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Relation to Rainfall.
Detailed information on the distribution of rainfall is not available, but the variation in the amount of it can be deduced, satisfactorily for the present purpose, from the study of the effects produced by it. The density of the vegetation in general, the abundance of epiphytes and bryophytes, all serve as useful indicators, while the presence or absence of a number of characteristic species indicates unmistakably the relative amount of precipitation. The Tararuas are not high enough to cause the rainfall to vary much from one place to another. In general the western slopes everywhere receive a somewhat higher precipitation than the eastern. The difference is greatest where the two sides are separated by greater distance and higher intervening ranges. Thus, in the central Tararuas, the general aspect of the warm temperate belt on the
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eastern side has little resemblance to that of the same belt on the western. However, the difference practically disappears within the lower half of the lower cold temperate belt. Higher still the conditions are not strictly comparable owing to the difference in illumination. The rainfall here, in all probability, exceeds 250 cm. per annum (Kidson, 1930). February is probably the driest month, when even the wettest ground of the warm subpolar may be parched for a few days at a time.
The distribution of the species of Nothofagus is of special interest. First of all the absence of N. cliffortioides is noteworthy. This species inhabits fairly dry localities in the cold temperate belt, but as such are absent from the Tararuas, so is the species. Its place for the greater part is occupied by N. Menziesii, which normally inhabits the wet cold temperate belt. North of Mount Dundas, however, where a peculiar combination of low rainfall and heavy fog prevail, both species are absent, and their place to some extent is taken by Dacrydium biforme. N. fusca tolerates a fairly wide range of precipitation and is widely distributed. It ranges mainly throughout the lower cold and upper warm temperate belt, but is practically absent from the latter on the western side, being ousted by Dacrydium cupressinum and Metrosideros robusta forest. Both N. truncata and N. Solandri, especially the latter, occupy much the driest portions of the mountains. They extend through the warm temperate belt south of Mount Alpha.
There are four distinguishable rainfall areas on the Tararuas, characterised by their own floristic elements.
The Southern Tararua Area, consisting of the Hutt River Basin and the Rimutaka Ranges, is the driest of the four areas. It is effectively screened by the range (now for the most part devoid of primitive vegetation) rising to some 600 m. and running along the coast from Wellington to Mount Kapakapanui. The northern boundary extends approximately from Mount Wainui over Mount Omega to Mount Reeves. The area is characterised by the presence of the following species: Nothofagus truncata, N. Solandri, Danthonia antarctica Cheesemanii, Raoulia glabra, Leucopogon fasciculatus, Cyathodes acerosa, Leptospermum scoparium, L. ericoides, Helichrysum glomeratum.
The Northern Tararua Area has its southern boundary ill-defined. The latter runs approximately over Mount Waiopehu, Mount Dundas, and Mount Bruce. The mountains north of this line lie in the rain-shadow of Mount Egmont and considerably inland. Being for the most part low, they intercept comparatively little rain. The characteristic species are: Dacrydium biforme, Phyllocladus alpinus, Veronica catarractae, Elatostema rugosa.
The Western and Eastern Tararua Areas occupy the central portion of the mountains. The difference in rainfall between the two areas is already indicated. In addition, in the Eastern Area, the sunshine is much more abundant at higher altitudes than in the Western, and hence there is a greater luxuriance of vegetation and more prolific flowering. It is convenient therefore to draw the boundary between the two areas approximately in a straight line over Mount Arěte and Mount Omega.
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The Western Tararua Area is characterised mainly by the abundance of epiphytes and luxuriance of undergrowth particularly in the warm temperate. The entanglements of Rhipogonum scandens and Freycinetia Banksii form an outstanding feature. Beilschmiedia tawa, Gleichenia Cunninghamii are abundant, while the almost complete absence of Veronica catarractae and Elatostema rugosa as well as Nothofagus truncata, N. Solandri, Cyathodes acerosa, and Leucopogon fasciculatus is noteworthy.
In the Eastern Tararua Area the last four species are more or less characteristic. The epiphytes are not nearly so profuse as in the Western Area, the undergrowth is much thinner, and the ground is littered with dead leaves. The forest floor generally is remarkably free of fallen logs. Apparently the decay proceeds here very rapidly as compared with the Western Area. There the ground is strewn with dead logs often as large as 2 m. in diameter. They are thickly covered with bryophytes, filmy ferns, and other plants which quickly form a thick layer of permanently wet humus, the latter acting as a blanket, preventing free circulation of air and slowing down the processes of decay. In the upper cold temperate belt of the Eastern Area the abundance of pendulous Osnea sp., which is practically absent elsewhere, is a striking feature.