Vegetation changes and their climatic implication for the late Pleistocene at Lake Poukawa, Hawkes Bay, New Zealand

Abstract

A detailed vegetation history extending back to the Last Interglacial (Marine Isotope Substage 5e) is presented for Lake Poukawa, Hawkes Bay, New Zealand. This history is based on palynological analyses of a 198-m core record, age secured by uranium/thorium (U/Th) and optically stimulated luminescence (OSL) dates as well as tephrochronology. Vegetation of the penultimate glacial (Isotope Stage 6) probably consisted of a very sparse shrubby herbland of stunted podocarps ( Phyllocladus sp.), daisies, grasses and sedges. Similar floras existed in each of the succeeding cool periods. Four interglacial/interstadial floras are preserved. The peak of the Last Interglacial (Substage 5e) was dominated by extensive lowland forest of tall forest podocarps ( Podocarpus/Prumnopitys), most probably matai ( Prumnopitys taxifolia) with red beech ( Nothofagus fusca), secondary forest trees ( Coprosma spp., Myrsine spp.) and tree ferns ( Cyathea spp.). In the Poukawa basin itself, a swamp forest of the podocarp ( Dacrycarpus dacrydioides) with the true palm ( Rhopalostylis sapida) expanded. Substage 5e was significantly warmer and/or moister than the present day. Substage 5c/5a was marked by montane forest of red and silver beeches ( N. fusca and N. menziesii), Phyllocladus spp. and secondary trees. A reduced representation of Podocarpus/ Prumnopitys possibly consists of montane podocarp ( Podocarpus hallii). We infer the temperature of Substage 5c/5a to have been 3.5–5 °C lower than the present day. Although Stage 3 flora are superficially similar to those of Substage 5c/5a, the beeches are relatively less dominant. The regional flora during Stage 1 is very similar to those of Substage 5e, but the density is lower with no swamp forest during the present interglaciation. This may represent a change in evapotranspiration balance around Poukawa, but may also be controlled by hydrological conditions within the basin.