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Information on Dwarf Grasslands

Developing management options
The majority of tussock grassland research has focussed on determining the vegetation history of the mountain lands, and how pastoral management practices have influenced vegetation succession patterns. Only recently has grassland research started to look at the issue of progressive successional changes occurring on retired land (i.e. what vegetation changes will take place when tussock grasslands are retired from grazing and burning). The majority of studies have focussed on determining the impacts of pastoral use on induced montane fescue tussock grasslands (Festuca novae-zealandiae), the most widespread short tussock grassland found to the east of the Main Divide. Few studies have focussed on dryland dwarf grassland communities. There are therefore still many information gaps to be filled, some of which are currently addressed in the FRST-funded research programme "Improved Management Systems for Tussock Grasslands". For more information about this research programme and how to become involved, contact the programme leader Dr Ockie Bosch at boscho@landcare.cri.nz.

In the meantime, the suggested management options have to draw on broad principles and "best guesses" by ecologists, or observations made by people who live and work in the tussock grasslands. Such "best guesses" may be appropriate in some situations, but not others. They will be updated as and when more information becomes available.

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Factors influencing the distribution and characteristics of tussock grasslands

for more information on environmental gradients that determine the distribution of tussock grassland types, or the management factors that have influenced the characteristics of tussock grasslands.

Vegetation change since human settlement

if you want further information on a generalised model for vegetation change, east of the Main Divide.

Dryland dwarf grasslands- a vegetation history

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, and helped to increase the extent of dwarf grasslands by preventing the revegetation of taller statured dryland short tussock grassland species.

In more recent times, other exotic plants, such as wilding pines, are establishing in dwarf grasslands, despite the severe soil mosture deficit.

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. In the last decade, vineyard establishment in Central Otago has further modified areas of dwarf grasslands.

The long-term control of rabbits in some areas of dwarf grassland is resulting in an increase of taller vegetation (mainly exotic), at the expense of the low-lying plants characteristic of dwarf grasslands.

Future threats to the extent of dwarf grasslands include the establishment of lifestyle blocks, and the associated change in vegetation cover as people plant trees, lawns and pasture in an effort to make their homes more sheltered.

Management of Dwarf Grasslands

 Little information is available regarding the impacts of management on dwarf grasslands. However, dwarf grasslands occur at very dry sites, as well as where excessive grazing by rabbits and/or stock has maintained a very open and low-growing vegetation cover. Patches of bare ground often accompany this community. In Central Otago, several small and rare annual herbs (e.g. Ceratocephalus pungens, Myosurus minimus, Myosotis minutiflora) grow in this habitat, and are dependant on the continued existence of an open and low-statured vegetation cover. Some level of grazing is therefore likely to be necessary to ensure that taller statured grasses and herbs do not take over (Walker et al. 1995). Such plants will compete for light, water and nutrients, and change the microclimate by increasing the surface roughness of the vegetation. Such changes are likely to occur where grazing ceases at all but the driest sites.

Vegetation succession

if you want more information on factors influencing vegetation succession (includes model for vegetation succession).

Buffering

The relationship an area managed for conservation purposes has with it's surrounding landscape is important. Ideally, the area should be surrounded by land which will buffer it from external influences which may compromise the conservation values.

The buffer zone may:
  • act as a fire break, thereby protecting neighbouring land from wild fires occurring in dense tussock grasslands, or preventing controlled fires on pastoral land inadvertently burning an area managed for conservation.
  • protect the area from inadvertent AOSTD drift from neighbouring land
  • protect the area in terms of the hydrological processes.
  • help reduce the rate of spread of some weed species into the area e.g. intensive grazing between a forest and an ungrazed tussock grassland helps reduce the establishment of wilding trees from that forest.

Weed control

Weed spread is likely to be a problem at low to mid altitudes because these grasslands have been highly modified by pastoral activities (grazing, trampling, aerial over-sowing and top dressing, burning) and may neighbour land which has a predominantly exotic plant cover. Monitoring of the area will be necessary to determine which weed species are present and the extent of the problem. An assessment of the feasibility of controlling the weeds will then have to be made.

The need to control weed species is dependent on the characteristics of the weed (e.g. will it overtop the tussocks; does its' dispersal mechanism make it an aggressive invader?) and their impact on the indigenous ecosystem in question. Tall woody weeds are considered to be the most troublesome weed in tussock grasslands, as they have the potential to overtop and shade-out the tussocks. Wilding trees are a growing problem in Otago and inland Canterbury.

The spread of Hieracium in short tussock grasslands

Irrespective of whether managed for pastoral use, or soley for conservation purposes, many short tussock grasslands have a significant component of hawkweed. A large number of studies have been conducted to investigate the relationship between grazing management and rate of Hieracium pilosella spread, and associated trends on vegetation composition. However, no studies have focussed specifically on tussock grasslands dominated by bristle tussock.

More information on hawkweeds:


Summary of study findings
Hieracium Management Information Module




Native woody plant regeneration

The regeneration of native tree and shrubland species is likely to occur in grasslands below the natural treeline. In most areas managed primarily for conservation purposes, succession to native woody species is desirable as shrubland communities are more stable than short tussock grasslands. The species composition of the woody plants will be dependant on which seeds are viable in the seed bank, or whether there are adult reproducing plants in the vicinity of the grassland.

Replanting of naitve shrubs may be considered where active management to maintain a dryland short tussock grassland is not feasible. Moist microsites, such as gullies and cool aspects, are considered to be the best starting point for such revegetation programmes.

Animal Pests

The historical as well as present day degradation of tussock grasslands has been attributed in part, to high rabbit numbers (see Cockayne 1919). Some species that are presumed extinct and for which rabbits have been implicated in their demise include Logania depressa, Myosotis laingii, and Stellaria elatinioides (P. De Lange, pers. comm. in Norbury 1996). Other species once widespread in the short tussock grasslands e.g. the highly palatable Gingidia montana, are restricted to rocky refugia that are inaccessible to grazing mammals (Allen et al. 1995).

In a short term exclosure study on degraded short tussock grasslands of Earnscleugh Station where rabbit numbers were high (30-76 rabbits/spotlight km), it was found that after one growing season, the exclosure of rabbits resulted in a six fold increase in pasture yield (Rose 1996). Most of the biomass consisted of exotic grasses and herbs, but a significant increase in hard tussock and blue tussock also occurred. It was also noted that with protection from grazing, both grasses and hawkweed flowered prolifically, whereas no flowering was observed in the grazed plots. Norbury and Norbury (1996) suggest that rabbits were probably restricting the spread of exotic grasses and hawkweed, and the regeneration of palatable native tussocks.

In coastal silver tussock grassland on Motunau Island, Canterbury, areas with high rabbit numbers were characterised by dead tussock stumps, bare ground and barley grass. Areas with low rabbit numbers had healthy tussocks with a diversity of intertussock species, and a dense litter layer. After rabbit control, existing tussocks recovered and tussock seedlings were evident (Mason 1967).

In Central Otago, rabbit grazing has undermined and destroyed silver tussocks growing at the Cromwell Chafer Beetle Reserve (Barratt and Patrick 1992).

On Molesworth Station, Marlborough, Moore (1976) observed rabbits killing fescue tussocks (Festuca novae-zelandiae) by eating tillers and physically uprooting plants. With rabbit control, the palatable blue tussock recovered well, as did fescue tussock. Similar observations have been made by more recently some farmers in the Mackenzie Basin following intensive rabbit control during the Rabbit and Land Management Programme (1990 - 1995).

Some native plants appear to thrive in the open disturbed ground induced by grazing. At Flat Top Hill, Alexandra, Walker et al. (1995) noted that the rare native annual herbs- Myosurus minimus subsp. novae-zelandiae, Ceratocephalus pungens and Myosotis pygmaea var. minutiflora were present despite a history of high stocking rates and high rabbit numbers. Rabbit grazing here is likely to be beneficial to the conservation of these species by suppressing the spread of exotic grasses and herbs which might otherwise colonise the bare ground that these native annuals require.

Rabbit grazing appears to beneficial in controlling some woody weeds, including thyme in Central Otago (Wilkinson et al. 1979; Fraser, 1985; Walker 1994), sweet briar (Moore 1976; Ogle 1990) and broom (Moore 1976).

Monitoring

The management guidelines suggested are based on the best available information, and are often only "best guesses". To ensure that the management practices are achieving the desired management goal, it is crucial that the vegetation condition and composition, weed status, and animal pests are monitored. Management may have to be changed in response to the findings of such on-going monitoring.

Botanical information

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