The Role of Litter and Humus Layer as a Mortality Factor of Seedlings of Japanese Red Pine, Pinus densiflora Sieb. et Zucc.

Abstract

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...