Evidence for tree-line fluctuation from Late Glacial-Holocene pollen diagrams from the subalpine zone of Mt Hauhungatahi, Tongariro National Park, New Zealand

Abstract

Pollen diagrams from four sites along an altitudinal sequence on Mt Hauhungatahi support fossil wood data (Ogden et al., 1997) in suggesting a fluctuating Holocene tree-line not exceeding the altitude reached in the early Holocene. Tree-line forest at 1340-1390 m during the periods 10 100-7500 (calibrated) BP and 5400-3800 BP was associated with patchy sub-alpine scrub communities above and below this altitude. Rapid decline of this Halocarpus-Phyllocladus forest at c. 7500 BP, and again c. 3800 BP was probably due to volcanism. During 7500-5400 BP and 3800-1718 BP tree-line forest was replaced by sub-alpine scrub. The failure of forest to replace scrub during these two periods implies a long-lasting influence of the event which destroyed the forest, a continuation of disturbance events, or changed environmental conditions.

After the Taupo volcanic eruption (1718 BP) expansion of Libocedrus indicates an upwards movement of forest species into sub-alpine scrub, followed by a decline. Volcanism has probably affected the vegetation of Mt Hauhungatahi directly and indirectly (through effects on soil drainage) throughout the Holocene. Results are consistent with increased climatic variability since 7500 BP, and support the hypothesis that disturbance events can have persistent long-term effects on community composition and species distribution patterns.

Keywords

Palynology, Late Glacial, Holocene, tree-line, disturbance, volcanism, Taupo Tephra eruption, Tongariro National Park.