Evidence for Lateglacial and Holocene tree-line fluctuations from pollen diagrams from the Subalpine zone on 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 and 5400–3800 cal. BP was associated with patchy Subalpine scrub communities above and below this altitude. Rapid decline of this Halocarpus-Phyllocladus forest at c. 7500 cal. BP, and again c. 3800 cal. BP was probably due to volcan ism. During 7500–5400 and 3800–1718 cal. BP tree-line forest was replaced by Subalpine 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 cal. BP) expansion of Libocedrus indicates an upwards movement of forest species into Subalpine 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 cal. BP, and support the hypothesis that disturbance events can have persistent long-term effects on community composition and species distribution patterns.