Serpentine Vegetation

What is generally known as serpentine vegetation occurs in localised areas throughout the world on soils derived from ultramafic rocks. These soils provide a difficult medium for plants, as they are highly infertile with low levels of potassium, phosphorus and calcium and often high concentrations of iron and magnesium and the toxic metals nickel and chrome.

As a result, in forested areas serpentine vegetation often has a very different appearance from the vegetation adjacent to or even surrounding it (Fig. 89). The serpentine vegetation is more open with fewer, somewhat stunted trees and more shrubs and herbs. Some of the species on serpentine, although often reduced in stature, are shared with non-serpentine forests, shrublands and grasslands; others are unique and are known as serpentine endemics.

In the South Island there is a belt of ultramafic rock, known as the 'mineral belt', which runs through D'Urville Island, then to the east of Nelson and ends south of the Wairau River.¹²⁷ A similar belt is found far to the south at the northern fringes of Fiordland. The two belts are considered to have been continuous at one time until separated by page 160lateral movement along the alpine fault.

In the North Island the only occurrences of ultramafic rock are at the northern tip along a few kilometres of the Surville Cliffs near North Cape and at the tip of the Peninsula a little to the south.

The serpentine flora at the Surville Cliffs has been investigated recently and 144 species have been recorded.¹²⁸ Two unnamed shrub species of Hebe and Coprosma are considered to be endemic as well as 10 varieties, mostly of shrubs. There are patches of low forest on the cliffs comprising common local species of which pohutukawa (Metrosideros excelsa) is the dominant. Elsewhere the cover consists mainly of low shrubs draped with the string-like stems of the parasitic Cassytha paniculata. The only conspicuous herbs are a toetoe (Cortaderia splendens), Astelia banksii and Phormium tenax.

A curious feature of a number of the shrubs at this locality is their 'semi-lianoid' habit. Their 'elongated stems do not climb, but scramble or trail down the cliffs through other plants'. Most of the shrubs exhibiting this habit are considered to be varieties of wider ranging species.