The Vegetation of New Zealand
Chapter V. — The Climate of New Zealand Proper
The Climate of New Zealand Proper.
Such meteorological statistics as are available are not of much value for ecological plant-geography, nevertheless they give an indication of the general climate to which the plant-communities are exposed and they are of service for comparative purposes. But in a sparsely populated young country such accuracy or thoroughness cannot be expected as in the Old World.
Almost the whole of the data is derived from observations made in the lowlands and there are no statistics concerning the high mountains for altitudes exceeding about 600 m. But, apart altogether from instrumental observations, the members of a plant-community can tell a good deal concerning the various climatic factors on which, in part, its structure and activities depend. So, too, the behaviour of indigenous species abroad and of both indigenous and exotic species in New Zealand furnishes meteorological information of no small value. Finally, my studies of the vegetation carried out at all seasons for some 40 years have enabled me to gain some knowledge of the climate of many localities for which no other data are available.
In what follows only the climate of North Island, South Island and Stewart Island is dealt with, that of the Outlying Islands being discussed when treating of the vegetation and floras of each group. Also, the special details concerning the Botanical Districts are removed from this chapter to their more suitable position in Part IV, and climate is frequently referred to when dealing with the communities.page 53
New Zealand possesses, for the most part, a maritime climate, situated as it is remote from other lands in the widest ocean of the globe with no part of the area more than 128 km. from the sea. There are however marked differences in climate, owing firstly to the region extending through 25 degrees of latitude and secondly to the lofty mountain chains of the main islands lying athwart the prevailing winds. "Aspect" in a wide sense therefore has a remarkable influence both on rainfall and temperature, not only as to average annual amounts, but also in every atmospheric disturbance that passes over the land. Ecologically it is a primary cause of the wide-spread continuous formations — forest and tussock-grassland. Aspect in a narrow sense regulates climate or modifies its effects and so affects both plant-distribution and the composition and structure of communities.
With regard to rainfall, that of South Island, in the west, is extremely high, while, on the contrary, parts of the eastern districts are, in comparison, very dry. North Island has a maximum rainfall — almost a rainy season indeed — in the winter months, but South Island shows a remarkable evenness in its monthly averages. Periods of drought occur at times in the eastern districts, such being commonest in spring and summer, in the north, and in autumn and winter, in the south. Although the total average rainfall, especially in the east, decreases with increase of latitude, yet the number of rainy days is greater in the south than in the north. This arises through the frequent occurrence of atmospheric disturbances in the latitude of the "forties", but the northern districts are under the influence of occasional cyclonic disturbances of tropical origin which travel from north-west to south-east over North Island. Occasionally, extensive "Lows", decreasing northwards, account for much warm and moist weather but do not usually bring about a heavy precipitation. The cyclone track will often pass to the northward of New Zealand; sometimes it crosses as low as Cook Strait; occasionally it comes from the north-east to the East Cape and then passes down the east coast before taking an eastward route under the guiding and controlling influences of the prevailing westerly winds of these higher latitudes. The upper winds are almost invariably westerly and a divergence to a southwesterly direction from the west usually precedes a marked change in weather conditions, which the forecaster values as a guide to the subsequent swing of atmospheric pressure. High pressure, or the anticyclone, may be regarded as the controlling factor of weather conditions and on the edge and between these high pressure systems are found the "Lows".
The frequency of the above disturbances judging from the average of 9 years are as follows: (1) For the cyclone or monsoon of marked intensity — Spring 2; Summer 1.8; Autumn 3.3; Winter 5.3. (2) For the westerly or antarctic low — Spring 6.3; Summer 4.3; Autumn 5.2; Winter 4.7.
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.
Latitude, insolation, proximity to the ocean, or the large inland lakes, and height above sea-level are the determining major factors with regard to temperature. Especially are the oceanic influences a master-factor with regard to both summer heat and winter cold, upon both of which they exercise a moderating effect. Indeed, extremes of heat and cold, such as occur at similar latitudes in the Northern Hemisphere, are absent throughout New Zealand at every altitudinal belt. The west coast of South Island lies open to the prevailing westerly winds and is more humid and equable than the eastern botanical districts which, generally speaking, possess a more or less continental climate with a considerable range of temperature. Near the coast of North Island frosts, even on the grass, are of rare occurrence, but further south, and inland throughout, they are often experienced. Special details as to temperature are given in Part IV when treating of the different botanical districts and in the section dealing with the vegetation of the high mountains.
The meteorological seasons are later than the astronomical. Thus July is usually the coldest and wettest month in the year, while January is the driest and warmest. The seasons may be roughly divided as follows: — spring, — September, October, November; summer, — December, January, February; autumn, — March, April, May; winter, — June, July, August. But such divisions are somewhat misleading from the phytogeographical standpoint, altitude, latitude and aspect being controlling factors with regard to seasonal changes.
Taking the mean maximum and minimum temperatures for the hottest (January) and coldest (July) months for a number of localities — all at about sea-level unless height is given — and proceeding from north to south they are as follows:—Auckland 23° C, 10° C.; Rotorua, (276 m. alt.) 24°, 2.1°; Napier 24°, 5°; Moumahaki (south of EW.) 23.5°, 5.7°; Wellington 21°, 4°; Nelson 23.8°, 2.8°; Hokitika 19.7°, 1.8°; Lincoln (E.) 22.2°, 1.8°; Queenstown (SO., alt. 301 m.) 19.1°, — 1.1°; Dunedin 19°, 2.8°; Invercargill 19.2°, 1°.
Central Otago (NO.) has the reputation of experiencing the coldest winters of New Zealand proper and many exaggerated statements have been page 57made, which, however, are disproved by the inability of any indigenous plant to be cultivated in the open in the colder parts of Europe or North America. All the same, the area in question in certain localities is far colder in winter than most parts of South Island at the same altitude. Thus, at Eweburn on the Maniototo Plain at 420 m. altitude, the average annual minimum for 15 consecutive years is — 12.1° C. and on one year 16.6° was reached, other low minima for different years being — 12.8°, 14.4° and — 15°. In North Island, —12.2° has been recorded at Waiotapu on the Volcanic Plateau at an altitude of about 300 m. Doubtless, in the high mountains lower temperatures are reached but the evidence derived from the cultivation of New Zealand subalpine and alpine plants in Europe shows that a temperature of — 18° C. is more than the majority can tolerate. In Great Britain, New Zealand plants of all kinds can be grown well in parts of Cornwall and Devon, but in England and Scotland generally many are only half-hardy. Mr. C. T. Crawford of St. Andrews, Fife, has sent me a list of New Zealand plants which he cultivates, 76 being perfectly hardy, the lowest shade temperature being nearly — 14° C. and on the grass nearly-—17° C. At the Royal Botanic Garden, Edinburgh, almost any New Zealand high-mountain plant can be successfully cultivated.
In New Zealand itself an exceptional frost, particularly if maintained for a number of days in succession, damages or kills outright many indigenous species. Thus, in 1923, at Queenstown "though there was almost constant frost for six weeks (L. Cockayne, Festschrift zum siebzigsten Geburtstage von Karl von Goebel, 1925: 77–78) and many supposedly hardy plants were killed, so far as I could ascertain, the thermometer did not fall below — 11° C. But, even if the cold were greater, it certainly cannot have nearly reached — 17° C. for Eucalyptus Gunnii, juvenile E. globulus and Pinus radiata were undamaged". On the other hand, the following species which ascend to above the forest-line were killed or damaged: — Phormium Colensoi, Weinmannia racemosa, Myrtus pedunculata, Leptospermum scoparium, Nothopanax Colensoi, Gaultheria perplexa, Olearia arborescens, Shawia paniculata (grows on rock, unprotected by a snow covering at 1200 m. alt. on the Seaward Kaikoura Mountains), Senecio cassinioides (never descends to the lowlands) and Senecio elaeagnifolius. Also nearly all the purely lowland species were either killed outright or more or less damaged including such as were in their natural habitats.
Just as Central Otago has the greatest winter cold, so is its average and maximum summer temperature greater than in any other part of New Zealand proper. Thus in 1922 the maximum shade temperatures for January and February were respectively 38.8° C. and 36.6°, while from Dec. 1921 to March 1922, inclusive, the temperature on 23 days was over 32.2°, on 47 from 26.6° to 31.7°, on 40 days from 21.1° to 26.1° and on 11 days from 15.5° to 20.5°. Even in May (last month of autumn) a shade temperature page 58of 20.5° may be reached and in September (first month of spring) one of 23.8". The following rather high (for New Zealand) temperatures are recorded occasionally for various localities: — Rotorua 33.3°, Waiotapu 31.1°, Waihi (T.) 30°, Starborough (NE.) 35°; Hanmer 36.8°; Lake Coleridge (E., 366 m. alt.) 30°; Tapanui (SO., 150 m. alt.) 36.6°; and Invercargill 30.5°.
|Possible sunshine on the||Auckland Hrs. min.||Wellington Hrs. min.||Dunedin Hrs. min.|
The next table shows the average amount of sunshine at various places in proceeding from north to south. The comparative paucity in the South Otago district (Dunedin and Invercargill) is in part reflected by the high-mountain species at a low altitude and the facility with which that class of plants (indigenous and exotic) can be cultivated.
Wind, especially in certain botanical districts, is a most important ecological factor. Generally speaking its effect becomes more intense the further south one proceeds or the higher one ascends. It is also of great moment on the coast especially on the west and on small islands.
The westerly winds of South Island are of special moment. Striking the western mountain wall, the wind loses its moisture in passing over the high lands and descends on the east as a hot wind sweeping through the river valleys and over the gravel plains, raising transpiration to its maximum. Though this hot north-west wind occurs to the east of the Southern Alps as a whole, in places its effect is greatly lessened through interception by ranges to the east of the Divide, so that its maximum strength is limited to the valleys and plains of the North-eastern and Eastern districts, where it even reaches the coast-line, and to the North Otago district. Perhaps its greatest intensity is experienced on the Canterbury Plain and the montane valleys extending westwards from its upper part to the Divide. Such a "Canterbury nor-wester", as it is called, sweeping through the river-gorges page 59bursts with all its fury upon the plain — a hot, dry wind, its progress marked by clouds of sand and silt rising out of the wide beds of the glacial rivers. In the west clouds hang over the distant mountains indicating the rainstorm that is raging there, but over the plain is a clear blue sky while a burning sun strikes down. On plants the leaves hang flaccid, in orchards the trees are stripped of their fruit, everywhere the surface of the ground if unprotected by vegetation becomes dry as dust and the soil of ploughed fields may be blown away. On the dunes sand in clouds is carried back to the sea, sandhills are bodily removed and the rope-like entangled stems of Desmoschoenus, metres in length, laid bare. On mountain-passes and exposed ridges the fury of the storm reaches its height, it is impossible to stand upright, small stones are hurled through the air.
The south-west wind — of far wider range — is of equal ecologic importance and frequently brings with it squalls of a subantarctic character, leading to snow on the mountains, or even to a heavy downpour, but at other times rain is wanting while a furious gale extending to the southern parts of North Island rages for one or two days at a time. The change from north-west to south-west is quite sudden the temperature dropping many degrees and conditions approximating to those of midwinter may occur in the middle of summer. Obviously such sudden changes are of great physiological importance.
The average velocity of the wind in the following tables is from records of the Robinson anemometer in kilometers per day.
|Auckland (11 years)||Wellington (16 years)||Hokitika (16 years)||Lincoln (13 years)|
|Average per day||288||451||216||262|
|Maximum velocity for one day||1558||1920||1108||1547|