A 2000 year history of vegetation and landscape change in Hawke's Bay, North Island, New Zealand

Sediment cores from four lakes in the Tutira and Putere districts of Hawke's Bay, North Island, New Zealand, are analysed for the remains of pollen, charcoal, tephra and erosion pulses to reconstruct a 2000 year history of vegetation and landscape change. The Hawke's Bay region is disturbed fi'equently by earthquakes, volcanic eruptions, cyclonic storms, droughts and fire. This thesis determines how the vegetation and soil stability have responded to some of these disturbances, through detailed palaeoecological investigations of lake sediment cores. Studies of surface pollen and differential pollen and spore preservation were undettaken to enhance the interpretations made from the palaeoecological record. Because New Zealand has only been settled by Polynesians relatively recently, the effects of natural disturbance on the vegetation and landscape can be assessed under similar climatic conditions to the present, but in the absence of cultural change. The effects of human settlement on a previously uninhabited landscape are assessed and compared with previously occurring natural disturbances. Over the 2000 years investigated, the pollen records show three distinct types of vegetation were present in Hawke's Bay. These are summarised below, together with the principal conclusions drawn from the analyses: (1) Pre-deforestation vegetation: 1850 to 600 BP Frequent disturbance caused ongoing changes in the composition of the lowland podocarphardwood forests of Hawke's Bay. Natural fires have not previously been considered as a primary cause of forest disturbance in the North Island during the late Holocene, but the results show forests were disturbed frequently by lightning-strike fires associated with cyclic and episodic droughts, and with volcanic eruptions. Despite the frequency of these fires and the low specific adaptation to fire in the New Zealand flora, full and relatively rapid forest redevelopment occurred after each fire episode indicating a degree of tolerance to fire. The 1850 BP Taupo eruption was the largest single disturbance event during this period, but full forest redevelopment occurred within c. 200 years. Storms and erosion producing events caused no measurable changes in pollen composition. (2) Polynesian deforestation: 600 BP Widespread deforestation occurred in Hawke's Bay c.600 BP as a result of Polynesian settlement, after which bracken became the dominant vegetation type until it was cleared by European settlers in AD 1840-1870. Polynesian deforestation was the largest and most sustained disturbance to have occurred in Hawke's Bay since the end of the last glaciation. Polynesian deforestation may have coincided with or followed a severe, prolonged drought. Droughts and the increased frequency and scale of both intentional and accidental fires, invoked widespread and rapid forest clearance. Throughout the Polynesian period soil erosion was minimised, partly because of the deeply penetrating bracken rhizomes which maintained the strength of the soil structure and the dense fern canopy which protected the soils from raindrop impact. Despite the rapid clearance of large areas of forest, the landscape and bracken/shrubland vegetation remained relatively stable for 500 years. (3) European settlement: AD 1870 Since European settlement in AD 1870, erosion caused by intense rainstorms is the most significant and frequently occurring type of disturbance in Hawke's Bay. Erosion occurred more often during the European period when the catchments were under grassland, compared with both the forest and bracken/shrubland vegetation phases. These results agree with contemporary studies of landslide erosion under similar vegetation types. In the European section of the cores, there is approximately six times more erosion pulse sediment than there is in the forested period, and about two to four times more than in the Polynesian period. Since pasture became the dominant vegetation type, the soft-rock hill country of the Tutira and Putere districts has become more vulnerable to erosion and landslides. The pollen and spore content of surface samples (mosses, soils and surface lake sediments) was analysed to help define the different sources of pollen and spores in the ancient lake sediments. Surface lake sediment pollen spectra represent an amalgamation of both contemporaneous and secondary inwashed pollen and spores of mixed age. The bracken and tree fern spores in the surface sediments were highly corroded but the pollen was well preserved. In contrast, pollen and spores in mosses were well preserved and almost entirely derived from local, contemporaneous source plants. Soils contained a lower diversity of mixed aged pollen and spores which were highly corroded, except for the most resistant types such as tree fern spores. These variations in pollen and spore preservation are attributed to their different sources in the catchment. In the surface lake sediments most of the bracken spores were derived from stored spores in the soils; most of the pollen was derived from the contemporaneous plants; and corroded tree fern spores were presumed to have been eroded from river bank sediments. Differential pollen and spore preservation analyses were used to determine relationships between the categOlY of preservation and type of deposit. Results indicate corroded bracken spores are strongly associated with inwashed soils. In lake basins that receive inwashed soils, these soils can continue to supply old, corroded bracken spores to the pollen spectra, even though the fern may be absent or scarce in the catchment. Differential preservation analysis allows the proportion of these corroded bracken spores to be estimated and removed from the pollen sum, permitting a more accurate representation of this fern spore in the fossil pollen record.