Forest Vines to Snow Tussocks: The Story of New Zealand Plants
At the time of European settlement some lowland areas, particularly in the North Island, were occupied by bracken fern or shrubland dominated by manuka (Leptospermum scoparium) and/or kanuka (Kunzea ericoides). It is now believed that these areas were originally largely under forest, which was destroyed and prevented from returning by repeated fires of both Maori and natural origin. Many of the fires caused by the Maori were accidental, no doubt, but others were deliberate to encourage the growth of bracken fern whose rhizomes were an important source of starch.
Much of the eastern side of the South Island, as well as the Southland Plain, supported native grassland: short tussock grassland on the driest sites (the Marlborough Plains and adjacent hill country, the Canterbury Plains and foothills, and the Central Otago hill country) and tall tussock grassland of red tussock (Chionochloa rubra) on the moister Southland plains and eastern Otago hills. For some time it was assumed that the short tussock grassland was the original vegetation cover; the regions in which it predominated seeming too dry to support forest. When exotic trees were planted and thrived, there must have been some doubt about this view, but it was not until relatively recent times that evidence came to light indicating that these areas of short tussock grassland originally supported forests and forest/tussock shrubland mosaics. This evidence, to quote Molloy,117 includes 'surface logs and forest wind throw hummocks and hollows in now tree-less areas; buried wood, charcoal, and other plant remains; buried (forest) soil profiles; and a number of relict plants and soils'. It is suggested that forests, particularly in the east of the country, became more susceptible to fire following a change to more variable climates with periodic droughts after about 1800 years ago.118
Figure 81 Short tussock grassland dominated by fescue-tussock (Festuca novaezelandiae) on the Mt. Hay Station, Tekapo, central South Island. Photo: E. J. Godley.
Figure 82 Short tussock grassland mostly of Festuca novae-zelandiae on hills north of the upper Waitaki River.
Drought resistant shrubs grow on rock outcrops and along the bottoms of gullies where more moisture is available.
Photo: J. W. Dawson.
Figure 83 Matagouri or 'Wild Irishman' (Discaria toumatou) showing spines and flowers.
Photo: J. W. Dawson.
One of the driest regions supporting short tussock grassland was the Central Otago hill country and here overgrazing with sheep and the later depredations of rabbits reduced the landscape to virtual desert (Fig. 86). The tussocks were largely replaced by grey patches of Raoulia australis, unflatteringly termed 'scabweed'.page 151
Let us return to the red tussock grassland at the moister southern end of the South Island. A recent study121 of fossil pollen from bogs built up over the past 12000 years on the Longwood Range in Southland, indicates that forests also covered the southern end of New Zealand until about 1000 years ago. The species represented suggest that kahikatea (Dacrycarpus dacrydioides)-dominated forest occupied moister and swampy sites, rimu (Dacrydium cupressinum)-dominated forest drier sites and silver beech (Nothofagus menziesii) forest the higher altitudes.
Figure 85 (above) Aciphylla subflabellata. Seedhead with most of the bract segments aligned vertically to form a cage around the seeds.
Photo: J. W. Dawson.
Thus it seems that in their original state, before the increasing frequency of natural and human-caused fires over the past few thousand years, the lowlands of New Zealand had few sites not occupied by forests. In such open habitats as there were, establishment of trees would have been precluded by some localised environmental factor: periodic flooding and disturbance of parts of riverbeds normally above water level; excessive salinity in some coastal sites; excessive drainage and sparse soil of cliffs and bluffs in the drier eastern part of the country, and the infertile soils, containing varying concentrations of toxic metals (nickel, chrome, magnesium), derived from the ultramafic rocks more commonly known as 'serpentine'.
Lowland bogs and swamps would also have been treeless, but as these alter with time under the influence of plant cover and generally give way to forest, they have already been considered in Chapter 4.