Lateglacial and Holocene vegetation and climatic change on Auckland Island, Subantarctic New Zealand

Auckland Island is a small uninhabited Subantarctic island south (c.51°S) of the New Zealand mainland. It is the southernmost outpost of tall forest in the southwest Pacific. Pollen analysis of two closely adjacent peat cores near tree-line documents climate change and forest dynamics during the Lateglacial and Holocene. After the retreat of glaciers before 15 000 14C yr BP, mountain tundra communities dominated on mineral soils under a cooler (c.3°C lower) climate than now. By 11 000 14C yr BP, a transition to Chionochloa 14C yr BP shrub/grassland became dominant. Forest tree species (Metrosideros umbellata, Dracophyllum longifolium and Raukaua simplex) spread slowly from 10–000 14C yr BP, but did not form tall forest until 5500–4000 14C yr BP, despite southern ocean temperatures being warmer in the early Holocene. We suggest that warm, cloudy, low-radiation environments inhibited forest growth during the early Holocene in this intensely oceanic setting, through promoting saturated soils and reducing net photosynthesis. It was only in the later Holocene, when increased westerly windflow brought sunnier, although cooler and windier, climates, that forest expansion occurred on sheltered lowland sites. The forest at the study site collapsed to scrub at least twice within the last 2000 years, most likely because of extended periods of saturated soils.