|
Silver tussock
Poa cita
Diagnostic features for identification in the field
- A dense shiny fawn green (dead leaves are mixed in with fresh green ones) tussock -30 to 70cm tall
- Leaves tightly folded, leathery, smooth above, fringed with hairs along the margins below; tip sharp.
- Leaf sheath creamy brown, shiny, with rough margin.
- Ligule is leathery, very short and even.
- Flower heads (30-100cm) do not stand proud of leaves. Panicle is open, slender, rough, with twisted branches. Awns absent.
References:
- Jane, G. 1995. Field Guide to Grasses. A. Alpine Grasses
- Wilson, H.J. 1996: Wild plants of Mount Cook National Park, Manuka Press, Christchurch
Description of community and habitat
Silver tussocks are found from the coast in central and southern new Zealand, to the penalpine zone. Although widespread in a range of grassland and shrubland communities, it seldom dominate the vegetation.
Silver tussocks prefer a high fertility environment. They are therefore associated with three classes of habitat:
- Where soils are naturally fertile, coarse textured and friable, e.g. recent terraces, dry north facing slopes in semi arid areas, and in river beds
- In hollows which act as nutrient flush zones, both in semi arid and wet climates
- On hillslopes and older terraces where there has been soil disturbance and top dressing with fertiliser
In the semiarid parts of Central Otago, remnants of pre-European short tussock grasslands which once occupied the dry terraces, valley floors and low hills can still be found. In the Maniototo Ecological District silver tussock grasslands occur on low to mid altitude sites between 750 to 900m, on gentle free draining hill slopes and low lying flat ground. The cover of silver tussocks in these grassland is variable, and is associated with small shrubs, blue tussock, fescue tussock, and pasture herbs and grasses. C. rigida and C. rubra subsp. cuprea tussocks can also be present. Much of this vegetation type has been converted to pasture by oversowing and topdressing (Grove, 1994).
Silver tussocks are commonly found in moister areas, where low to mid altitude short tussock grasslands have been oversown and topdressed. While silver tussocks may dominate these grasslands, there are usually few other native species present. Silver tussock appears to thrive in competition with the oversown pastoral species, where other native plants fail. Matagouri (Discaria toumatou) is often associated with this community.
This information was taken from Wardle, P.; 1991. Vegetation of New Zealand, Cambridge Press, Cambridge.
Other species present
Silver tussocks can form a canopy with shrubs including native matagouri (Discaria toumatou), broom (Carmichaelia sp.) and Olearia bullata, and the exotic sweet briar (Rosa rubiginosa). Other native species may include blue tussock (poa colensoi), fescue tussock (Festuca novae-zelandiae), narrow-leaved snow tussock (C. rigida), copper tussock (C. rubra subsp. cuprea), blue wheat grass (Elymus solandri), plume grass (Dichelache crinita) and abundant exotic pastoral grasses and herbs.
This information was taken from Wardle, P.; 1991. Vegetation of New Zealand, Cambridge Press, Cambridge.
Vegetation History
Induced silver tussock grasslands
Much of the high country was covered in beech forest until about 700 to 900 years ago, whilst the moister parts of inland basins such as the Mackenzie Basin and Central Otago lost it's podocarp and beech forest cover around 1000 years and 2000 ago respectively. Even the driest parts of the South Island would have supported a low conifer-broadleaf forest 3000 years before present (McGlone,1989). However, deforestation by natural fires occurred around 2500y.b.p. and a change to a cooler and drier climate prevented it's regeneration. It is therefore likely that when Maori first came to inland Otago, the landscape was already substantially deforested and covered in low scrub and grassland.
Silver tussocks were presumably present in this mixed shrubland-grassland community, and was able to migrate upslope during the period of deforestation. At the same time, tall tussock species were migrating downslope from their alpine environments, replacing the short tussock grasslands.
Since European settlement, these tall tussock grasslands have been repeatedly burnt and grazed, often with the aim to reduce the cover of the tall tussocks in favour of the more palatable short tussock species. More recently, many of these short tussock grasslands have been the focus of pastoral development programmes, which has increased the soil fertility and introduced exotic pastoral species. Where present, silver tussocks have responded well to the boost in soil fertility, and can become the dominant tussock present, as other native species fail to compete with the more vigorous exotic species.
This information was taken from Wardle, P.; 1991. Vegetation of New Zealand, Cambridge Press, Cambridge.
Dryland silver tussock grasslands
Practically all the vegetation of lowland intermontane basins of Central Otago would have been modified since the arrival of man approximately 1000 years ago. Evidence from buried wood, charcoal and pollen samples suggest that the Maniototo District and adjacent mountain slopes and uplands below the climatic tree line were forested ( Molloy et al. 1963) except on some large areas of saline soils. McGlone (1989) has decribed the vegetation of the very driest districts in the South Island that existed 3000 years before the present as a low conifer-broadleaf forest. Toatoa (Phyllocladus alpinus), Hall's totara (Podocarpus hallii), broadleaf (Griselinia littoralis), kowhai (Sophora microphylla), kanuka (Kunzea ericoides and Myrsine australis made up this distinctive forest type.
Buried charcoal and pollen analysis of sites in Central Otago indicate that deforestation by natural fires began at least 2500 y.b.p., with a change to a cooler and drier climate preventing regeneration. Such natural deforestation was patchy rather than extensive throughout the region, and confined to the drier valley floors and lower slopes of areas such as the Maniototo and adjacent ecological districts (McGlone 1989), where the annual rainfall was below 500mm, or on NW slopes in areas receiving up to 800mm annual rainfall (Wardle 1991). Thus the climax vegetation of the dry lowland basins, at the time when the first Polynesians arrived, would have been low scrub and grassland (short tussocks and sward grasses).
These grasslands were further modified by early pastoralists, who burnt the vegetation to improve accessibility, and grazed large numbers of sheep. The introduction of the rabbit, whose numbers reached plague proportions, further exacerbated the problem of grassland depletion. In the 1970's, Land Development Programmes encouraged the oversowing and top dressing of tussock grasslands to improve pastoral productivity, resulting in an increase in exotic plant species. Other exotic plants (e.g. wilding trees, broom, gorse) have spread into dryland short tussock grassland areas.
Many areas which once supported dryland short tussock grasslands have been the focus of large-scale irrigation schemes. Such areas have been transformed to improved pasture and orchards.
Few areas of dryland short tussock grassland are in existence today.
| |
Site Contents