Aranuian vegetation history of the Arrowsmith Range, Canterbury I. Pollen diagrams, plant macrofossils, and buried soils from Prospect Hill

Abstract

Two pollen analyses are described from sediments in the mires at Quagmire Tarn and Windy Tarn, on the Prospect Hill plateau, upper Rakaia Valley, Canterbury. The sediments span most of the Aranuian period. Macrofossils from Quagmire Tarn supplement the pollen data. The vegetation sequences from the two sites are generally similar, although the Windy Tarn sequence begins later than that of Quagmire Tarn and differs in some details. The post-Otiran dominant pollen assemblages at Quagmire Tarn are: Poaceae and Coprosma (beginning probably about 13 000 yr B.P.) → Halocarpus (about 10000 yr B.P.) → { Phyllocladus and “Podocarpus” (just after 10 000 yr B.P.) → Nothofagus menziesii and N. fusca-type (about 2000 yr B.P.) → Poaceae, Cyperaceae, Asteraceae, and Apiaceae (beginning about 860 yr B.P.). Changes in the sediments, pollen assemblages, and macrofossils are correlated with the early Aranuian glacial history of the region, with subsequent climatic changes, and with the fire history. The fire history is deduced from approximate ages estimated from depths of charcoal horizons in the Quagmire Tarn sediment column and radiocarbon dates for charcoal from palaeosols on the edge of the Prospect Hill plateau. Fires are thought to have occurred at Prospect Hill about 5800, 3800, 3500, 2600, and 860 yr B.P. Climatic shifts are thought to be the cause of changes from Poaceae, Coprosma (cold, dry) → Halocarpus (cool, moist) → Phyllocladus, “Podocarpus” (evenly mild, moist). The time-transgressive expansion of Nothofagus spp. in the Canterbury mountains requires a more complex explanation which includes the proximity of stands of these species during the Otira Glaciation and slow Aranuian migration, the requirement for infection of seedlings by ectomycorrhizas, and possible local shifts across climatic thresholds (towards more variable conditions). The replacement by grassland of most of the woody vegetation of the Prospect Hill plateau and adjacent areas was accomplished after one or more major fires within the last 1000 years.