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Low Altitude Environments
Low altitude areas in humid parts of the South Island, are characterised by relatively warm soil temperatures, while a summer soil moisture shortage may be experienced only from time to time. Such areas have a good growing seasons, and have usually already been developed for intensive or extensive farming. Tussock grasslands in humid low altitude areas are generally very modified, or no longer in existence. The soil fertility and distribution at low altitudes are a reflection of the rainfall, and hence amount of leaching that has taken place. Brown grey earths are found in semi-arid areas, and are relatively fertile. In areas with slightly more rainfall, yellow grey earths occur, which exhibit more soil development than the brown grey earths, but are not as leached as those soils found in high rainfall areas. Here, leached yellow brown earths are found. On outwash plains in semi-arid and subhumid areas, poor fertility outwash soils develop.
Inland basins of Central Otago, Mackenzie Basin and Marlborough are characterised by hot soil temperatures and prolonged summer drought conditions.
Why tussock grasslands are hereIn pre human times, tussock species would naturally have occurred where it was too dry, wet or frosty for forest species to grow. In the low altitude areas, short tussock species would have been relatively common in savanna vegetation occupying the dry intermontane basins where the low rainfall (< 500 mm/yr) either prevented forest establishment, or resulted in the rate of colonisation to be very slow. When forest cover was removed by natural fires on neighbouring hillslopes, the tussock species migrated upslope, later to be succeeded by tall tussock species, and then woody vegetation. This pattern of plant succession would have been similar in humid areas, though the rate of succession would have been much faster. In more humid areas, tall tussock species would have occupied (and still do) areas too wet for forest colonisation (e.g. copper tussocks of Southland). By the time the first European settlers arrived, most of the forest cover had been removed, being initially replaced at low altitudes, by short tussock grasslands, and some tall tussock grasslands. Subsequent land development for farming has seen the majority of these grasslands cultivated, or at least, severely modified. Tussock grasslands can be found in those areas which are least suitable for farm development or where irrigation water has not been available. Such grasslands are usually relatively small in extent; highly modified with many dryland weed species present, and are characterised by a depleted short tussock cover. References:
Connor, H.E. 1964: Tussock grassland communities in the middle Rakaia valley, Canterbury, New Zealand. NZ J Botany 3: 261-276.
Floate, M.J.S. Ed. 1992:Guide to tussock grassland farming. Chapters 2 & 3 AgResearch, Mosgiel, Invermay.
Molloy, B.P.J.; Burrows, C.J.; Cox, J.E.; Johnston, J.A.; Wardle, P. 1963: Distribution of subfossil forest remains, eastern South Island, New Zealand. New Zealand journal of Botany 1:68-77.Wardle, P. 1991: Vegetation of New Zealand. Cambridge Press.