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Relation to Temperature.
In the study of the vegetation of a large area it is convenient to subdivide the latter into a number of smaller units determined by the more important environmental factors. Of these on the Tararuas, as in most other mountainous places, temperature is the most important single variable factor to consider. Next come rainfall and others associated with it. Since the Tararuas extend only some 150 km. from north to south, the latitudinal variation cannot be significant. Further, according to Kidson (1931), the isotherms with slight gradient run more or less parallel to the axis of the mountains. The altitudinal variation of .5° C. per 100 m. over 1500 m. on the other hand is appreciable. Whatever the manner in which temperature affects the plants, the effects themselves can be readily studied, for example, the altitudinal delimination of the range of species. This in its turn gives the means of delimiting botanical belts, which may be termed temperature belts as distinct from those determined by other factors. On the Tararuas the belts are as shown in the table below. The principles on which modification of terminology is based are discussed by the author in “Some Correlations Between
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Vegetation and Climate in New Zealand” (1938). L. Cockayne's (1928) terminology adapted especially for the Tararuas is also shown for comparison.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
| Cockayne's Description of Belts. | Altitude in m. | Modified Description of Belts. | |
| Alpine | 1800 | Upper (absent) | Warm subpolar |
| Alpine | 1500 | Warm subpolar | |
| 1200 | Lower | Cold temperature | |
| Subalpine Upper | 900 | Upper | Cold temperature |
| Lower | 900 | Cold temperature | |
| 600 | Upper | Cold temperature | |
| Lower | Warm temperate | ||
| Montane | 300 | Upper | Warm temperate |
| Lowland | Lower | Warm temperate |
Thus on the Tararuas there are three primary belts: Warm Temperate from sea level to 600 m., Cold Temperate from 600 m. to 1200 m., and Warm Subpolar from 1200 m. to the highest peaks. Each of these can be satisfactorily subdivided into upper and lower belts of 300 m. each. There are a number of species which extend through several belts. Many species also have an ill-defined lower range limit. The upper limit, however, in most cases is very sharp. The disappearance not only of a species, but of whole groups of them is generally complete well within 25 m. of an arbitrary limit line.
The characteristic species of the warm temperate belt are: Beilschmiedia tawa, Hedycarya arborea, Knightia excelsa, Melicytus ramiflorus, Rhipogonum scandens, Pseudowintera axillaris, Nothopanax arboreum, Freycinetia Banksii, Clematis indivisa, Pittosporum Edgerleyi, Nothofagus truncata, N. Solandri, Mida salicifolia, Cyathodes acerosa, Metrosideros robusta, M. scandens, Astelia Solandri, Earina autumnalis, Dendrobium Cunninghamii, Uncinia uncinata, Isolepis cernua, Cyathea dealbata, Polystichum Richardi, Danthonia antarctica Cheesemanii. All these species disappear nearly at the same altitude.
The following characteristic species of the cold temperate belt do not, as a rule, descend into the lower belt: Pseudowintera colorata, Coprosma foetidissima, Olearia lacunosa, O. Colensoi, O. arborescens, Senecio elaeagnifolius, Dracophyllum Urvilleanum, Pittosporum rigidum, Nothopanax Colensoi, Gahnia pauciflora, Danthonia Cunninghamii, Astelia nervosa, Uncinia filiformis, Isolepis aucklandicus, Libertia pulchella, Nertera setulosa, N. dichondraefolia.
None of the above species, nor indeed any of the other woody species of the temperate belt, with the exception of Olearia Colensoi,
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occurs above this belt. O. Colensoi, however, often extends well into the lower warm subpolar belt. Libertia pulchella and Uncinia filiformis are restricted to the lower cold temperate. Nothofagus Menziesii is one of the most characteristic species of the cold temperate, but in fairly dry places, where plenty of light reaches the ground, it occurs in abundance as low down as the upper warm temperate. Nothofagus fusca ranges generally from lower cold temperate to upper warm temperate. A few species including Weinmannia racemosa, Coprosma australis, Aristotelia racemosa, Rubus cissoides, Dacrydium cupressinum, Fuchsia excorticata range from the lower cold temperate to the sea-level.
Of the higher belts only the lower warm subpolar is represented on the Tararuas. The following species are characteristic of this belt: Dracophyllum rosmarinifolium, Senecio Bidwillii, Hebe Astoni, Raoulia rubra, R. grandiflora, Phyllachne Colensoi, Celmisia hieracifolia, Danthonia flavescens, D. —-, Astelia Cockaynei, Celmisia spectabilis, Anisotome dissecta, and A. aromatica are often the dominant species, particularly the first three. Aciphylla Colensoi conspicua, Ranunculus geraniifolius, and many others are plentiful.