Alpine Plant Succession on Granite-porphyry Rubble Slopes of Mt. Shirouma, the Northern Japanese Alps, in Relation to Slope Development

Abstract

In the alpine zone of the Japanese Alps, distributions of alpine plant communities are closely related to lithology. There are clear contrasts among the combinations of vegetation and lithology. However, the author found that several different plant communities often occur in areas of the same lithology. In a wind-exposed rubble slope on granite-porphyry area of Mt. Shirouma, three kinds of plant communities were distinguished : sparse lichen community, lichen and moss community, and wind-exposed herb-heath community partly covered with Pinus pumila scrub. These communities are believed to be in different serial stages of succession. To clarify the cause of this distribution, the age of angular debris production on the slope was examined. W eathering rind thickness in angular rock fragments on rubble slope was used to speculate the age of debris production. Debris covered with sparse lichen was produced during the Neoglaciation period (2, 000-3, 500 y. BP), while debris covered with lichen and moss was formed in the late Glacial period (10, 000-11, 000 y. BP). In the area covered with wind-exposed herb-heath and Pinus pumila scrub, the debris were produced in the Last Glacial Period. These ages mean the time costed for the plant succession on these rubble fields. It seems to be necessary 3, 000 years for settling the sparse lichen community, about 10, 000 years for the lichen and moss community, and more than 10, 000 years for the wind-exposed herb-heath and Pinus pumila scrub community. The progress of succession is extremly slow in the area of this lithology, because fine matrix which is important for seed germination and root extention is lacking in these rubble slopes, in addition to severe climatic conditions in winter.