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Influences of forest decline on various properties of soils on Mt. Hirugatake, Tanzawa Mountains, Kanto District, Japan

Abstract To investigate the changes in the vegetation, soil profile morphology, and some chemical properties of the soils under different conditions of forest decline, six representative study sites were set up on Mt. Hirugatake, Tanzawa Mountains, Kanto district, Japan, where serious forest decline had been observed since the 1970s for broadleaved deciduous trees. The selection of the study sites under different degrees of forest decline was based on the interpretation of vegetation changes in aerial photographs taken in 1947, 1967, 1977, and 1993, and also on vegetation surveys at and around the selected sites. The results obtained were as follows. The soil profile morphology was similar among the study sites in terms of depth, sequence, and content of scoria fragments in each horizon. However, with the disappearance of tree layers of Fagus crenata and associated trees, the thickness of the O horizon had decreased and the distribution of medium to coarse roots had also decreased, especially at the advanced stage of forest decline. The increase in the hardness of the surface soils and the decrease in the soil organic matter content tended to occur at the very early stage of forest decline. As for the changes in the chemical properties of the soil samples associated with forest decline, the pH and contents of exchangeable bases did not show a definite tendency while the content of exchangeable AI seemed to decrease, especially for the A horizon. Accordingly, it is unlikely that the acidification of soils was the major factor responsible for forest decline in the study area, in contrast to the results obtained in investigations of forest decline areas conducted in Europe.