Divaricating TheoriesThe puzzle of our small-leaved shrubs, moa and the ice ages

Author: Warren Brewer

Visitors sometimes comment on the marked presence of small-leaved, twiggy shrubs in our forest. Our flora contains a high proportion of these small woody shrubs which have a tight interlocking branching pattern. They are called “divaricating shrubs”. They have small leaves and tough wiry stems with a dense meshwork of branches with multiple growing points inside the shrub. They often have a brownish colour, giving an almost dead-looking appearance.

Some of our trees also pass through a divaricating juvenile stage, changing to a less tangled normal growth above 3m height. These plants all lack sharp spines. Collectively they are a special New Zealand feature. Our flora contains over 50 species of these divaricates (about 10% of all our woody plants). How has this growth pattern come about?

Two theories have been postulated:

1. Browsing Moa:

This huge two-legged bird came to occupy the role of a four-legged grazing mammal in New Zealand. It would have spent many hours picking leaves and twigs off low trees and shrubs. Moa favoured fertile river flats, forest edges and wetlands rather than deep forests. Highly palatable and nutritious shrubs growing in this habitat would have suffered extensive damage from these birds. They, therefore, had to evolve a mechanism to survive – hence their divaricating nature. Small leaves and a brownish colour would have made them look less attractive and dense interlocking branches would have deterred browsing. The lack of thorns and spines can be explained by the fact that whereas they would be effective at repelling the sensitive lips and tongue of a browsing mammal this would not work when dealing with the strong horny beak of the moa. Its beak would have acted like secateurs nipping off thorns etc which would have been easily crushed by the stones stored in its gizzard. Those trees with a divaricating juvenile form would be able to change to the adult stage above 3m height which would be out of reach of the largest browsing moa.

Left image: South Island Giant Moa, Dinornis robustus (foreground) and Pachyornis elephantopus (background)Credit: Joseph Smit

2. Climate Change:

The last ice age (Pleistocene) circa 2 million years ago caused world climates to go through a series of fluctuations with long cold glacial spells interrupted by warmer interglacial. It was a highly significant time for the final moulding of New Zealand’s present flora. Our vegetation lost the last traces of its Australian look (known that eucalypts, proteas etc once grew here from the evidence of fossil pollen deposits). The small-leaved divaricating form would be well designed to adapt to the cold, windy arid climate prevailing. This theory suggests that the growth form would act as a defence against wind abrasion and frost damage as the whole plant may have acted as a heat trap keeping delicate growing points within a protective shield. Small leaf size would have led to less moisture loss through evaporation thus counteracting the arid conditions.

Botanists continue to argue these two theories. Perhaps they both contain merit and actually reinforce each other as the need to grow this way would have been even greater if it helped plants to survive both adverse climate change and excessive moa browsing.

Left image: Twiggy coprosma with ripe black fruit, photo credit Warren BrewerRight image: Muehlenbeckia, photo credit Kay Milton