The formation of peatlands in various kinds of alluvial lowlands in Japan were studied for this article. Peatlands were mainly formed during the period between the latest Pleistocene and the earliest Holocene, and also in the latter half of Holocene. The thickness and extent of peat formations as well as when they began forming are different for each type of alluvial lowland. Peat layers are well developed in alluvial lowlands such as drowned valley, barrier and delta types. Peat layers in the lowlands of drowned valley type are thick and their formation started earlier. In the barrier type lowlands, especially in northern Japan, peatlands are very extensive, and the peat layers are more than 5 meters thick in some places. Peatlands in the delta type lowlands are relatively large but the formation of peat layers started later than the barrier and drowned valley types. There are few peatlands in the alluvial fan type lowlands. The peat layers in the coastal plain type are thin and relatively small.
The development of subalpine conifer forests in Japan is closely related to climatic changes since the Last Glacial Age. However, it is also related to the development of mountain slopes. So, the relation between slope development and subalpine conifer forests is discussed first. Block slopes, which were formed during the Last or late Glacial Age, were distributed extensively in subalpine zones. Though most of these block slopes were covered with subalpine conifer forests, some parts were treeless. There, only saplings or isolated short trees, such as Larix leptolepis are able to observed. It is considered that the lack of forest is due to the delay of plant succession on extensive block slopes. Treeless block slopes are distributed, for example, near the Sensui Pass in the southern Japanese Alps, and on Mt. Kimpu in the Kanto Range. In both areas, the lower limits of block slopes were at about 2000 m a. s. I. Fossil periglacial block slopes are dissected at their lower limits. Such parts, named " the postglacial dissected line " by Hatano (1986), offer unstable habitats for plants. Betit/a ermanii shrubs are often observed in such habitats. They seem to be adapted to an unstable geomorphic conditions. Forest line extends to about 2500-2600 m in central Japan. However, considering the climatic conditions it could ascend to about 2800 m. Normally, the limiting factor for forest growth is considered a mean temperature of approximately 10°C for the warmest month of the year. But in the Japanese Alps, an isotherme of 10°C for the warmest month runs about 2800 m. So the height of present forest line is lower than the estimated limit. The author concluded that the presence of periglacial block slopes probably caused the lower forest line.
Some representative conifers ar:e discussed in this and the following article from the viewpoint of taxonomy and phytogeography. In this paper, short phytogeographical notes on all eight coniferous families are given, five of which have subalpine species. Then, the distribution patterns of the genus Larix and Tsuga were examined with reference of distribution maps. In both genera, the distribution of the primitive groups, Larix sect. Multiseriales and Tsuga sect. Heopeuce or sect. Hesperopeuce, is restricted in East Asia and the Pacific side of North America; while whereas, the distribution of the advanced species, e.g. Larzx sibirica and Tsuga laricina, is more extensive.
The process and causes of the differentiation of the mountains in their development of Abies mariesii forests were discussed based on the results of ecological geography and paleoecology studies. A. mariesii is adapted to the wet and snowy oceanic climate, and makes stands on mesic sites which develop in such climates. A. mariesii tolerates poorly drained soils well and as a result, is distributed around flats or slightly inclined slopes where water from melted snow collects. As the Last Glacial Age ended, blank spaces where the quasi-alpine zone had originated were formed by the decline of subarctic coniferous forests along the Japan Sea coast because of the remarkable environmental changes. A. mariesii was a minor species during the continental climate of the Last Glacial Age, but it expanded its range by filling up the blank niche during the oceanic climate of the Postglacial Age. The core stands of the expansion inhabited poorly drained and less sharply inclined slopes at relatively lower altitudes. Therefore, the extent of less sharply inclined slopes controled the success of the range expansion of A. mariesii on each mountain.