Geographical Botany of New Zealand.
Explanatory of a Series of Essays by
Sir David Monro, and Messrs. Travers and Buchanan.
Read before the Wellington Philosophical Society, September 15, 1868.
In communicating the following Essays, I will take the opportunity of explaining briefly, by a single example, the chief physical peculiarities which regulate the distribution of the vegetation in the South Island.
The accompanying diagram (see Illustration) is an ideal section across the island, between latitudes 40° 30′ and 46° 30′ S. The greatest altitude met with, in such a section, will be 10,000 feet, but the mean elevation of the ridges that connect the various summits is barely 5000 feet, while in these occur breaks, or “passes,” in the mountain chain, which by permitting the passage of the western winds, give rise to local modifications of the flora, at the points where they lead out on the eastern slope. The best known of these are, the pass from the head of the Wanaka Lake, by which Dr. Haast crossed to Jackson's Bay; and the Greenstone Pass, leading from the Wakatipu Lake to Martin's Bay. Another pass, only a few miles in length, crosses the narrowest part of the Southern Alps, between the head of the sounds and arms of the Te Anau Lake.
These breaks in the mountain chain have all about the same altitude of less than 2000 feet above the sea level, which is sufficiently low to admit of the transfusion of many species of plants.
From the fact, that these “passes” follow longitudinal valleys, with a succession of short gorges at right angles to their general course, and do not coincide with straight transverse depressions, the influence which in the latter case they would have exercised on the climate of the interior, is greatly reduced; nevertheless, the mild and genial climate that is experienced in the neighbourhood of the Wanaka and Wakatipu Lakes, is to be attributed, in a great measure, to the existence of these deeply cut notches in the mountains.
From comparative meteorological observations, it would appear that four times the quantity of rain falls on the west than on the east coast; and this, as may be expected, produces a marked difference in the character of the flora. Moreover owing to the influence of the mountains which thus intercept the moist winds, a comparatively arid district occurs in the interior of the province, which approaches to within a variable
distance of the east coast, according to the form of the surface, and the prevalence of dry shingly soils.
Whatever may have been the original botanical features of this district, it is now, at least, characterized by an almost total absence of forest. That heavy timber, at one time, grew upon the ranges, is indicated by the occurrence of large half-consumed logs; but it is very improbable from the nature of the soil on the flat basin-like plains, that they ever supported any other kind of vegetation than grasses and low scrub.
In addition, therefore, to the division of the Flora into Zones according to altitude; it is intended, in the diagram, to represent the three parallel disricts of the West, Central, and Eastern parts of the province.
Zone A, consists of mixed bush. A 1, is the forest of the West Coast, that clothes steep slopes and confined valleys, which open directly on the sea without the intervention of open or alluvial land. It presents no essential difference of character, between the sea level and an altitude of 4200 feet (which is the highest limit ever reached by the sylvan zone in any part of Otago), while in many instances it ceases, without obvious reason, at a much lower elevation.
As indicated by the prevalence of certain trees, the western forest may be sub-divided as follows:
(a) Along the shore there is a profusion of shrub Veronicas and Olearias, and large Iron Wood trees with gnarled branches.
(b) The flat land and low spurs are covered with the common species of Pines and Birch, such as Rimu, Totara, Weinmannia (Karmahi), and Fagus (Tawai), with a dense undergrowth, and many tree ferns, among which the most remarkable is the Mamaku (Cyathea medullaris), a tree Fern only found in Otago, on the west coast, though common in the northern provinces.
(c) From 1000 feet upwards, the Pines diminish in number but are still well grown; and in addition to the common kinds, the Toa-toa or Celery Pine (Phyllocladus alpinus) becomes abundant. The Rata (called Iron wood in the South) acquires an immense size on the loftier ridges; but, on the whole, the trees which predominate are the Red and White Birches, which gradually become dwarfed, and form, along with the heath-like and composite shrubs, the belt of dense scrub which always marks the upper limit of the forests. The most striking feature of this western bush is, however, the wonderful luxuriance of Cryptogamic plants, every shrub and tree being loaded with damp lichens, mosses, and fungi, which rapidly destroy the timber.
A 2. The mixed bush of the East Coast presents a marked difference from that on the West; on the whole the timber is better grown, and of larger size, and there is a much greater variety of pines. Its most characteristic development is around Dunedin, where it is rare to meet any Iron wood, Karmahi, or Birch, which are so common on the west (Black and White Birch being wholly wanting).
Zone B. In crossing the Alpine ranges to the eastern side, the upper limit of the forest is at a less elevation, reaching to only 3500 feet; and along the eastern slope of the mountains, consists nearly wholly of Birch trees, with patches of mixed bush in favorable situations, by the sides of the larger lakes. The valleys in this district are generally occupied by a dense scrub, consisting of Veronicas, Cassinias, and Olearias, resem-
bling somewhat the Sub-alpine flora, while the woods are skirted by a luxuriant growth of the handsome Lace-bark tree (Plagianthus Lyallii) with its delicate green leaves and large white flowers.
Zone C 1. As has been already observed, the influence of the mountains in intercepting moist westerly winds, has given to the district in the central part of the Island, a Flora which possesses almost a continental or arid type.
Cryptogamic plants, which are generally so abundant in New Zealand, are almost wanting, being largely reduced in relative per-centage to the Phanerogamic flora.
The plains are covered with grasses, the roots of which are gathered into tufts or tussocks, intermixed with the Bayonet grass or “Wild Spaniard” (Aciphylla Colensoi), Toumatakuru (Discaria Toumatou), and a great variety of the New Zealand brooms (Carmichælia), with small Orchids, Pimeleas, and ericaceous plants.
Advancing towards the east, the grasses acquire a different character, indicating the influence of moisture derived from the eastern sea-board.
The yellow tussock predominates, and is intermixed with a rank growth of fern (Pteris aquilina, var. esculenta), Tutu (Coriaria) several species, Flax (Phormium tenax), and the soft leaved Spear grass (Aciphylla squarrosa), all of which latter plants rarely occur in the interior or lake district.
Zone D 1. This is the zone of Sub-alpine plants, which is especially distinguished by the prevalence of large bunches of Snow grass, which term includes various species of Danthonia and Agrostia, intermixed with scrubby patches of Dracophyllum and other heaths, many beautiful Veronicas, and a great variety of composite plants, principally Celmesias, Senecios, and Cassinias. It is comparatively narrow on the western slope, where it ranges from 4200 to 5500 feet; but on the opposite side of the mountains it expands, so as to embrace from 3500 to 6000 feet, its lower limit conforming to the winter snow line of the region, which in accordance with meteorological laws, declines in altitude as we advance from the western sea board into the interior of the island; while on the other hand the summer snow line rises in elevation.
It is in this zone of the western ranges, that the large species of Ranunculus with its magnificent peltate leaves and flowers, form such a striking object in the Alpine landscape (R. Lyallii and R. Traversii.)
D 2. Where the open country of the eastern plains rises above an altitude of 3000 feet, there also, Snow grass, Veronicas, Celmisias, and other plants characteristic of this zone, displace the more nutritious pasturage of the lower grounds.
Zone E, is the Alpine region, which for a great part of every year is completely covered with snow, and where, in consequence, all the flowering plants (of which there are many species of great beauty) that form wood are nearly stemless, and in most cases are aggregated in the form of hard hassock-like patches, in consequence of the slipping action of the snow on the sides of the mountains, forming what are known as patch plants.
This zone ranges from 6000 feet upwards, the highest plants having been obtained at a little over 8000 feet, but which, however, is probably not the extreme limit of vegetation in this latitude.