The Metrosideros genus contains around 50 species and is one of the most widely distributed plants in the Pacific.
The subgenus Mearnsia contains the shrubs, creepers and climbers and includes New Zealand species such as White and Scarlet Rata. It is generally only found on fragments of the old Gondwanan landmass: New Zealand, New Caledonia and New Guinea.
The subgenus Metrosideros contains trees such as New Zealand’s Pōhutukawa, Northern and Southern Rata. They are supremely adapted to dispersing long distances, producing thousands upon thousands of tiny seeds that can be held aloft on the slightest breeze. Not only that, the seeds are incredibly hardy and can survive up to a month of immersion in salt water and temperatures well below freezing.
New Zealand’s Southern Rata (Metrosideros umbellata) is found at the base of the Metrosideros phylogenetic tree, giving support to the theory that New Zealand may have been the point of origin for Metrosideros trees. Over millions of years, sporadic waves of dispersal spread the genus out of New Zealand and across the Pacific. Today it has an extensive range including sub-antarctic islands, the Bonin islands near Japan, and the Hawaiian islands – one of the most remote island groups in the world.
Explaining the dispersal of Metrosideros to Hawaii was a puzzle to researchers who had to account for its travel across the “doldrums” of the equator. Wind patterns here are separate from the northern and southern hemispheres and can at times remain dead calm. One potential explanation is that Metrosideros seeds first made their way from New Zealand to the Marquesas islands, just south of the equator. Using the island as a stepping stone, the seeds were picked up by high atmospheric winds and carried over the equator to Hawaii.
Another feature of Metrosideros trees making them supreme dispersers is a range of adaptations to a pioneering lifestyle. This means that once they arrive at an island they can establish themselves in harsh, hostile environments where other plants couldn’t survive. In Hawaii, Metrosideros polymorpha is the most common native tree and is the first species to grow on lava flows that pour out of the many active volcanoes. It can tolerate an incredibly broad range of environments and is highly variable, ranging from a stunted shrub-like tree in poor conditions, to a forest tree 20 – 25m tall when conditions are more favourable. In New Zealand, Pōhutukawa colonises rocky coastal areas and cliff faces, and provides a habitat for less tolerant species.
Photos courtesy of Edin Whitehead