Late Quaternary vegetation history of central North Island, New Zealand
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
23 sites in the central North Island of Otiran and Aranuian age (Last Glaciation and post-glacial) were pollen analysed. Three regions were examined in detail: Taranaki, Taupo-Tongariro and Bay of Plenty. For most of the Otiran period (c. 70 000 BP-14 000 BP) the central North Island was covered with scrub-grassland communities. Forest was scarce and scattered. Within this period two major interstadials are recognized. During an early Otiran interstadial, which reached its maximum at about an estimated 60 000 BP, both podocarp-hardwood forest and Nothofagus forest were abundant. Although perhaps 2°C cooler than present, the climate seems to have been mild and moist during this period. A second interstadial lasted from about 45 000 BP to 25 000 BP. It was a time of diverse vegetation and climates. Although scrub was the dominant vegetation over most of the region, there were substantial areas of podocarp-hardwood forest, Nothofagus forest and also some grassland. In some localities it appears that all these vegetation types co-existed. Forest was most abundant from 32 - 27 000 BP. The climate was considerably cooler than that of today with mean annual temperatures up to 4°C lower. Rainfall was lower than at present, but still adequate for podocarp forest growth in some regions. From after 25 000 BP until 14 000 BP grassland/scrubland associations were almost totally dominant. Some forest survived throughout this period, but mainly in hilly and mountainous regions. The grassland/scrubland associations appear to have been very uniform throughout, and there is no evidence for better developed vegetation in the northern regions. The climate was much cooler than today, perhaps 5°C cooler but, more importantly, appears to have been harsh and variable. Rainfall was much lower than at present, especially in the east. In Taranaki, forest dominated by podocarpus spicatus replaced grassland between 13 000 and 12 000 BP. At about 10 000 BP Dacrydium cupressinum and Ascarina lucida became abundant and remained so until about 5000 BP. After this date Ascarina declined while Knightia excels and Dacrydium colensoi spread. After 3000 BP D. colensoi became scarce and today is no longer found in the region. In Hawkes Bay Podocarpus spicatus/totara forest replaced the previous grass and scrub communities by 10 000 BP, but probably not much earlier. These forests remained dominant with only minor changes until the Polynesian burnings of the last millenium. The early Aranuian (14 000 - 10 000 BP) was drier than present, but rainfall was adequate to support podocarp forest in most places, except the drier east coast districts. Increase in rainfall, rather than the well-attested Aranuian warming, was the main reason for the spread of forest. The period from 10 000 BP to 5000 BP was the mildest and wettest of the last 70 000 years. Since 5000 BP there has been a drift to a cooler, droughtier climate.