The Late Quaternary History of Kauri (Agathis australis) in New Zealand and Its Climatic Significance

Abundant sub-fossil wood of kauri Agathis australis (Salisb.) is preserved in peat swamps throughout the present distribution of the species in North Island, New Zealand. Analysis of 107 radiocarbon dates on this material shows they fall mainly into two groups. A northern group represents late Pleistocene interstadial forests, while a southern group relates to a mid to late Holocene expansion to the current southern limits of the species at c. 38⚬ S. Wood growth rates in the interstadial samples, compared to modern trees, suggest that kauri was stressed by lower temperatures and a wetter environment. With further temperature reduction in the late Otiran stadial (last glacial maximum) growth may have been limited to c. four summer months. The scarcity of kauri pollen from stratigraphic samples and the rarity of wood dating from this period support the contention that kauri was much reduced in abundance during the last glacial maximum. Kauri expansion in the early Holocene may have been delayed by moist cloudy summer conditions, which are not favourable for kauri growth, and by lack of suitable regeneration opportunities in the more equable climate of that period. In the mid to late Holocene, after c. 7000 BP, kauri spread southwards at rates of up to 197 m yr1, reaching its southern limit by c. 3000 BP. This apparent southwards migration probably represents the expansion of small populations on favourable north-facing slopes or ridge-top sites scattered throughout the area to the north of Auckland, rather than a simple wave-like migration from a hypothetical northern refuge. It implies a change in the disturbance regime, with greater frequency of landscape-scale forest destruction by fire or windstorm, favouring the regeneration of karui, and drier sunnier summers enhancing tree growht rates. Kauri remained abundant at the southern limits in the Waikato from 3000 to c. 2000 BP, but there is a suggestion of a northwards contraction since then. This southern decline is not supported palynologically until c. 700 BP, when anthropogenic fires reduced forest cover over much of New Zealand.