Several plant groups which are herbaceous or small shrubs in the northern hemisphere have evolved into trees in New Zealand. These include tree-sized daisies, lilies, fuchsia and veronica (hebe). New Zealand also has giant buttercups, and forget-me-nots 1.5 metres tall. On offshore islands, too, there is a tendency for gigantism – megaherbs (large-leaved flowering plants) are prominent on the subantarctic islands. Many have smaller alpine relatives on mainland New Zealand.
Many New Zealand trees can live to a great age – kauri to 1,700 years, miro to 1,400, rimu to 1,200 and rātā to 1,100. Perching astelia lilies can live at least 60 years.
Most flowering plants on earth are hermaphrodites – male and female sex organs appear on the same flower. In the New Zealand bush, however, many species have single-sex flowers. These may be on the same plant (for example, kauri and kawakawa), or may be found as separate male and female plants (for example, most conifers, coprosmas, astelia and clematis).
Plants that have evolved from two species of the same genus are more common than in many other lands. This ability to hybridise freely means that it is easier for new species to arise.
New Zealand has over 50 species of small-leaved shrubs and low-growing trees with densely interlaced wiry, highly tensile stems. Among them are conifers, daisies, myrtles, brooms, pittosporums, and coprosmas. Collectively known as ‘divaricating shrubs’, their branches are spread apart at a wide angle.
While this feature is found elsewhere, it is nowhere as prominent as in New Zealand, where it occurs in around 10% of woody plants and has evolved independently in 18 plant families.
Theories about divaricating plants
Some claim that these small-leaved plants with tangled stems evolved in this way to avoid being eaten by moa. In experiments, when offered these plants, emus and ostriches got so little nourishment that they would have died of starvation. Was divarication an adaptation by the plant to deter grazing birds?
This theory is challenged by others who think that divaricating plants evolved in response to climate. They argue that divarication is an adaptation to a dry, windy and frosty climate, either recently, or during past glaciations. Tangled branches may serve as both a windbreak and frost screen, and reduce water loss from the plant.
Many New Zealand trees have different leaves as saplings and as adults. Ribbonwood, mataī, lancewood, kaikomako, some hebes, certain species of kōwhai, and several other common trees and shrubs have small or fibrous leaves as saplings. But once grown to 2–3 metres (the height to which moa could reach), they develop larger leaves. Some botanists suggest this growth pattern evolved to prevent moa from eating the saplings.
Evergreen, red and brown leaves
New Zealand also has very few annual or deciduous plants, perhaps because of the lack of a strongly seasonal climate. Most plants are evergreen, and their leaves some shade of green, but there are curious exceptions.
Some young bush plants are dark, with brownish leaves that merge with their shadowy backgrounds, making them difficult to see. Examples of seedlings with brown leaves are lancewoods, young kauri and some sapling kahikatea and mataī. Several forest and alpine plants are so devoid of leaves that they appear to be dead.
The mountain horopito often has red and yellow blotches on its leaves. In some cases whole hillsides can appear reddish at the higher altitudes where this plant lives. Research suggests red leaves may be a protective measure against harsh ultraviolet radiation.