Ｔｅ Ｎｇａｅ Ｒｏａｄ Ｔｅｐｈｒａ Ｓｅｃｔｉｏｎ（ニュージーランド）における火山灰土壌の植物珪酸体分析 過去２万年間の土壌と植生の関係

Abstract

Opal phytolith analysis using A horizons of present and buried soils at Te Ngae Road Tephra Section in Rotorua Basin, New Zealand, was carried out to clarify the relationship between vegetation and volcanic ash soils during the last 20, 000yrs. Silica bodies in some New Zealand native trees were also examined. The results are summarized as follows.1) For the period between 20, 000 and 11, 250y.B.P. grass was the main source of phytolith. The very low amount of phytolith in soils formed before the deposition of the Rerewhakaaitu Ash (14, 700y.B.P.) indicates that these soils developed under scattered grassland or in forests poor in phytolith-supplying trees. During the formation period of soil in the Waiohau Ash (11, 250-7, 330y.B.P.) the main sources of phytolith were grasses and trees. The clear rise of cauliflower-head-like phytolith originating from trees suggested that the climate was wetter and milder than at an earlier stage. Between 7, 330 and 930y.B.P. tree-origin phytolith was dominant. This indicates that forest vegetation completely covered soils. After the deposition of the Kaharoa Ash (930y.B.P.), grass, trees, and ferns were sources of phytolith. The increase of grasses and ferns was due to Polynesian settlements with a resultant reduction in forest cover and increase in fernland and grassland.2) Holocene volcanic ash soils buried beneath the Kaharoa Ash contain very low amounts of humus because these soils developed under forest vegetation without grasses and ferns. After the deposition of the Kaharoa Ash, widespread grassland and fernland introduced by Polynesian settlements provided the volcanic ash soils with a black humus horizon.3) It is said that volcanic ash soils with thick black humus horizons (Koroboku soils) were formed under grass vegetation and slow tephra deposition of slow rate. The fact that few Kuroboku soils are distributed in New Zealand might be explained by the fact that until the deposition of the Kaharoa Ash (930y.B.P.) there was little human activity which destroyed forests and induced grass vegetation.4) Small cauliflower-head-like phytoliths were found in the leaves of Rewarewa (Knightia excelsa) and the wood of Kohekohe (Dysoxylum spectabile). These trees seemed to be one of the sources of tree-origin phytolith (small grain type) in the soils of New Zealand.