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Ultrabasic boulderfields (centre) near Junction Hill, Gorge River, South Westland, with red tussock ([Chionochloa rubra]) and pink pine ([Halocarpus biformis]) (Rowan Buxton)

Ultrabasic boulderfields (centre) near Junction Hill, Gorge River, South Westland, with red tussock ([Chionochloa rubra]) and pink pine ([Halocarpus biformis]) (Rowan Buxton)

Boulderfields occur where deposits of large (> 0.25 m) fragments of fallen detritus accumulate on slopes. Ultrabasic rocks contain very little quartz or feldspar and are composed essentially of ferromagnesium silicates, metal oxides, and native metals (Anon. 1962) that weather to soils with low concentrations of major nutrients and high concentrations of toxic metals such as nickel, chromium, and cobalt. Where there is very little interstitial fine material, boulderfields may be colonised only very slowly by lichens and mosses. Where the underlying rooting medium is suitable, woody plants and large herbs such as flax (Phormium spp.) may grow between the boulders. Pockets of fines can provide shaded, mesic habitats that allow herbaceous plants to persist. With time, boulderfields may be invaded from the margins by mat-forming plants and also by others spreading vegetatively. Boulderfields provide fire breaks that allow remnants of vegetation to persist in otherwise disturbed landscapes.

Below treeline vegetation includes scattered trees of mountain beech (Fuscospora cliffortioides) and several small podocarps such as yellow-silver pine (Lepidothamnus intermedius). Above timberline small-leaved shrubs (Coprosma spp., Dracophyllum spp., Leptospermum scoparium), and tussocks (Gahnia procera, Chionochloa defracta) are important, with a few herbs (e.g. Montia australasica and Notothlaspi spp.).

Synonyms

Ultramafic boulderfields, serpentine boulderfields

Notable flora and fauna

Rare and threatened plants include nationally critical Red Hills forget-me-not (Myosotis laeta) and Veronica rigidula var. sulcata, and nationally endangered Pittosporum serpentinum. Naturally uncommon species include Carex berggrenii, C. devia, North Cape sedge (Carex ophiolithica), Travers sedge (Carex traversii), Red Hills snow tussock (Chionochloa defracta), Veronica urvilleana, Red Hills button daisy (Leptinella pyrethrifolia var. linearifolia), Monroe’s forget-me-not (Myosotis monroi), Montia racemosa, Surville Cliffs haloragis (Haloragis erecta subsp. cartilaginea), Speden's mountain daisy (Celmisia spedenii), Ultramafic cushion poa (Poa acicularifolia subsp. ophitalis) and Pimelea suteri. Range-restricted species include Pittosporum pimeleoides subsp. majus, and Colobanthus aff. wallii (AK232551) “serpentine”.

Threat status

Not threatened (Holdaway et al. 2012)

Threats

Boulderfields exclude the majority of weed species but Pinus spp. invade the Red Hills Ridge, Richmond Range, Nelson-Marlborough. Deer and possums are widespread while goats and pigs are locally common in Nelson-Marlborough. Historically, some areas of serpentine (a type of ultrabasic rock) were mined (e.g. Cobb Valley asbestos mine), but there are no mines currently in operation. Many ultrabasic areas have been burnt and they are still susceptible to fire, especially in drier interior sites of Nelson-Marlborough.

Where do they occur?

Landscapes formed from ultrabasic rocks are found in Northland, Nelson-Marlborough, and western Otago-Southland. Areas of boulderfields are very limited in extent (Lee 1992), e.g. Red Hills, Marlborough, and Red Mountain, north-west Otago.

Further reading

Anon 1962. Dictionary of geological terms. National Academy of Sciences for the American Geological Institute, New York, Doubleday.

Dawson JW 1988. Serpentine Vegetation in Forest Vines to Snow Tussocks: The Story of New Zealand Plants . Victoria University Press, Wellington.

Lee WG 1992. New Zealand ultramafics . In: Roberts BA, Proctor J eds The ecology of areas with serpentinized rocks. A world view. The Netherlands, Kluwer. Pp. 375-418.

Wardle P. 1991. Vegetation of New Zealand . Cambridge University Press. Pp. 375-396.