カラハリ砂層分布域東縁部におけるウッドランド環境の成立

抄録

This paper discusses the processes of woodland environment formation in Western and Northwestern Zambia in a part of the eastern marginal zone of the Kalahari Sands which spread over the inland Central and Southern Africa.
Present-day climatic condition of the area is considered adequate for the existence of dry evergreen forests contrary to actually dominant open deciduous woodlands. Small patches of dry evergreen forest are scattered in woodland on the plateau composed of the Kalahari Sands with very porous and oligotrophic Arenosols. On the other hand the adjacent bedrock plateau which have a little more nutritious Ferralsols is covered extensively by mosaic of deciduous woodland and cultivated land. Vegetation-ecological survey has demonstrated that existing small dry evergreen forest patches show very stable community structure maintained by natural renewal in contrast with repeatedly intervened structure of woodland communities. Palaeoenvironmental analysis based on surface-geological, geomorphological, and pedological evidences has concluded the prevalence of dryer climate in Latest Pleistocene time around 20, 000y. B. P., when the uppermost member of the Upper Kalahari Sands were redeposited on the plateau, and the following climatic humidification around the Pleistocene-Holocene boundary about 10, 000y. B. P., when the lower terrace was formed along the Kabompo River. Radiocarbon dating of the charcoal-rich layers which, intercalated in surface sands under woodland, indicate the occurrence of extensive burning provides about 2, 500y. B. P. and 1, 000y. B. P. An evidence of accelerated erosion due to deforestation since 400y. B. P. has been given in the Nyika Plateau, a famous former iron-smelting zone on the Zambia-Malawi border. All the material as above is concordant with previously-known palynological and archaeological records and supports a hypothesis as follows.
Dry evergreen forest seems to have (re) appeared in the area in humified climate of around 10, 000y. B. P. and to have gradually replaced by deciduous woodland mainly through human activity during recent thousands of years, particularly several hundred years. Although slight climatic aridification in the time may have affected the vegetational change, arrival of Later Iron Age farmers and diffusion of a type of slush-and-burn cultivation such as citemene system have more greatly contributed to the recession of Zambezian dry evergreen forest and appearance and maintenance of some types of open deciduous woodlands. Cultivators' preference of Ferralsols rather than very porous and oligotrophic Arenosols has resulted in the uneven remaining of small patches of dry evergreen forest on the plateau composed of the Kalahari Sands, although the Kalahari Sands' water-retaining character in dry season may provide a little suitable condition for existence of forest.