The Vegetation of New Zealand
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°.