and synchronization of the heading time which has been shifted to early part from middle or late part of August. In general, for the rice diseases, the development of the new races have been the most serious which are largely resulted from the introduction of the new varieties, but it also seems to be related with the prolonged periods of the favorable condition associated with the shifted growing seasons. In general, the diseases and insect pest problems have become much more variable and complex, and control measures should be based on the thorough knowledge of the ecology of the pest organisms, that is, effects of various environmental factors on the disease cycle; spore release, spore deposition, infection, colonization and sporulation of the disease organisms, and those on the development, reproductive potentials, dispersal, age specific responses of the insects. The well organized real-time pest management systems, such as alfalfa weevil management system developed at the Purdue University in U.S., is the prime importance for the implementation of the pest management principles.

• The Sea:JOURNAL OF THE KOREAN SOCIETY OF OCEANOGRAPHY
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• v.20 no.1
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• pp.43-52
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• 2015

The growth and population dynamics of eelgrass (Zostera marina) due to changes in sediment composition were examined in the lower intertidal zone of the Seomjin Estuary, Korea. We surveyed environmental factors such as water temperature, underwater irradiance, main types and organic content of sediment, tidal exposure, and nutrient concentrations in the water column and sediment pore water, in relation to the shoot density, biomass, morphological characteristics, and growth of Z. marina inhabiting lower intertidal zones. The survey was conducted monthly from May to December of 2004 and 2009. The water temperature showed obvious seasonal trends in both study years. Underwater irradiance was significantly higher in 2009 than in 2004. Tidal exposure was not significantly different between 2004 and 2009. The sediment was muddy-sand in 2004 but became sandy and with a significantly lower organic content in 2009. Water column $NH_4{^+}$ concentrations were significantly higher in 2004 than in 2009. Sediment pore water $NO_3{^-}+NO_2{^-}$ concentrations were significantly higher in 2009 than in 2004. Other nutrient concentrations did not differ significantly between 2004 and 2009. Morphological characteristics, including eelgrass length and leaf width were significantly lower in 2009 than in 2004. Eelgrass shoot height, leaf length, and sheath length showed typical seasonal patterns, increasing in early summer and decreasing in autumn, in both years. Vegetative shoot density was not significantly different between 2004 and 2009, while the biomass of individual plant parts and the total biomass were significantly lower in 2009. Eelgrass leaf productivity did not differ between years, but leaf turnover time was significantly shorter in 2009 than in 2004. Eelgrass downsizing and decreased turnover time in 2009 compared to 2004 indicate more effective adaptations to the stress of long-term changes in sediment composition. Overall, results suggest that changes in sediment composition can be a limiting factor for seagrass growth in the intertidal zone.