A LATE HOLOCENE POLLEN RECORD OF ENVIRONMENTAL CHANGES FROM WHANGAPOUA ESTUARY ON GREAT BARRIER ISLAND, NORTHERN NEW ZEALAND

Pollen and sediment in cores taken from Whangapoua indicate a sequence of vegetation changes and environmental disturbance since c. 3500 cal. yr B.P. Three major zones can be distinguished in eleven cores. The lower zone (pre-human impact, 3500-800 cal. yr B.P) is characterised by a phase of marine sedimentation. The local environment at 3500 cal. yr B.P. was a tidal flat surrounded by conifer-hardwood forest (Dacrydium, Libocedrus, and Prumnopitys) including Cyathea, which indicates a moist warm climate. After 1700 cal. yr B.P., Agathis and Phyllocladus became more common, and more shrubs such as Myrsine appeared in the sub-canopy, suggesting the climate was becoming drier. Around 1500 cal. yr B.P., the appearance of Avicennia pollen marks the start of a successional vegetation sequence, and coincides with charcoal fragments, which may have been a result of natural fires. The subsequent, intermediate zone (Polynesian, 800 cal. yr B.P. to 1840 AD) encompasses large-scale human impact in the region and the transition of the core sites from marine to freshwater. The associated decline in tree pollen, a result of deforestation by fire, coincides with a sharp and sustained increase in charcoal and Pteridium spores. Higher sedimentation rates at the same time indicate increased erosion of surrounding hills. Around 350 cal. yr B.P., a sharp decrease in Leptocarpus and an increase in Baumea pollen indicate the change from salt marsh to freshwater swamp. The uppermost zone (European, 1840 AD to present) is marked by the appearance of Pinus and an increase in Poaceae pollen. A local change from Baumea to Leptospermum/Gleichenia to Typha swamp is coincident with continued deforestation during this period and a further increase in the rate of erosion. The driving factor for plant succession in the Whangapoua during the late Holocene thus appears to have been siltation caused by human impact.