The forest history of Mt Hauhungatahi, Tongariro National Park

Abstract

The aims of the study were to examine: 1) the individualistic species colonisation of an altitudinal gradient during the Holocene, and 2) the influence of the catastrophic Taupo Pumice eruption of c. 1714 BP on subsequent vegetation patterns. Palynological methods were mainly used, supplemented by population age-structure analysis of modern tree stands. Moss samples from plant communities on an altitudinal gradient from montane forest to sub-alpine tussockland on Mt Hauhungatahi were analysed for pollen content to quantify modern pollen/vegetation relationships. Cluster analysis of pollen percentages from the sites indicated five distinctive vegetation types: 1) montane forest, 2) sub-alpine forest, 3) sub-alpine scrub, 4) sub-alpine fernland, and 5) sub-alpine tussockland. Ordination arranged sites along an open to densely structured vegetation gradient, ie from fernland and tussockland to forest and dense scrub. Stepwise discriminant analysis selected the pollen proportion of Myrsine (sub-alpine scrub), Gleichenia (sub-alpine fernland), Libocedrus (sub-alpine forest), and Restionaceae (sub-alpine tussockland) as highly discriminatory variables in relation to vegetation type. High VJeinmannia, Nestegis, and Cyathea smithii-type pollen values characterised the montane forest. Comparison of basal area and pollen data for sites in forest indicated that Phyllocladus, Weinmannia, Nestegis, Coprosma, and Cyathea smithii-type were over-represented. Dacrydium, Podocarpus, Halocarpus, and Libocedrus were under-represented. Comparing pollen with basal area within forest sites, two out of five sites showed a significant positive correlation between species ranked by basal area and pollen. Across forest sites, Phyllocladus, Nestegis, Dacrydium, and Coprosma pollen counts showed a significant positive correlation with basal area. Twelve pollen diagrams were constructed from peat/soil cores taken at different altitudes on Mt Hauhungatahi and Gibson's Swamp. Linear discriminant function analysis predicted the vegetation type represented by fossil pollen assemblages. Good differentiation was made between open (fernland and tussockland) and closed (dense scrub and forest) environments.The present tree-line on Hauhungatahi fluctuates between 1100-1250 m. Between c. 10,200-9000 BP the tree-line was between 800-1140 m and dominated by Phyllocladus aspleniifolius var. alpinus and Halocarpus (probably H. biformis). Montane forest in the region was dominted by Prumnopitys taxifolia. During the period c. 9000-7000 BP the tree-line advanced at least 290 m as a result of climatic change to less variable conditions. Fossil wood showed that this higher forest was comprised of mainly Halocarpus biformis, with some Phyllocladus aspleniifolius var. alpinus and Libocedrus bidwillii. In the Horopito-Ohakune area, Dacrydium cupressinum replaced Prumnopitys taxifolia as the forest dominant in the early Holocene, and tree-ferns and hardwood trees expanded, also suggesting change to less variable conditions. During the period c. 7000-1714 BP the tree-line on Hauhungatahi retreated c. 170 m indicating a return to more variable climatic conditions. Forest within broad depressions retreated further as a result of drowning out possibly due to the onset of colder, wetter winters. In the Horopito-Ohakune area Dacrydium cupressinum and tree-ferns declined, Prumnopitys taxifolia regained some of its former dominance, and hardwood species continued to expand, all suggesting a change to more variable conditions. Immediately following the Taupo Pumice eruption of c. 1714 BP Libocedrus bidwillii expanded in upper montane and sub-alpine forest on Hauhungatahi. Lidocedrus also increased at Gibson's Swamp, suggesting a regional response of this species where it was present in sufficient numbers to colonise new sites. By providing open sites, the eruption may have simply facilitated this expansion which may have already been underway as a result of more variable conditions during 3000-2000 BP. Age structures of present Libocedrus stands on Hauhungatahi were obtained from increment cores. These show mainly episodic regeneration, probably as a result of windthrow, and do not suggest synchronous regeneration failure in recent centuries as proposed for some South Island conifer stands. There is evidence of localised cohort establishment in the sub-alpine forest following forest destruction (most likely by wind-storm) probably during the period AD 1740-1750. Extensive forest destruction by fire commenced in the region c. 650 BP resulting in a general increase in Weinmannia racemosa in forests near Hauhungatahi. Weinmannia invaded forest on Hauhungatahi at the expense of Libocedrus in upper montane forest, and Prumnopitys spp. and Nestegis cunninghamii in lower montane forest. Areas of sub alpine vegetation on the upper slopes of Hauhungatahi were burnt after c. 450-400 BP. In the Horopito-Ohakune area logging for podocarps after AD 1850 resulted in an increase in the abundance of Weinmannia racemose. Disturbance, interacting with climatic change and endogenous factors, has been a major determinant of forest structure and composition at Hauhungatahi since the late glacial. The evidence supports an individualistic view of plant communities and non-equilibrium based theories of community composition.