Biological flora of New Zealand 14: Metrosideros excelsa, pōhutukawa, New Zealand Christmas tree

We review the biology and ecology of Metrosideros excelsa (Myrtaceae), an endemic angiosperm evergreen tree. Metrosideros excelsa belongs to a conspicuous and widely distributed Pacific Basin genus, with centres of diversity in both New Zealand and New Caledonia. Metrosideros excelsa is an iconic tree species that forms a significant component of northern New Zealand's exposed coastal headland and cliff vegetation. Where conditions are more favourable, M. excelsa forms tall coastal forest, ranging from simple young high-density stands to diverse mature forest. Inland, M. excelsa stands are confined to the margins of lakes and rivers on the Central Volcanic Plateau, where some may originate from early Māori plantings. Metrosideros excelsa is reliant on stochastic disturbance events (e.g. landslides, volcanic eruptions) to create open sites necessary for regeneration. Mass flowering (December–January), followed by abundant production of wind-dispersed seed maximises chance colonisation of such sites. Since human settlement in New Zealand, the distribution of M. excelsa forest has declined by c. 90% and the southern limit of the species has retreated north. Natural regeneration on the mainland is limited by the infrequency of large-scale disturbances and increased anthropogenic and herbivore pressures. Consequently, M. excelsa forest has become rare and localised on the mainland; monitoring and active management are fundamental to the species' long-term conservation.