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Narrow-leaved snow tussock Chionochloa rigida

Diagnostic features for identification in the field

Descriptions of narrow-leaved snow tussock are complicated by the fact that this species is now considered to form a complex with broad-leaved snow tussock (C. flavescens). There are five recognised taxa (including three broad-leaved ones which are described under C. flavescens), which vary in locality and habitat. However, the general description below will differentiate the narrow-leaved members of this complex from the other Chionochloa tussock species.

A large green or tawny green tussock, usually less than 150cm tall. Tussocks may be smaller with narrow leaves, where soil fertility is reduced or at higher altitudes.

Leaves are relatively broad, which droop. They are tough and glossy, and lack a conspicuous midrib above or beneath. The leaves tend to trail on the ground, so that the exposed soil shows a swept effect.

The sheaths are a lighter colour than other species, being pale green to light brown to orange, often purple tinted. Young sheaths are white-hairy, while dead sheaths break crosswise.

The flower heads are borne well above the leaves.

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

Description of community and habitat

Chionochloa rigida occurs as a complex with broad-leaved snow tussock (C. flavescens), with five taxa being recognised. The two narrow-leaved snow tussock taxa are discussed here. Narrow-leaved snow tussock is the most common grassland community to the east of the Main Divide, and south of Rakaia valley. In addition, one taxa occurs in Westland. For further information on the taxa of the C.flavescens-C. rigida complex, see Wardle (1991).

Narrow-leaved snow tussock has the widest altitudinal range and temperature tolerance of all the Chionochloa species.

East of the Main Divide, South Island
Narrow-leaved snow tussock grassland is the dominant vegetation cover at mid altitude on mountain slopes south of the Rakaia River, and the hill tops of Banks Peninsula. It's current distribution is a shadow of it's extent prior to European settlement. Tussock size is influenced by the harshness of the climate in which it grows. Where present near sea-level in south-east Otago, tussocks are up to 1.5m tall, but reduce to 0.3m tall at high altitudes.

The abundance and composition of species growing in narrow-leaved snow tussocks is influenced by the degree of disturbance that the grassland has experienced. Grasslands which have a dense canopy of narrow-leaved snow tussocks, naturally develop a thick layer of litter and humus, which prevent widespread establishment of intertussock grasses and herbs. By contrast, grasslands which have been opened up by grazing disturbance have a reduced tussock cover, with a greater cover and abundance of intertussock herbs and grasses. Open canopied grasslands are widespread today, especially at mid-altitude. Dense stands are more commonly found at higher altitudes or elsewhere where human impacts have been less.

In Otago and Canterbury, there are two broad types of C. rigida grassland. The higher altitude grassland has alpine fescue (Festuca matthewsii) and blue tussock (Poa colensoi) associated with it, and tends to be less modified than the lower altitude narrow-leaved snow tussock grassland type, but exotic species may be common, depending on the management history. At lower altitudes, fescue tussock (Festuca novae-zelandiae) is associated with the narrow-leaved snow tussock. This grassland type tends to be more modified as a result of pastoral development. Tussock cover can be open, and exotic pastoral species can dominate the intertussock spaces. In Canterbury mountain cotton daisy (Celmisia spectabilis) is a common associate, especially in more modified grasslands.

West of the Main Divide, South Island
A narrow-leaved snow tussock taxa is also found in Westland, where leached soils form over rolling gentle terrain in the lower penalpine zone.

North Island
Narrow-leaved snow tussock grasslands are not present in the North Island.

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

Other species present

 East of the Main Divide, South Island
Dense stands of narrow-leaved snow tussocks naturally provide little opportunity for other grasses and herbs to become established in the spaces between tussocks. However, the common tussock grassland species may be present in low numbers e.g. blue tussock (Poa colensoi), fescue tussock (Festuca novae-zelandiae or F. matthewsii), Viola cunninghamii, Wahlenbergia albomarginata.

The more open canopied narrow leaved tussock grasslands have a greater cover of intertussock species. Mat daisies (Raoulia subsericea) becomes increasingly common as the grassland is opened up. Other grasses which may be present include Deyeuxia avenoides, Elymus solandri, and the exotic brown top (Agrostis capillaris) and sweet vernal (Anthoxanthum oderatum). Mouse-eared hawkweed (Hieracium pilosella) is becoming increasingly common, especially in open canopied grasslands. The most common shrub associated with narrow-leaved tussock grassland is matagouri (Discaria toumatou) which is especially common where soil fertility has been increased through the aerial application of fertiliser. Native broom (Carmichaelia spp), and Leucopogon colensoi can be locally important.

Between the Rakaia River, and North of the Two Thumb Range/Hunter Hills area, C. rigida tussocks are associated with fescue tussock (Festuca novae-zelandiae) and mountain daisy (Celmisia spectabilis). Beyond the Two thumbs/Hunter Hills area, Celmisia spectabilis is no longer important, and fescue tussock is replaced by alpine fescue tussock (Festuca matthewsii) to the west, in South Canterbury, especially with increasing altitude. West of the Main Divide, South Island
The narrow-leaved snow tussock taxa found in Westland is associated with Dracophyllum longifolium and D. uniflorum scrub.

North Island
Narrow-leaved snow tussock grasslands do not occur in North Island.

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

Vegetation History

 Much of the mountainlands to the east of the Main Divide were covered in forests prior to 700 - 900 years ago. These were largely destroyed by natural and Polynesian fires prior to European settlement, leading to a migration of tussock species into previously forested areas. Initially, short tussock species are thought to have migrated from enclaves within the forest, upslope. This was followed by a slower migration of Chionochloa species (including C. rigida) naturally occupying alpine areas above the timberline, downslope into penalpine, subalpine and even montane environments. Subsequent modification of these tall tussock grasslands in subalpine and montane zones, through pastoral activities (grazing and burning) has lead to a transformation to short tussock grasslands.

Those narrow-leaved snow tussock grasslands found west of the Main Divide are less likely to have been modified as a result of browsing by feral animals, such as deer.

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





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