Contributions to the quaternary history of the New Zealand flora

Abstract

Long pollen diagrams from Cass (Canteibury) and Bell Hill (north Westland) illustrate a similar succession from grassland-shrubland through shrubland to forest. However, the expression of this succession in terms of plant communities is different. Shrubland communities at Cass were dominated by Dacrydium bidwillii or D. biforme and Phyllocladus, whereas Coprosma and Myrsine dominated at Bell Hill. Podocarpus and Nothofagus were the important forest trees at Cass in contrast to Weinmannia and Dacrydium cupressinum at Bell Hill. These contrasts reflect marked climatic differences between the eastern and western sides of the main divide. There are other less extreme differences in patterns of vegetation development within each region. Thus Podocarpus forest was present in lowland south and central Canterbury throughout the postglacial forest phase until its destruction by fire some 600 years ago. In north Canterbury a mosaic Podocarpus-Nothofagus forest had developed before the fires and at Cass, about 600m altitude, Nothofagus replaced mixed Podocarpus forest-Phyllodadus shrubland sometime after 8,000 years ago. At a few sites the major trends of forest history have been dated by radiocarbon assay. The available dates suggest that forest was not re-established in the South Island before about 10,000 years ago, and that its spread into different areas was not necessarily synchronous. The later spread of Nothofagus was not synchronous. Nothofagus was already dominant at Tophouse, near Nelson Lakes National Park, 7,800 years ago and began to spread into the upper Maruia Valley and Crooked Mary Creek areas about 6,500 and 6,000 years ago, respectively, and to Bell Hill about 2,400 years ago. This slow rate of spread emphasises that Nothofagus is still recovering from the disruptive effect of the Otira Glaciation upon its former distribution, especially in the central South Island. Thus, the major changes in vegetation, as illustrated by the pollen diagrams, do not necessarily reflect major changes in climate. Pollen rones are defined on the basis of major changes in the pollen curves. Five zones are recognised, although not all are necessarily represented in each pollen diagram. Because of site and regional differences, and because the number of pollen diagrams is still limited, the zones are treated on a local basis. The five zones recognised are: The period of time involved in the present study is of equivalent status to other stages of the Quaternary. Accordingly the term “post-glacial” is only used in an informal context, and the geological stage name “Aranuian” replaces it. This usage conforms with practice in New Zealand and in the Northern Hemisphere.