In a Tangle: the Debate over New Zealand’s Small Leaved Shrubs

For more than a hundred years, botanists have puzzled over a floristic mystery: Why does New Zealand have so many shrubs with tiny leaves and interlacing branches? This may seem a strange question – it’s not the first thing that comes to mind when thinking of the flora of New Zealand. The country is known for its wet Gondwanan rainforests dripping with ferns and mosses, and for its famous horticultural exports like the bold-leaved flax (harakeke, Phormium spp.) and cabbage tree (tī, Cordyline spp.). But another unique feature is a peculiar abundance of shrubs that form a zig-zagging maze of flexible, spineless stems bearing miniaturised leaves.

Grey scrub - diversity hidden in plain sight.

These plants are normally called divaricating, meaning ‘branching at wide angles’ which doesn’t describe all of them, but it stuck. Sometimes the interlacing isn’t so much due to angles but actually because, unlike most plants, the leading stem doesn’t produce the hormones that normally supress side shoots, and so a tangle ensues. The plants are spineless, with stems that are near impossible to snap, and the leaves are always extremely reduced when compared to their recent ancestors. Unlike most plants, they’re often leafier within the frame of the plant, and twiggier on the outside.

Divaricates inhabit a variety of habitats from forest understory to coastal banks. They are most abundant in the drier areas of the South Island where they form communities of soft mounding forms, called grey scrub, a name that’s both slightly demeaning but also accurate in its economy. When growing en masse the subtle shifts in tone of their fine foliage can make beautiful arrangements that are evocative of many things: billowing smoke, a soft-focus photograph, thunderstorms. Up close they transform into works of three-dimensional Euclidian geometry, with simple patterns repeating ad infinitum. Sometimes these patterns look artificially angular, at other times they curve gracefully like falling water. Often, before they grow dense, young seedlings have a bonsai-like quality to them, looking older than their years, as if tortured by the elements. Many species make profoundly beautiful (and reliable) plants for the garden – sadly, few are available outside of the country.

This bizarre syndrome seems to be quite unique to New Zealand, making up a whopping ten percent of the woody species of the country.”

The form evolved separately at least 18 times in unrelated groups of plants as diverse as the pea family, daisy family, coffee family, even a species of conifer does it. And each divaricating plant is closely related to normal plants with normal leaves and structure. The result is there are at least 60 unrelated plants that, in the words of one botanist, all “look similar enough to make you despair at ever putting names to them.” It’s as if the plants are attending some strange botanical costume party where the theme is ‘dress up as your favourite barbed wire entanglement’.

Sophora prostrata, of the pea family (Fabaceae). Image by Robert Champion
Cascading stems of Myrsine divaricata, one of the few divaricating trees that does not revert to original foliage. Image by Robert Champion

So, what’s the evolutionary problem that these unrelated plants have solved? And what is particular about New Zealand that has produced an epidemic of this form? Two hypotheses have been put forward.

The first is based around the moa – giant flightless birds that were chief herbivores before their extinction caused by humans. Unlike every other substantial land mass, New Zealand had no land mammals prior to humans, and so birds dominated its ecosystems in an unparalleled way, taking on many roles that would normally be filled by mammals (including those of grazing herbivores like deer, antelopes or kangaroos). Birds and mammals eat very differently. Birds generally strip plant material by clamping, pulling and snapping, rather than the cutting and grinding of teeth. Plus, a hard beak isn’t as bothered by common defences like thorns and rough foliage, unlike the soft fleshy mouth of a mammal. And thus, an entirely different set of defences evolved: tiny leaves protected within tangled, flexible branches made them inconvenient for toothless beaks to devour, especially when difficult to snap off. This provided an advantage over the plant’s original form. The largest moa could browse foliage to a height of just over three metres.

Interestingly, the trees that divaricate usually abandon their tangled juvenile form once reaching this height, reverting to their normal leaves and shape.”

The second theory is that divarication is an adaptation to the foul mix of climatic extremes that were brought by the recent ice ages of the Pleistocene. Prior to this, New Zealand was warm and populated by plants that were largely of tropical descent. These plants didn’t have the typical strategies to cope with these new conditions (such having needle leaves or dropping foliage in winter). Thus, the divaricating form is what some tropical plants do when they’re having a go in a climate that is anything but tropical. The twiggy stems sheltered the internal leaves from either the high winds, intense light, frost damage or drought (some scientists arguing the importance of certain factors over others).

Many people love the story of the moa driving plant evolution, buoyed by the thought of their ghosts still haunting the landscape, in spite of their extinction.”

Many scientists side with a form of the climatic theory, arguing that it’s better supported by the data, although the debate isn’t over yet. In 2007 biologists undertook a botanical survey of the only other large island that was dominated by grazing birds: Madagascar, whose elephant birds covered the island until human arrival. It turns out that, here too, there are many plants with divaricating characteristics, but these are absent in closely related plants of neighbouring South Africa.

In reality, as some authors suggest, it doesn’t have to be a case of either/or. Natural selection doesn’t require one single cause for the evolution of a trait (especially a set of traits as complex as this). For example, humans found many advantages in walking upright – carrying tools, spotting predators, increasing walking efficiency. But if the moa theory is true, then perhaps these plants are a glimpse of a parallel universe where birds are in charge instead of mammals, and every continent is a tangled sea of small-leaved shrubs.

Robert Champion is the founder of TARN, a landscape design firm based in Sydney. His love of plants developed early at two diametrically opposed locations: his grandparents’ park-like orchard, and a hut on a wind-beaten bluff within rugged New Zealand bush. He seeks to harmonise the emotional with the analytical. WEBSITE/ INSTAGRAM

Korokio (Corokia cotoneaster; front) and a divaricating Coprosma (rear). Image by Robert Champion
Prostrate kōwhai (Sophora prostrata). Image by Robert Champion