千葉県八千代市新川低地における最終氷期後期以降の植生変遷 [in Japanese] 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 [in Japanese]
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八千代市新川低地におけるボーリングコアの花粉分析を行い,八千代市周辺における最終氷期最寒冷層準直前から現在に至る植生変遷を,ほぼ連続的に明らかにすることができた.すなわち,八千代市周辺における現在までの約3万年間の森林植生の変遷は,下部より主として1.<i>Picea</i>と<i>Haploxylon</i>よりなるマツ科針葉樹林期(約3万年前),2.マツ科の針葉樹と<i>Lepidobalanus</i>よりなる針広混交林期(約1.7～1.2万年前),3.主として<i>Lepidobalanus</i>よりなる落葉広葉樹林期および4.<i>Cryptomeria-Diploxylon</i>林期の4期に区分できる.全層準にわたって<i>Cyclobalanopsis</i>の含有率は15%に達せず,<i>Lepidobalanus</i>のそれを超えることもない.八千代市周辺の極相林とされる照葉樹林が広がった時期は見い出されない.しかし,少量の<i>Cyclobalanopsis, Camellia, Illicium</i>等の照葉樹林要素は,最終氷期最寒冷期前後から本地域周辺に分布していたと思われ,最終氷期の房総半島南部における照葉樹林の残存を強く示唆している.<i>Cyclobalanopsis</i>の含有率の増加には,<i>Cryptomeria, Juglans-Pterocarya, Ulmus-Zelkova</i>等の好湿性分類群が微増しており,落葉広葉樹林から照葉樹林への移行は,湿潤気候の始まりに伴う現象と考えられる.八千代市周辺において,照葉樹林の拡大を阻害した要因は土地的乾燥である可能性が大きい.
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 <i>Pinus</i> (<i>Haploxylon</i>) and <i>Picea</i>. In the second stage (ca. 17, 000-12, 000yrs BP) there were coniferous and broadleaved mixed forests which consisted of <i>Haploxylon, Picea</i>, and <i>Quercus</i> (<i>Lepidobalanus</i>). 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 <i>Lepidobalanus</i> along with others such as <i>Ulmus-Zelkova, Celtis-Aphananthe</i>, and <i>Carpinus</i>. The lower part of the fourth stage was characterized by the expansion of <i>Cryptomeria</i> 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 <i>Pinus</i> (<i>Diploxylon</i>) forest. Through all stages, the content of the <i>Quercus</i> (<i>Cyclobalanopsis</i>) 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 <i>Cyclobalanopsis, Illicium</i>, and <i>Camellia</i>, 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 <i>Myrica, Camellia</i>, and <i>Podocarpus</i> pollen, the percentage of <i>Cyclobalanopis</i> slightly increased; at the same time, wet-like taxa, for example <i>Cryptomeria, Juglans-Pterocarya</i>, and <i>Ulmus-Zelkova</i>, 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.
- The Quat. Res.
The Quat. Res. 37(4), 283-298, 1998-10-31
Japan Association for Quaternary Research