New Zealand has about 2,500 native vascular plant species (seed plants, ferns and lycophytes). About 500 of these belong exclusively to the high mountains, with another 100 at lower altitudes only under certain circumstances. This makes 600 alpine, subalpine or nival species, belonging to 45 families and about 120 genera. Another 350 or so species are found both in the lowlands and highlands, bringing the mountain flora to a total of nearly 1,000 species.
The origin of New Zealand’s high altitude plants is something of a mystery. The mountains are geologically young and geographically isolated. Most of the alpine species occur nowhere else, although there are affinities to plant life in Australia, New Guinea and South America.
The movements that produced today’s uplifted land began only a few million years ago. Before that, for tens of millions of years, the New Zealand region was low-lying and warm. There were mountains earlier still, in early Cretaceous times more than 100 million years ago, before New Zealand split from the supercontinent Gondwana. But these mountains had been worn away by late Cretaceous and early Tertiary times. A Cretaceous mountain flora could not have survived such a long period of warmth and low relief. From what, then, has New Zealand’s unique modern mountain flora developed? There are several possibilities, probably all of which played a part.
One possibility is that a few cool-climate plants survived on infertile upland soils. Some of the Cretaceous mountain flora may have been able to adapt to warmth and low altitude, and then expand into the new mountain habitats of the Pleistocene period. Even if the Cretaceous mountain vegetation was almost extinguished, lowland plants may have given rise to innumerable new species when mountain habitats became unoccupied. Certainly many high altitude plants have close relatives in the lowlands. But others do not. Hectorella caespitosa, for example, a high-altitude cushion plant, is never found below the alpine zone and its only known relative is found in the Kerguelen Islands in the subantarctic. For Hectorella at least, we must assume it survived through the warm period of the Tertiary on cooler lands further south.
Another possibility is that ancestral forms have reached New Zealand by long-distance dispersal across the sea, mostly from or via Australia. A few migrant species could have evolved explosively to fill unoccupied niches on the rising mountains. Willowherbs (genus Epilobium) are an example. Its worldwide distribution and taxonomy means it could not have originated in New Zealand, but there are a large number of native species there. Many of these are mountain plants, but all are so closely related that it seems certain they evolved from a single ancestor in the geologically recent past.