Aranuian vegetation history of the Arrowsmith Range, Canterbury, New Zealand

Abstract

The vegetation history of three upland valleys in the Arrowsmith Range is deduced from pollen and macrofossil analysis of buried plant debris and peat, deposited at intervals through the Aranuian period. The evidence is supplemented by information from dated charcoal layers in the soils and from surface logs, knowledge of the modern pollen rain, and the present composition and distribution of the woody plant flora. The vegetation history is interpreted in the light of episodes of glacial advance and retreat and periods of alluviation. There are no direct data for reconstruction of early Aranuian (c. 14 000–10 000 yr B.P.) vegetation although probably it was mainly herbaceous. From the beginning of the middle Aranuian (10 000–6000 yr B.P.) Phyllocladus alpinus assumed dominance. This lasted until very late in the late Aranuian (6000–0 yr B.P.). In the latter part of the middle Aranuian, Podocarpus hallii was common and Prumnopitys taxifolia was almost certainly present, together with various angiosperm tree species. Floristic and geomorphic evidence indicate that climate was wetter than at present in the middle Aranuian. Herbaceous elements increased in the pollen floras near the end of the middle Aranuian and during the late Aranuian. This may have been associated with periodic natural fires during a time of drier climate. About 1850 yr B.P., and during the Polynesian occupation (800–600 yr B.P.), major fire episodes removed almost all tall woody vegetation, initiating the development of the present predominantly snow tussock (Chionochloa) grassland cover, with some extensive scrub stands. Hoheria lyallii is the only common tree now. Other trees and some shrubs are rare or locally distributed. Phyllocladus alpinus occurs only as single plants or small patches.