The relationship between ectomycorrhizal development and mortality from pine wilt disease was studied in an artificial Pinus thunbergii Parl. stand on a slope. The development of ectomycorrhizae and the survival of the trees showed the same tendency, which suggests a correlation between mycorrhizal development and resistance to pine wilt disease. The development of pine roots and mycorrhizae was greater in the upper part of the slope. The ratio of mycorrhizae to the total of mycorrhizae and fine taproots was also higher in the upper part of the slope. Tree mortality was clearly biased and more trees survived in the upper part of the slope than in the middle and the lower parts. There was no significant difference between the upper and the lower part of the slope in the number of feeding wounds made by the pine sawyer beetle, which demonstrates the opportunity of infection with this disease. There was no clear correlation between the development of mycorrhizae and the composition of the soil substrate such as total carbon, nitrogen, and phosphorus. The abundant mycorrhizae in the upper part of the slope, which mitigate drought stress, may also have decreased the rate of tree mortality.
The impact of forest fire in 1997 and 1998 on the mycorrhzae was studied at the dipterocarp forest in East Kalimantan, Indonesia. In unaffected forest more than half of total ectomycorrhizae distributed in the organic layer but in the fire-affected forest one and a half years after the fire, total amount of mycorrhizae was smaller and they were found in deeper soil. The number of morphotypes of mycorrhizae was also smaller in fire-affected forest. An artificial plantation established after clear cut and prescribed burn had the largest amount of ectomycorrhizae, but the diversity was smaller than the unaffected forest. After four years, mycorrhizae had not yet recovered in fire-affected forest although the organic layer had recovered. Pioneer species of mycorrhizal fungi (early-stage fungi) developed sporocarps around the surviving hosts in severely affected forest and this suggests the start of secondary succession of mycorrhizae. In moderately affected forest, the species composition of mycorrhizal fungi which fruited during the study was similar to those of unaffected forest. This fact suggests that such sites may be the refugia of symbionts and be important in reforestation.
The process of decrease in seedlings of Japanese red pine (Pinus densiflora Sieb. et Zucc.) and the effect of litter and humus layers on the mortality were studied in their stand and adjacent bare land. After the rainy season, only small number of individuals died in the bare land but more seedlings died in the forest. The number of seedlings killed by drought was greater in the forest than in the bare land. The population of naturally occurring current year seedlings was larger in the forest than in the bare land, but that of 1-to 5-year-old seedlings was smaller. Severe summer drought exceeding the permanent wilting point was observed only in the forest F-H horizon where greater part of seedling roots distributed, this may be one of major mortality factors. Inhibitory substances to seedling growth in fresh fallen pine needles were also suggested. Key words: F-H horizon, growth inhibitor, Pinus densiflora, seedling mortality, water characteristics Japanese red pine, Pinus densiflora Sieb. et Zucc., is one of the most common pioneer tree species in Japan and colonizes well under severe conditions such as drought and poor nutrient. This species regenerates well on bare land but the seedlings seldom survive in their own stands even when the canopy is open. The factors causing the loss of seeds and seedlings have been studied from various viewpoints. The loss of seeds is reported to be caused by aminals (Rim and Shidei, 1974) but they did not mention mortality of seedlings after germination. The major mortality factors of seedlings have been reported to be drought , shortage of light (Ishikawa, 1933;Suyama and Nakamura, 1988), or fungal disease (Kurata, 1947(Kurata, , 1973 in the forest. It is often said that the reason why P. densiflora cannot regenerate in the forest is due to its shade intolerant property, but the seedlings of P. densiflora do not need strong light and the canopy of pine stands are often so sparse (Shidei, 1963) that the floor generally receives enough light to allow the seedlings to live. But even in such sparse forest there are few pine seedlings on the floor as well as in relatively dense forest reported formerly (Suyama and Nakamura, 1988). So it is difficult to attribute the mortality of seedlings in initial 1-2 years only to the shortage of light.Another difference between forest and bare land is the presence of litter and humus layer. In this study, we investigated the effect of this organic layer on the mortality of pine seedlings. We studied the survival rate and mortality factors of seedlings, water ' conditions of top soil and the effect of water solublelsubstances of leaves of coexisting tree species and the litter on the growth of pine seedlings. Materials and Methods 1 Study siteAll field studies were carried out in Kamigamo ExperiPresent address: Biological Environment Institute, Kansai Environmental Engineering Center Co., Ltd., Kyoto 611, Japan.Correspond auther. an abandoned, 2 to 3 m wide path where almost no organic soil was present and 1-to 5-year-old seedlings of P. densif...
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