Vegetation, fire and climate change in southwestern South Island (45°S), New Zealand since the Last Glacial Maximum

Full text: I report pollen, spore and charcoal records from Lake Von (45°14'S; 168°17'E, 685 m.a.s.l), a small closed-basin lake located in southwestern South Island of New Zealand. These records allow the examination of past vegetation and climate changes during and since the Last Glacial Maximum (LGM). The record indicates dominance of alpine vegetation with cold-tolerant shrubs, herbs and ferns between ~18.0-16.7 ka, followed by a gradual transition toward Coprosma-dominated subalpine vegetation with the local presence of subalpine/montane hygrophilous trees between ~16.7-14.8 ka. These data suggest an early stage with cold and relatively wet conditions between ~18.0-16.7 ka under moderate southern westerly winds (SWW) influence, followed by cold temperate conditions and a slight increase in precipitation between ~16.7-14.8 ka under strong SWW influence. I observe a rise in vegetation characteristic of alpine environments between ~14.8- 12.6 ka at the expense of Coprosma, accompanied by increases in cold-tolerant hygrophilous species and lake levels. This was followed by the increase and dominance of Coprosma between ~12.6-10.8 ka, along with declines in alpine vegetation, cold-tolerant hygrophilous trees and lake levels. I interpret these data as indicating decline in temperatures and enhanced precipitation between ~14.8- 12.6 ka under stronger SWW influence, followed by warming and reduced precipitation between ~12.6-10.8 ka under weak SWW influence. Conspicuous increases in Halocarpus and Phyllocladus between ~10.8-7.2 ka, along with increases in drought-resistant conifers and fire activity, suggest a further warming pulse and reduced precipitation under weaker SWW influence. The record shows an increase in the cool-temperate silver beech (Lophozonia menziesii) between ~7.2-3.7 ka, along with the hygrophilous conifer Dacrydium cupressinum and relatively low abundance but with high variability in the warm-temperate tall conifers. Within this multi-millennial-scale interval, I detect low-magnitude centennial-scale decline in Lophozonia menziesii and Dacrydium cupressinum between ~6.0-5.2 and ~4.4-4.1 ka, concurrent with increase in the drought-tolerant Prumnopitys taxifolia. These changes were followed by decline in Lophozonia menziesii and Dacrydium cupressinum between ~3.7-2.9 ka, sustained increase in Lophozonia menziesii, Dacrydium cupressinum and drought-intolerant species between ~2.9-1.9 ka, and a rapid and prominent rise in Fuscospora between ~1.9-0.56 ka. I interpret these data as a decline in temperature and a sustained rise in precipitation between ~7.2-3.7 ka under increased SWW influence. Superimposed upon, and following this multi-millennial climate trend, I detect alternating dry and wet oscillations of millennial- and centennial-scale with low precipitation between ~6.0-5.2, ~4.4-4.1, ~3.7-2.9, and ~1.9-0.56 ka under decreased SWW influence, and wet periods in the intervening intervals suggesting increased precipitation under strong SWW influence. Finally, the records show increased fire activity during the last ~600 years, coeval with a decline in arboreal pollen and rise in vegetation indicative of human disturbance (Māori). The climate trends detected from Lake Von record are coherent with variations of the SWW identified in other terrestrial mid-latitude records, suggesting synchronic and symmetric changes in the SWW during and since the LGM. These results might constitute the best empirical evidence in southern New Zealand to demonstrate high variability of the SWW during and since the LGM