Aranuian pollen diagrams from Montane Canterbury New Zealand

Abstract

Detailed site histories are developed from pollen analyses at six sites in three areas of montane Canterbury: Upper Rakaia Valley, Mt. Somers, Lake Sumner. A post-glacial (Aranuian) vegetational and climatic history for central montane Canterbury is developed from these site histories and pollen analyses published from other areas. The site histories relate broadly to existing knowledge, but it is shown that pollen diagrams from montane areas may not be taken as being directly representative of the regional vegetation. Forest in montane Canterbury became widespread in the upper Rakaia Valley 10,000 years ago. The subsequent spread of beech forest (species of the Nothofagus fusca pollen group) in montane Canterbury occurred about 6,000 years ago in the Waimakariri and Hurunui catchments; more than 4,500 years ago in the Harper tributary of the Rakaia River; and about 1,000 years ago in the vicinity of Prospect Hill in the upper Rakaia Valley. The isolated occurrence of silver beech (N. menziesii) in the Lake Stream tributary of the Rakaia River has a probable history of about 8,000 years, and at Prospect Hill, a local history of 2,000 years. Beech forests of the Hurunui catchment originated from a northern mixed beech source, while the beech forests of the Waimakariri and Rakaia catchments, and Mt. Somers, originated mainly from mountain beech (N. solandri var. cliffortioides) sources, scattered most probably in the foothills of the central Canterbury Alps. Present evidence suggests that there was a marked improvement in climate 10,000 years ago from cold early Aranuian conditions. It is thought that climatic conditions were most equable between 10,000 and about 6,000 yr B.P. when precipitation was higher than at present. Conditions deteriorated at about 6,000 yr B.P. becoming drier and less equable, approaching present conditions. Pollen and charcoal evidence of European, Polynesian, and prehistoric fires in the study areas contributes to the history man-caused and natural fires in Canterbury. Polynesian fires in the Upper Rakaia - Lake Heron - Mt. Somers region are seen as the coup de grace in a long established history of decline of montane podocarp forest there.