Y. Deng

Holocene vegetation, environment, and tephra recorded from Lake Pupuke, Auckland, New Zealand

Abstract Lake Pupuke provides a near‐complete, high‐resolution environmental record of the Holocene from northern New Zealand. Tephra beds constrain the timing of a range of proxy indicators of environmental change, and demonstrate errors in a radiocarbon chronology. Agathis australis forest progressively increases from c. 7000 yr BP and, in conjunction with indicators of reduced biomass productivity, support a model of long‐term climate change to drier conditions over the Holocene. However, except for Agathis, conifer‐hardwood forest dominated mainly by Dacrydium cupressinum shows little change throughout the pre‐human Holocene, suggesting environmental stability. Dramatic vegetation change occurred only within the last millennium as a result of large‐scale Polynesian deforestation by fire. This happened a short time before the local eruption of c. 638 cal. yr BP Rangitoto Tephra. The identification of two eruptions of tephra from Rangitoto volcano has implications for future hazard planning in the Auckland region, because the volcanoes were previously considered single event centres. Changes in atmospheric circulation since the Late Glacial, possibly causing lower frequency of distal ashfall in Auckland during the Holocene, complicates the use of long‐term records in hazard frequency assessment.

Vegetation changes since early Maori fires in Waipoua Forest, Northern New Zealand

The presence of abundant charcoal in sedimentary deposits is one of the key indicators of early Maori presence in New Zealand. However, it is often difficult to distinguish natural fire from anthropogenic. Studies of sedimentary charcoal and pollen in the Tarahoka clearing (or waerenga) in Waipoua Forest, where intentional burning is supported by the oral history of Te Iwi O Te Roroa, were undertaken with the belief that they would provide a level of detail, which could aid interpretation elsewhere. Vegetation plots and dendrochronological studies of trees on the clearing margins date the cessation of burning and subsequent invasion by woody plants. The radiocarbon and palynological results indicate that the clearing was created by fire ca. AD 1460. Although people were probably present to windward in the Waipoua valley before this time, they left no palynological signature at the study site. The date for the formation of the clearing agrees with others indicating population increase at this time, and with oral tradition for the arrival in the Waipoua valley of Manumanu 1, the ancestor of Te Iwi O Te Roroa. The maintenance of the clearing in seral vegetation by fire for >300 years supports the tradition that it was used as a kiwi (Apterix australis) hunting site. During the period of European contact, fire intensity appears to have declined, while fire frequency may have increased, favouring the spread of bracken (rahurahu, Pteridium esculentum), an important food source. Intentional firing probably ceased ca. 1900, by which time the local Maori population was in decline and European gum-diggers were camped in the clearing. The postulated sequence of formation, use and abandonment of the clearing requires confirmation by investigation of similar nearby sites using the same combination of methods.

A palaeoenvironmental record of natural and human change from the Auckland Isthmus, New Zealand, during the late Holocene

Abstract A multi‐proxy analysis of a sediment core from Waiatarua, Auckland Isthmus, adds to an environmental history from the local wetland spanning the Late Glacial to modern times. Several distal tephra were recorded in the core: 8.5 ka Rotoma (reworked), 6.1 ka Tuhua (primary and reworked), most likely the 1.8 ka Taupo (the latter is previously unreported for the Auckland Isthmus), and one unidentified, possibly 665 yr BP Kaharoa. Pollen and diatom analyses of the core show that during the period c. 6000‐c. 4800 yr BP, the site was a lake fringed with Cyperaceae/Leptospermum swamp. The lake became progressively shallower after c. 4800 yr BP, probably due to hydroseral infilling. Surrounding the lake was forest dominated by Dacrydium, Prumnopitys, Metrosideros, and Nestegis. Transition to the Polynesian era appears unclear because the site probably endured a hiatus due to destruction of peat by burning in European times.

A palynological study of Polynesian and European effects on vegetation in Coromandel, New Zealand, showing the variability between four records from a single swamp

Abstract Seven cores were extracted from a river terrace swamp in the forested Kauaeranga valley, Coromandel Peninsula, New Zealand. High‐resolution (c. 36–73 yr interval) pollen records were obtained from four of the cores and aged by radiocarbon dating and with stratigraphic reference to the 665 ± 15 14C yr BP Kaharoa Tephra. The records span the last c. 1800 yr and show that the vegetation consisted of lowland podocarp‐hardwood forest before deforestation by burning occurred. The pattern of deforestation at Kauaeranga, indicated by the abrupt dominance of Pteridium with concurrent increased charcoal, is typical of pollen records associated with early Polynesian settlement in New Zealand. Peaks of Pteridium and charcoal were also found in sediments deposited after European settlement. Different cores show marked palynological and stratigraphic differences relative to the Kaharoa Tephra, most importantly with regard to the timing of deforestation. Deforestation occurred close to the Kaharoa, at a calculated age of c. 750 BP in one core but well above the Kaharoa (c. 480 BP) in another. The stratigraphic unconformities between cores are attributed to variable fluvial processes causing an uneven deposition of sediments within the swamp.

Application of palynology to describe vegetation succession in estuarine wetlands on Great Barrier Island, northern New Zealand

Abstract Question: This paper compares published palynological studies from coastal swamps containing the same suite of species. We ask the following questions: (1) does succession follow the same pathways in different swamp systems, or (2) at different times? If not, (3) how variable are the patterns and (4) what are the likely driving factors? Location: Great Barrier Island, Northern New Zealand. Methods: Eighteen pollen profile diagrams were studied from four estuarine wetlands, ranging from mangroves to swamp forest. Recognition of a transition between vegetation stages was by subjective consideration of the relative abundances of pollen of key indicator species at different depths in the sedimentary sequence. Results: A linear sequence of vegetation communities beginning with mangroves and followed by estuarine marsh communities composed of Juncus kraussii, Leptocarpus similis and Baumea juncea was recognised in almost all pollen diagrams. Further transitions, from Baumea to a terrestrial system of Leptospermum shrubland or Cordyline/Dacrycarpus swamp forest, followed two main pathways associated with autogenic accumulation of peat and terrigenous sediment input respectively. At Kaitoke and Awana the marine/freshwater transition occurred before the arrival of humans on Great Barrier Island. At Whangapoua, increased sedimentation followed anthropogenic burning of adjacent forest, and this transition was faster and is still in progress. Conclusion: Palynology and current vegetation zonation patterns concur to demonstrate that the marine sedimentation phase of estuarine succession is predictable and linear. Baumea marks the transition to the freshwater phase, in which varied successional patterns are determined by interactions between hydrology, sediment input, and peat accumulation. Natural and human disturbances drive sedimentation rates, and interact with autogenic factors, to dictate vegetation transitions in these later stages. The intensive impact (mainly burning) during Polynesian times had a much greater effect on estuaries and swamps than the pre-Polynesian natural processes, greatly accelerating plant succession.

The vegetation sequence at Whangapoua Estuary, Great Barrier Island, New Zealand

Abstract One hundred and eight vegetation plots were measured along six transects, running from estuarine mudflats to freshwater swamp in Whangapoua Estuary. These were analysed to define the patterns of plant communities, and evaluate their relationships with environmental gradients. TWINSPAN classification of the vegetation plots and species suggests that the three main vegetation zones (mangroves, salt meadow/marsh, and freshwater swamp) can be divided into six broad vegetation communities named as follows: (A) Avicennia marina, (B) Juncus kraussii sea rush, (C) Leptocarpus similis salt meadow, (D) Baumea juncea sedges, (E) Leptospermum scoparium shrubland, (F) Typha orientalis/Cordyline australis swamp forest. DECORANA ordination reveals a gradient of communities from mangrove through salt meadow to shrubland then to swamp forest. The pattern of species distribution from salt to freshwater results from the interaction between species and the physical constraints of the salinity and freshwater inundation gradients. This pattern accounts for c. 77% of the current floristic variation in the modern vegetation, indicated by the different vegetation communities exhibiting similar distribution patterns along the estuarine to freshwater gradient.

Preservation of a Late Glacial terrestrial and Holocene estuarine record on the margins of Kaipara Harbour, Northland, New Zealand

Abstract Subtidal to intertidal deposits from the margins of Kaipara Harbour in Northland preserve a c. 23 000 year incomplete sedimentary record of the transition from terrestrial to estuarine conditions. Cores are used to reconstruct the depositional setting for this transition, interpreted as a succession from dune and freshwater wetland to shallow estuarine environments. The fossil pollen record provides a proxy of Last Glacial Maximum and Late Glacial vegetation for the area. Stability of the Pleistocene dune landscape during the postglacial marine transgression is interpreted on the basis of strong dominance of tall forest taxa (Dacrydium) in the pollen record and soil development in dune sands, with preservation aided by location along the estuary margin. During the Holocene, reworking of the buried dune and wetland sediments has only reached to a depth of 1.5 m below the modern tidal flat. As such, the site provides a rare example of good preservation of Pleistocene deposits at the coast, where extensive reworking and loss of record are more typical.