Old tephra (>500 years) plains (= frost flats)
Old tephra plains form low-lying flats on the North Island’s Volcanic Plateau, created after vast quantities of pumice were washed off surrounding hills and filled basins and valleys after the monumental eruptions that shaped the plateau. These sites are at relatively high elevation (400-800 m a.s.l.), so are foci for cold-air ponding and suffer from year-round frosts; consequently they are commonly referred to as frost flats. The term ‘frost flat’ is sometimes also used to describe frosty, often strongly leached alluvial terraces and plains in the South Island (e.g. ‘Wilderness’ ecosystems), but these systems should not be confused with old tephra plains.The rhyolite pumice comprising old tephra plains is naturally infertile. Fire has also been important in shaping and maintaining the vegetation, which is typically dominated by monoao (Dracophyllum subulatum). Other diagnostic species for this ecosystem are coral lichen (Cladia retipora), Cladonia confusa, Deyeuxia avenoides, catsear (Hypochaeris radicata), silver tussock (Poa cita), woolly moss (Racomitrium lanuginosum), and Rytidosperma gracile (Smale 1990). Bog pine (Halocarpus bidwillii) rarely occurs, but remains in sites long protected from fire, and along with mountain toatoa (Phyllocladus alpinus) may have once been characteristic of this ecosystem.
Where do they occur?
Some of the best remaining examples are the heathlands at Rangitaiki and Otangimoana Conservation Areas on the southern Kaingaroa Plateau. Heathlands in west Taupo occur at Whenuakura Plain, Waihaha Ecological Area and Waipapa Ecological Area, Pureora Forest Park, and in Waituhi-Kuratau Scenic Reserve.
Notable flora and fauna
Threatened plants include the nationally vulnerable tent pole tree (Pittosporum turneri) (occurs in late successional systems) and Pimelea tomentosa. Threatened bryophytes include the naturally uncommon Crosbya nervosa and the sparse Pseudocephalozia lepidozioides.
There is an undescribed species of the mushroom Cystoderma from the Rangitaiki (PDD92285, Col. J.A. Cooper, 18/5/20111), which phylogenetic analysis places very close to Cystoderma andinum described from a páramo alpine tundra ecosystem of Ecuador.
Rare invertebrates include the Lepidoptera Ericodesma aerodana and the northern pimelea cutworm moth (Meterana pictula), which have Pimelea species as hosts, and Meterana grandiosa, whose larvae feed on small-leaved Olearia species (Patrick & Dugdale 2000).
Threat statusCritically endangered (Holdaway et al. 2012)
Old tephra plains are vulnerable to invasion by exotic weeds adapted to low soil nutrients such as the N-fixers gorse (Ulex europaeus) and Scotch broom (Cytisus scoparius), heather (Calluna vulgaris), pines (Pinus species), and species of pasture grass such as browntop (Agrostis capillaris) and herbs, e.g., mouse-ear hawkweed (Pilosella officinarum), in nutrient-enriched areas (e.g., around gull colonies; Smale et al. 2011). Adjacent agriculture and plantation forestry can also result in nutrient pulses that increase vulnerability to weed invasion. Rabbits and hares may be locally abundant. Where accessible, 4WD vehicles can be especially damaging to the low, scattered vegetation.
Patrick BH, Dugdale JS 2000. Conservation Status of the New Zealand Lepidoptera. Science For Conservation 136. Wellington, Department of Conservation. 33 p.
Smale MC 1990. Ecology of Dracophyllum subulatum-dominant heathland on frost flats at Rangitaiki and north Pureora, central North Island, New Zealand. New Zealand Journal of Botany 28: 225-248.
Smale MC, Fitzgerald N 2007. The frost flats of Rangitaiki. New Zealand Geographic. May issue.
Smale MC, Fitzgerald NB, Bartlam S 2013. Monitoring condition of frost flat heathlands, a critically threatened rare ecosystem in the Waikato region. Waikato Regional Council Technical Report 2013/56. 36 p.
Smale MC, Fitzgerald NB, S J Richardson 2011. Resilience to fire of Dracophyllum subulatum (Ericaceae) frost flat heathland, a rare ecosystem in central North Island, New Zealand. New Zealand Journal of Botany 49: 231-241.
Yeates GW, Schipper LA, Smale MC 2004. Site condition, fertility gradients and soil biological activity in a New Zealand frost-flat heathland. Pedobiologia 48: 129-137.
Mycological Notes 1 - Frost-Flat Fungi (Fungal Network of New Zealand)