A question of considerable moment in plant-geography is the occurrence of species which, though common enough elsewhere, are most limited in numbers in some particular locality.
The forest in the neighbourhood of Dunedin (South Otago Botanical District) supplies several interesting problems of this class, one of which, to be considered here, deals with that fundamental matter concerning New Zealand forests in general—the relation between subtropical forest composed of broad-leaved dictotylous trees and podocarps, and subantarctic forest where one or more species of Nothofagus dominate.
As will he seen further on, there are in the Dunedin area some twelve pieces of Nothofagus Menziesii forest, mostly quite small, indeed, out of all proportion to the original forest-covering of the area, and the question at once arises, “Is the silver southern-beech (N. Menziesii) a new arrival, or is it merely a survivor of a former host?” To attempt an answer to this important question is the main object of this paper. Secondary to this is the presentation of various matters referring to these Dunedin Nothofagus communities, and especially the placing on record of their distribution and composition.
Finally, in the light of our new knowledge concerning Nothofagus in the Dunedin area, we are in a position to examine critically L. Cockayne's bold theory (1921: 322-23 and 1926: 39) regarding the relation between the two great classes of New Zealand rain-forest—the subtropical and the subantarctic.
The said theory is based on the present latitudinal, altitudinal, and ecological distribution of Nothofagus forest in New Zealand, and it suggests that at one time the Nothofagus forest was the chief tree-community; but that it has been gradually replaced by the subtropical forest of Malayan origin, the Nothofagi having been slowly suppressed where the soil is comparatively fertile, and that the Nothofagus forests could only remain intact on the poorer ground or at higher altitudes where the climate is hostile to a majority of the subtropical forest species.
At the time of the settlement of Otago there can have been at no place near Dunedin any considerable area of Nothofagus. The whole of the slopes on both sides of the harbour were covered with dense subtropical rain-forest*; that on the west stretching from
[Footnote] *Hereafter cited as rain-forest, but it must be remembered that rain-forest in New Zealand includes both the subtropical and subantarctic forests.
the water's edge to the high ridges of Mount Cargill and Flagstaff Hill and extending northwards to beyond the Waikouaiti River.
With few breaks this forest covered the hills to the west of Dunedin and along the slopes north and west of the Taieri Plain. The gullies to the east of the plain were also thickly forest-clad, and the reserve at Taieri Mouth gives evidence of the once splendid rain-forest which at that time clothed the hills along the coast-line. In this extensive area Nothofagus Menziesii has previously been reported from the following places only, the authority for each locality being given in parenthesis:—(1) Ravensbourne, Dunedin Waterworks and West Taieri† (Dunedin Field Club, 1916: 21); (2) near Leith-Waitati Road (Watt, 1924: 674-75); (3) Mount Cargill, above Main North Road (Petrie, 1896: 573).
Exhaustive enquiries among bushmen and others brought to light a few more localities, and field-work has augmented the list, so that to the above we have been able to add the following:—(4) Pigeon Plat (a few trees only), (5) Fergusson Creek (a stand of considerable area in higher forest with remnants also near Leith-Waitati Road, and on south side of Double Hill), (6) Bethune Gully (a mature stand in the midst of rain-forest), (7) Flagstaff Creek (isolated trees along the creek bed), (8) Boulder Hill (stand on the south side of this hill), (9) Traquair Burn, Outram Glen (trees along creek bed, evidently a remnant), (10) Taieri Mouth (remnant of once large stand which has been cut down and milled), (11) Source of south branch of Waikouaiti River, (12) Silver Peaks (an extensive area of pure southern-beech forest).
No doubt other isolated trees or groups of trees are still unrecorded, and at many places they have been cut down. Several such spots have been pointed out to us by settlers,* but as all evidence of their existence is now destroyed their occurrence at these points cannot be accepted for the purposes of this paper.
The above newly-recorded areas were carefully examined, and in several of them the trees were counted, the diameters of their trunks measured, the occurrence of seedlings and saplings noted, and the age of saplings determined by counting their annual rings. Notes were also taken of the composition of the surrounding forest, of the undergrowth immediately under the Nothofagus trees and also that of the forest floor-covering, especially with regard to its light-reducing properties.
[Footnote] †After West Taieri comes the convenient “etc.,” which not merely tells nothing, but is misleading, since it suggests that other areas of Nothofagus Menziesii were well known, and that the species was fairly common.
[Footnote] *From experience we find that records of Nothofagus by settlers must be received with great caution, since besides species of Nothofagus many other trees are “birch”—as they call such—to them.