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Mid-ribbed snow tussock
Chionochloa pallens

Diagnostic features for identification in the field

A large green or tawny green tussock, usually less than 1m tall, but can range from 60cm to 2m high.

Leaves are soft and flat, with an obvious yellow midrib beneath.

The sheaths at the base of groups of leaves are pale-brown to purple, and hairless when young. Dead sheaths break cleanly, leaving truncated bases.

The flower heads are borne well above the leaves.

References:
  • Mark, A.F. and Adams, N. 1996: Alpine plants of New Zealand. Christchurch
  • Wilson, H.W. 1996: Wild plants of Mount Cook National Park, Manuka Press, Christchurch.

Description of community and habitat

Mid-ribbed snow tussock is the most widely distributed of all the Chionochloa species. It is commonly found in the penalpine zone (from 1100m), though they can extend up to elevations of 1800m. They occur at moist, well drained sites with weakly weathered colluvium, especially along and west of the Main Divide in the South Island, and on the axial ranges between the Manawatu Gorge and East Cape in the North Island, including Mt Hikurangi and Mt Egmont. These grasslands are less common on the eastern rainshadow ranges, and the granite mountains.

East of the Main Divide, South Island
Mid-ribbed snow tussock grasslands are restricted to the wetter ranges close to the Main Divide, where they occupy weakly weathered well drained soils at high altitude. In North Canterbury, Marlborough and Kaikoura Ranges, they are limited to weakly weathered soils on cool moist penalpine slopes.

These grasslands are found at high altitude on the Richmond Range, in association with C. australis, and on rocky well drained soils on Mt Stokes, where it merges with Olearia colensoi shrubs. In poorly draining areas, it hybridizes with North Island C. flavescens.

West of the Main Divide, South Island
In the west Nelson Mountains, Chionochloa pallens occurs with C. australis on well drained soils over marble and schist. This is the dominant grassland above 1500m where marble occurs. Here, stable soils have devveloped where calcium is leached as soon as it is released from the slowly weathering rock fragments. On the lower penalpine slopes, mid-ribbed snow tussocks occur with blue tussock (Poa colensoi) rather than C. australis.

Mid-ribbed snow tussocks also occur with broad-leaved snow tussocks (C. flavescens) on moderately steep slopes on marble and tertiary calcareous rocks, over a wide altitudinal range. These recent soils have good drainage and haven't been leached.

In Westland, mid-ribbed snow tussock grassland occur between 950 - 1550m a.s.l. on south facing, weakly weathered well-drained debris slopes . Tussock cover reduces significantly at exposed sites, where soils are shallow or stony, or where snow lies for longer. C. pallens can also extend down to 750m on fine textured recent soils on moraine alluvium and debris. However these areas have often been modified by grazing, resulting in the depletion of C. pallens. Avalanche grass (Poa cockayneana) or thousand island fern (Hypolepis millefolium) become the dominant species.

In Fjordland, mid-ribbed snow tussock occurs with C. oreophila on recent soils on debris fans and terraces.

North Island
C. pallens is found on recent or moving soil on steep slopes of the Tararua Range. On the northern axial ranges between the Manawatu Gorge and East Cape, these grasslands occur on well drained slopes from the forest limit to summits, except where fire, grazing or erosion have destroyed it. On broken terrain, this grassland merges into herbfields and low scrub vegetation.

Reference:Wardle, P. 1991. Vegetation of New Zealand, Cambridge Press, Cambridge

Other species present

East of the Main Divide, South Island
Mid-ribbed snow tussock is restricted to the wetter ranges close to the Main Divide, where similar species are found as those in Westland. At Mt Stokes, in the Marlborough Sounds-Nelson area, Olearia colensoi is associated with C. pallens.

West of the Main Divide, South Island
In the mountains of West Nelson, mid-ribbed snow tussocks can form an association with C. australis, which is replaced by blue tussock on steep lower-penalpine slopes. On moderately steep slopes over marble and tertiary calcareous rocks, it occurs with C.'robust', with avalanche grass (Poa cockayneana), Aciphylla ferox, Helichrysum bellidioides and Ranunculus insignis.

In Westland, between 950 and 1550m, openings within C. pallens grassland are occupied by Mount Cook lily (Ranunculus lyallii), Pratia angulata, Coprosma cheesemanii, and Aciphylla crenulata. On less favourable sites above 1150m, tussock cover is reduced to 10 -15%, with Microleana colensoi, Pentachondra pumila, Coprosma perpusilla, Celmisia verbascifolia, C. verpertina, and C.sessiliflorum characteristically occupying the openings. Below 750m, where grazing has depleted C. pallens, avalanche grass and thousand island fern (Hypolepis millefolium) become dominant, with alpine fescue tussock (Festuca matthewsii), Rytidosperma setifolium, Aceana anserinifolia, Celmisia sessilflorum, C. walkeri, Raoulia glabra and Helichrysum bellidioides present.

Chionochloa oreophila occurs with C. pallens on recent soils in Fjordland. Other species include Uncinia divaricata, Bulbinella gibbsii var. balanifera, Epilobium alsinoides var. atriplicifolium, Leptinella squallida, Luzula rufa and Oreomyrrhis colensoi.

North Island
C. pallens grows mainly at the higher altitudes of the Tararua Range, where it tends to co-dominate with Astelia nervosa, and Acipylla colenoi. Other characteristic species include Pratia angulata, Bulbinella gibbsii, Dracophyllum uniflorum, Uncinia ceaspitosa, prickly shield fern (Polystichum vestitum), and Coprosma depressa. On the northern axial ranges between manawatu gorge and East Cape, C. pallens grasslands occupy well-drained slopes between the forest limit and the summits. On broken terrain, or sites distrubed by fire, grazing or erosion, C. pallens has been replaced by herbfield or low scrub. Associated species are similar to those found on the Tararua Range. Dracophyllum recurvum replaces D. uniflorum.

Reference:Wardle, P. 1991. Vegetation of New Zealand, Cambridge Press, Cambridge

Vegetation History

These alpine grasslands are likely to have occupied similar habitats in the past as they do today. Because they occur in the higher rainfall areas, and at high altitudes, they have largely escaped the impacts of pastoral activities that have shaped the current distribution and condition of some other snow tussock grassland species. Mid-ribbed snow tussock grasslands in the western ranges of Otago and Canterbury, would at one time, have been used for extensive sheep grazing, but most of these areas have now been retired from grazing. The palatability of C. pallens (and associated herbs, including Aciphylla takahea, Ranunculus lyallii and Astelia petriei) makes it especially prone to depletion by grazing animals such as feral deer, chamois, thar and hares. Avalanche grass (Poa cockayneana) can become dominant where C. pallens has been grazed out.

Reference:Wardle, P. 1991. Vegetation of New Zealand, Cambridge Press, Cambridge





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