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Relation to Wind Force.
The mechanical force of wind becomes appreciable on the higher ridges and particularly about flat tops and in saddles. The north-westerly wind often attains here a velocity against which it is almost impossible to proceed even by crawling. The surface in such places is generally bare of all but ground-hugging plants such as cushions of Phyllachne Colensoi, Raoulia rubra, and rosettes of Anisotome aromatica and Oreomyrrhis andicola. If the wind is more restricted in its direction, numerous narrow channels are worn in the ground with intervening strips of Danthonia antarctica and, perhaps, some other herbs and shrubs.
An interesting effect can be seen on Mount. Omega, 1100 m. It is about the only place on the Tararuas where Nothofagus Menziesii is subjected to the direct force of extremely severe north-westerly gales descending from the main divide past Mount Hector. The wind is particularly concentrated at the broad top, which is swept practically clear of all vegetation, but behind every protruding rock N. Menziesii together with Dracophyllum Urvilleanum is to be found. The trees with tortuous and quite prostrate trunks stand not more than 50–100 cm. high and have perfectly streamlined “crowns.” The greatest thickness of trunks observed was 15 cm. in diameter, but this varies greatly because of irregular growth. Branches 5 cm. in diameter were cut from three different trees. Their age proved to be about 300 years each. Accurate counting of annual rings was impossible owing to their extremely small size, often less than 2 cells thick, and a high probability that many rings were entirely missing. It is noteworthy that Olearia Colensoi is plentiful on the leeward side, but is practically absent for some distance down on the windward side, its place being taken by Dracophyllum Urvilleanum. The latter is always more plentiful than the former in the drier places, so that it is probable the phenomenon, as far as these two species are concerned, is really brought about by the drying effect of the wind. However, the foliage of Dracophyllum is undoubtedly more suited to resist the force of the wind than that of Olearia.
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Practically everywhere at this maximum altitude for the timber line the forest roof has a peculiar levelled surface. This appears to be due to low temperatures and high insolation during clear weather causing transverse geotropism, rather than any direct or indirect action of wind. At least there does not seem to be any evidence to support the latter assumption.
There are three places observed in the cold temperate belt where forest is subjected to particularly severe descending north-westerly wind. These are the eastern slopes of Mount Baldy, just south of Mount Mitre, Mount Holdsworth, and Mount Matthews. The trees there never reach maturity, their diameters seldom exceeding 20 cm. The floor is strewn with decaying and still living overturned trees. During a high wind the trees rock violently, lifting boulders of considerable size upon their roots and dropping them down again with a deafening clatter. Uprooted trees up to 2 m. in diameter are not uncommon on the Tararuas everywhere, but these are, as a rule, either over-sized or over-aged trees.
As has been pointed out, the average velocity of the south-easterly wind is very low compared with that of the north-westerly, yet, on rare occasions, perhaps less often than once in a lifetime, it may attain great velocity. “During the night of the 1st and the morning of the 2nd (February, 1936) a deep cyclone, which had originated some days before as a tropical cyclone to the north-west of the New Hebrides, rapidly increased its spread of movement, travelled down the western side of the Auckland Peninsula, and thence across the centre of the North Island and away in a south-easterly direction.” (Dominion Meteorological Office, 1936.) On the Tararuas the south-easterly wind began to be felt about 8 a.m. on February 2 and it gained its maximum force between 10 a.m. and mid-day. In those two hours whole hillsides of forest were completely levelled. The main destruction occurred in the foothills in the Western Area and the south-western portion of the Northern Area. The wind swept downwards from over the main ranges, concentrating its force upon the eastern slopes and along the valleys. Small trees and forest giants suffered alike. Some of the wrenched-out roots stood on edge some 8 m. high and up to 20 m. in spread, leaving hollows in the ground up to 2 m. deep. Of mature trees Dacrydium cupressinum appears to be the only one which was capable of withstanding the blast. Although badly battered, there are many trees of this species to be seen standing alone amidst destruction. They owe their preservation partly, no doubt, to their superior root development, but, probably, largely to the pendulous nature of their branches, which offered relatively small resistance to the wind. Numerous trunks of Metrosideros robusta also stand, but they are mostly stripped of their crowns. A portion of the affected area is shown on the accompanying sketch-map.