Holocene vegetation, climate and history of a raised bog complex, northern New Zealand based on palynology, plant macrofossils and tephrochronology

Abstract

A Holocene history of vegetation, climate, and ombrogenous mire development is presented from pollen and plant macrofossil analyses of sediments at Kopouatai Bog, a large, raised, restiad bog in northern New Zealand. Tephra layers of established ages, supplemented by numerous radiocarbon dates, provide a secure chronology. The earliest peats, overlying last glacial sediments, and dated at c. 11700 radiocarbon years BP, with extensive accumulation after c. 10360 BP, are dominated by pollen of warm temperate podocarp-angiosperm forest, indicating a moist, mild early-Holocene climate. The bog began as a series of small soligenous mires within lowland podocarp-dominated swamp forest but was mostly oligotrophic by c. 8500 BP. Peat accumulation rates have varied spatially and temporally, averaging 0.9 mm yr-1 in central and southern areas. The deposition of deltaic muds in the northern part of the bog accompanied a marine transgression c. 6500-5000 BP, while elsewhere an associated groundwater table rise resulted in a temporary return to mesotrophic conditions. As the marine influence subsequently receded, the northern areas remained subject to regular flooding, but underwent rapid peat growth at a mean rate of 1.7 mm yr-1, while oligotrophic conditions returned to other parts of the bog. Regional vegetation developments indicate a change, c. 6000 BP, to drier, frostier conditions during the late Holocene. Ascarina lucida and Agathis australis may be used as regional pollen- stratigraphic markers for the early Holocene and late Holocene, respectively. The loss of tall trees and expansion of subcanopy species and seral vegetation in forests near Kopouatai Bog, just before the deposition of Kaharoa Tephra (c. 700 BP), are likely evidence for human activity dating from at least this time.