Forest Vines to Snow Tussocks: The Story of New Zealand Plants
These vary in their form quite markedly depending on the amount of eroded material — rock fragments, sand, silt — being brought down from the mountains. Where there is more material than the river can readily transport to the sea, the river bed is steadily raised. At the other extreme, where eroded material is slight and there is a sufficient fall to the sea, the river steadily cuts down into its bed. The best examples of the former type are the rivers traversing the Canterbury Plains (Fig. 87). These rivers have in fact built the Plains with the debris they have carried down from the Southern Alps. Their beds are sometimes page 154a few kilometres wide and are described as 'braided' as they exhibit a network of channels, of which only a few actually carry water at any particular time. As each water-filled channel builds up to a higher level than adjacent dry channels it may take only a slight freshening of water flow to divert the river into a new course at a lower level. Such channel changes, and occasional floods which fill the river bed from bank to bank, put riverbed plants at a constant risk of being swept away. When a portion of a riverbed has been stable and dry for a time plants begin to establish. First come species of Epilobium (E. brunnescens, E. melanocaulon and E. microphyllum) and flat circular patches, up to a metre in diameter, of Raoulia tenuicaulis. As wind-blown silt accumulates, Raoulia hookeri, R. australis and other epilobiums follow and, if stable conditions continue, taller plants appear: short tussock grasses, carmichaelias, hebes, parahebes, matagouri (Discaria toumatou) and others.123
Rivers that are cutting down have much narrower beds which are largely under water most of the time. Such rivers are often confined by quite steep banks or, in gorges, by high cliffs. Plants grow mostly on the banks or cliffs, although some may establish on 'islands' in the bed itself. Rivers of this type are quite common in the North Island and some parts of the South Island.
The alluvial flood plains beyond the river banks often support communities of small trees. The species concerned are not confined to such sites, but some are most frequent there, including ribbonwood (Plagianthus regius), and species of lacebark (Hoheria) of the family Malvaceae and perhaps most notably the small trees that bear New Zealand's best known flower, the kowhais (Sophora). Sophora microphylla is found throughout and S. tetraptera, with larger leaflets, in the eastern North Island. The flowers, which are pollinated by birds, are golden yellow, pendent and more or less tubular in form.
On the rocky banks and lower parts of cliffs of the river bed itself plants are subjected to more frequent floods and are much smaller. Common among herbaceous plants are ferns, including several species of Blechnum, and flowering plants such as the small-leaved, creeping species of Gunnera and the two subshrubby species of Jovellana, a genus closely related to Calceolaria. Among shrubs several willow-leaved koromikos (Hebe) are prominent, for example Hebe salicifolia and varieties of H. stricta, as well as several species of the related Parahebe of which one is appropriately named P. catarractae.page 155
A number of species of leafless broom (Carmichaelia) are often encountered in these riverbeds. They have slender but tough, flattened, more or less trailing stems with no leaves, or leaves for only part of the year. In the tropics there are often distinct communities of shrubs growing within the flood zone of streams.124 Some of the species grow nowhere else and are 'streamlined' with long, narrow leaves and tough, slender stems to minimise the force of the water during floods. Probably none of the New Zealand species is restricted to river beds, but some show similar modifications to those in the tropics. The narrow-leaved hebes look remarkably like the unrelated Metrosideros operculata of New Caledonian stream beds and the carmichaelias with their modified stems seem well suited to avoiding damage from floods. The species of Notospartium and Chordospartium, related to Carmichaelia, of the north-eastern South Island, have 'slender, drooping, leafless branches' and may also grow near rivers.
This leads to the speculation that in warmer Tertiary times when open, lowland habitats were in short supply, Hebe, Parahebe, and Carmichaelia may have been largely confined to riverbeds, diversifying into other open habitats when forests retreated during glaciations.