Late Quaternary palynological investigations into the history of vegetation and climate in northern New Zealand

Abstract

This thesis describes the vegetation and climatic changes over the past 20,000 years from pollen records at eight northern New Zealand lowland peat and lake sites, ranging from Taranaki to the Far North. The sites investigated are Umutekai Swamp (Taranaki), Lakes Rotomanuka, Rotokauri, and Okoroire (Waikato), Kopouatai Bog (Hauraki Plains), Lake Waiatarua (Auckland), Otakairangi Swamp (mid-Northland), and Trig Road Swamp (Far North). At sites from Auckland southwards, dating and correlation of the pollen records were enhanced by the occurrence of multiple tephra layers within the pollen-bearing sediments. The clearest picture of regional vegetation history and tightest chronologic control were obtained from the tephra-rich organic lake sediments of the Waikato lowlands. Holocene vegetation changes were broadly consistent throughout this northern New Zealand region and indicate climates, which were initially moist, mild and equable, but became increasingly variable and probably drier overall during the late Holocene. Podocarpangiosperm forest was always prominent and Agathis australis forest expanded throughout the region north of latitude 38° S during the last 6,000 years. Kauri was especially prominent in the Waikato region during the 1000 years or so following the Taupo eruption of c.1800 years ago. At pollen sites from Waikato, Hauraki Plains, and Auckland, palynological evidence suggests that people began clearing forests as early as 800 years ago, but probably not much earlier. Pollen records for the last glacial show less regional uniformity. South of Auckland, scattered tracts of Nothofagus or Libocedrus forest within a shrubland/grassland mosaic were succeeded, between c.14.5 and 10 ka by the regional expansion of podocarp-angiosperm forest, with Prumnopitys taxifolia initially prominent. North of Auckland, the pre-Holocene vegetation history is complicated by uncertain chronologies. Conifer-angiosperm forest with prominent A. austalis grew in the Far North during the last glacial, while in mid-Northland, a substantial period of Nothofagus forest, shrub and grassland communities may correspond to either the entire last glacial or to the late glacial. Local variations in vegetation cover were maintained to some extent independently of regional climate, influenced by site specific factors including edaphic controls, hydroseral succession, and local hydrological changes caused by, e.g., lahar or lava flow, fluvial activity and sea level change. The influence of these local factors is most evident for the late glacial, during which period podocarp-angiosperm forest spread throughout northern New Zealand generally, but with considerable variation in timing even between nearby sites. Fire appears to have been an important factor in vegetation change throughout the period investigated, not just during the human deforestation era; peat swamp communities show a long history of association with fire, while in dryland vegetation, Agathis australis appears to have been especially affected by burning. No unequivocal evidence was found for postglacial latitudinal migrations, but several plants show significant altitudinal range expansions during the last glacial compared with their present distributions in northern New Zealand, viz., Nothofagus menziesii, Libocedrus bidwillii, Phyllocladus aspleniifolius, and Halocarpus spp. Thus although vegetation communities at each locality have changed substantially over time, the flora of northern New Zealand remained essentially the same during the c.20,000 years before the human era. Interpretation of the pollen records was assisted by principal components analysis (PCA) and by referring to modern pollen data and pollen-vegetation comparisons obtained from Waipoua Forest, Northland. PCA provides an efficient means of summarising and portraying large pollen datasets, and helps to clarify underlying environmental factors and temporal trends. PCA also generally supports pollen diagram zonations determined by eye. The Waipoua study indicated that the relationship between pollen and tree abundances is highly variable within forests, dependent largely on local site characteristics, especially the masking effect of strong local pollen sources. Nevertheless, quantified pollen-vegetation relationships averaged for the study area mostly accord with results from previous New Zealand modern pollen rain studies, while adding new information for the pollen representation of several prominent northern species. The Waipoua study indicates that pollen spectra rich in Agathis may be found where trees grow nearby, but Agathis pollen appears to be less widely dispersed than pollen of other New Zealand anemophilous taxa. At several swamp sites, correlation of tephra layers and pollen-stratigraphic events reveals problems with radiocarbon chronologies which can not be satisfactorily resolved except by assuming contamination by modern carbon. Sites with a history of hydroseral succession, where swamp communities have developed in former lake basins, are especially prone to this contamination, presumably because root penetration of older sediments provides channels for downward movement of younger carbon. In such situations it may be unwise to date and correlate pre-Holocene sediments on the basis of radiocarbon alone. Periods of hiatus are not uncommon in lake and swamp profiles from northern New Zealand and it is possible that the record of the last glacial is missing or strongly compressed at many Northland sites. Sedimentation rates also varied markedly between and within sequences, precluding the accurate estimation of pollen accumulation rates except at the Waikato lake sites where tephra sequences provide detailed chronological resolution. Even here, however, pollen concentration and accumulation rates appear to have been highly susceptible to short-term fluctuations in the sedimentation regime.