Vegetational History since the Latter Period of the Last Glacial Age around the Lowland along the Shinkawa River in Yachiyo City, Chiba Prefecture, Central Japan

The vegetational history since ca. 30, 000yrs BP around the Shinkawa lowland, was clarified by pollen analysis. From the pollen diagram, we found four forest stages. The first stage (ca. 30, 000yrs BP) was characterized by coniferous trees such as Pinus (Haploxylon) and Picea. In the second stage (ca. 17, 000-12, 000yrs BP) there were coniferous and broadleaved mixed forests which consisted of Haploxylon, Picea, and Quercus (Lepidobalanus). During the first and second stages, paleoclimate was equivalent to the climate of northern part of the cool-temperate zone. But in the second stage, deciduous broadleaved trees were expanded, suggesting that the paleoclimate became warmer little by little. The third stage was the warmest. The forests in the third stage were warm-temperate deciduous broadleaved forests, which consisted mainly of Lepidobalanus along with others such as Ulmus-Zelkova, Celtis-Aphananthe, and Carpinus. The lower part of the fourth stage was characterized by the expansion of Cryptomeria forests, which suggested that the paleoclimate became cooler and slightly wetter. The vegetational destruction by human action began at the middle part of this stage. With this destruction, natural deciduous broadleaved forests disappeared rapidly, followed by expansion of the secondary Pinus (Diploxylon) forest. Through all stages, the content of the Quercus (Cyclobalanopsis) pollens was less than 15%. Lucidophyllous forests, which are considered as climax forests in this region, did not spread. But at the second stage, immediately after the coldest period of the Last Glacial Age, small quantities of lucidophyllous members, such as Cyclobalanopsis, Illicium, and Camellia, appeared. This suggests the existence of lucidophyllous forests during the Last Glacial Age in coastal regions of the southern part of the Boso peninsula. At the upper part of the third stage, with very small quantities of Myrica, Camellia, and Podocarpus pollen, the percentage of Cyclobalanopis slightly increased; at the same time, wet-like taxa, for example Cryptomeria, Juglans-Pterocarya, and Ulmus-Zelkova, increased also. The beginning of a wet and cool climate brought the expansion of lucidophyllous forests. Thus, it seems that dry soil conditions, especially in winter, hindered the expansion of lucidophyllous forests in the Kanto Plain.