Vegetation history and environmental significance of pre-peat and surficial peat deposits at Ohinewai, Lower Waikato lowland

Abstract

The Ohinewai peatlands occupy 15 km² near Lake Waikare, south Auckland. The peats average 6–9 m in depth, and overlie a grey-green lacustrine mud (3–11 m thick). The lacustrine mud was deposited in a proto-Lake Waikare between 20,000 and 7,000 yr bp, and pollen analysis shows that open shrubland-grassland vegetation was dominant for at least a portion of this time. The basal peat ranges in age from 7,000–6,000 yr bp. The peat formed in an oligotrophic raised bog during the drainage and contraction of the proto-Lake Waikare, a process which may have been caused by a drop in sea level. Pollen analysis of the peat shows that conifer-hardwood forest, similar to that of the present, was dominant throughout. Ascarina lucida and Dacrydium colensoi, trees which are extinct, or nearly so, in the region today, were present until at least 2,000 yr BP. There was little change in the forest between 7,000 and 2,000 yr bp, except for a marked increase in Agathis austrahs at around 3,000 yr BP, which may indicate a trend towards a drier climate. Fires were common on the bog at all times.