The Vegetation of New Zealand
The rainfall of New Zealand proper bears a striking relation page 54to the physical configuration of the land, and records gathered throughout the country during a period of more than 60 years present a certain regularity which clearly shows the dominating influence of the mountain ranges. In South Island, the lofty Southern Alps, together with the ranges of the Northwestern district, lie broadside to the prevailing westerly winds, and on their windward slopes are condensed the vapours which have been gathered by the breezes sweeping over vast stretches of ocean. On the Westland coastal plain, and on the adjacent rugged and precipitous slopes, the rainfall averages from 250 to 500 cm. per annum, while on the leeside of the great mountain barrier the climate is, in comparison extremely dry and, in places, the rainfall is only one tenth of that on the west. There are in fact two distinct climates that of the west strongly favourable to forest, and that of the east altogether antagonistic to that type of vegetation and in harmony with tussock-grassland (Fig. 39), but the latter climate, as will be seen, owes in part its character to the wind-factor.
While South Island isohyets stretch east and west, those of North Island are more irregular in form, but demonstrate that the rainfall itself is more regular over the land as a whole and less extreme in a comparison between the different botanical districts. But here again the control of the mountains and plains over "precipitation is apparent, the contours of the rainfall areas coinciding more or less with the configuration of the country, the heavier downpours occurring in proximity to Mount Egmont, the central volcanoes, the Dividing Range and the higher summits in general.
The mean annual rainfall of New Zealand proper, as derived from means of representative stations in various parts of the islands, is about 121 cm., but the seasonal falls are far from uniform throughout. The following averages taken from the climatological tables give some idea of the rainfall and its distribution throughout the year for the two main islands, but the first two tables, though useful for comparison with similar statistics for other countries, are of very little phytogeographical moment.
|North Island||Auckland||New Plymouth||Gisborne||Wellington|
|61 years||37 years||36 years||56 years|
|37 years||35 years||55 years||18 years|
From the phytogeographical standpoint the number of rainy days is of far greater moment than the amount of the downpour, the following table then is of special interest.
Annual mean totals: — Auckland 180.4; New Plymouth 189.7; Gisborne 153.8; Wellington 167.7; Christchurch 119.4; Hokitika 179.2; Dunedin 163.3; Invercargill 180.0.
The question of snow naturally comes along with that of rain, but as it is discussed from the ecological standpoint when dealing with the high-mountain vegetation in the first chapter of that section, only a brief statement need be made here. In the subalpine and alpine belts of all the islands the winter snow-fall is very heavy and there is a continuous covering for some months, the length of time it remains depending upon aspect and altitude. In the montane belt of South Island there are occasionally heavy falls reaching up to one metre in depth. The sheep-farmer knows all about the relation of his run to snow and divides it into "winter" (snow-free) and "summer (snow for months) country" — the area of the former determining the number of sheep the run can carry. In the lowlands snow page 56is almost unknown in North Island, but from the North-eastern district southwards, every few years it may lie for one or two days at sea-level. On the west snow at sea level is rare.