Question: Following a volcanic eruption of ca. 232 AD, known as the Taupo eruption, the emergent conifer Libocedrus bidwillii expanded on Mt. Hauhungatahi, upwards above the current tree-line, and downwards into the mixed montane forest. We ask: (1) if current age-structures at different altitudes support the patterns predicted by the temporal stand replacement model, with cohort senescence and progressively depleting recruitment at ca. 600 year intervals (average cohort age) since the eruption: and (2) if the case history of the population sheds light on the persistence of mixed conifer-hardwood forests in general.

Location: Mt. Hauhungatahi, Tongariro National Park, New Zealand.

Methods: The species composition and structure of seven stands covering the altitudinal range of Libocedrus bidwillii, were quantified. Libocedrus trees were cored, and regression equations used to predict ages. Cohorts were identified.

Results: Libocedrus densities and basal areas, and the abundance of seedlings and saplings, peaked at different altitudes. At the species' lower limits there has been no recruitment for ca. 550 years, and the angiosperm Weinmannia racemosa has gained dominance. In the tree line and sub-alpine forest stands, a low level of continuous regeneration has been boosted by periodic cohort recruitment following exogenous disturbances.

Conclusions: In the montane zone, the Libocedrus age structure, and its replacement by Weinmannia, are consistent with a model of depleting cohorts separated by ca. 600 years since the Taupo eruption. At higher altitudes more frequent disturbances and reduced competition have allowed Libocedrus persistence. Comparison with other studies suggests long-term relationships between gymnosperms and angiosperms are mediated by the scale and frequency of disturbance.

Nomenclature: Indigenous gymnosperms and dicotyledons: Allan (1961) except as modified in Connor & Edgar (1987); monocots: Moore & Edgar (1970); pteridophytes: Brownsey & Smith-Dodsworth (1989).