Abstract

The peat bogs of the Awarua Plains, in Southland, formerly very extensive, are fast disappearing as a result of drainage and cultivation. The plant cover contained many plants which elsewhere were always restricted to subalpine areas. These included both the flowering plants and the cryptogams, of which only the former have previously been catalogued. The present paper serves to complete a record of a unique plant formation before its final replacement and disappearance.

Prior to the European settlement of Southland and, indeed, for the first two decades of the present century, the Awarua Plains, extending from Invercargill to Bluff Harbour and for some miles to the north, was largely occupied by an extensive series of swamps and bogs covering some thousands of acres. Botanically the area held a special interest from the presence of a vegetation markedly sub-alpine in character though occurring almost at sea level. Smaller areas of a similar nature occurred as far north as Owaka, near Makarewa, and again in Stewart Island, but none were so extensive, or populated by such varied florula, even though subalpine species at sea level are still more numerous in the south of Stewart Island.

In 1927 a very comprehensive catalogue of the pteridophytes and flowering plants of the Awarua Plains was compiled by J. Crosby Smith and published in Vol. 58 of the Transactions of the New Zealand Institute; but no account of the even larger cryptogamic flora has hitherto been made, notwithstanding the rapid replacement of these bogs by arable land consequent on drainage and cultivation. In a few years the opportunity will have vanished.

L. Cockayne in “The Vegetation of New Zealand” (2nd edition, p. 202) attributes the existence of extensive areas of bog in the west and south of the South Island to “an abundant rainfall, a comparatively low summer temperature, and frequent cloudy skies”, and in the case of the Awarua Plains we might add to poor drainage owing to their low elevation above sea-level.

These Awarua Plains were occupied by a number of distinct plant associations, one of the most extensive of which had as its dominant species Donatia novaezealandiae, a species elsewhere restricted to subalpine bogs. In this association the subalpine element was strongly represented both in the phanerogamic and in the cryptogamic sections of the vegetation. The occurrence here in quantity of Herpolirion novae zealandiae, Oreostylidium subulatum, Oreobolus pectinatus, Carpha alpina, and many other species elsewhere montane and subalpine gave these bogs a unique character, as did the presence of Siphula medioxima and of Cladonia sullivani in the associated lichens. Besides the species elsewhere restricted to subalpine areas, there were numerous others which were commonest in such localities though sometimes found at lower levels.

Over much of the area, the peat, six to eight feet deep, overlies a layer of white gravel, but the depth thins out almost to vanishing point in some localities. Drainage channels have been cut and as a consequence the peat has partially dried out and greatly shrunk, leaving the Donatia cushions considerably elevated above the lowered surface. As a consequence, these have slowly died, and today scarcely a cushion remains in areas where they formerly existed in hundreds. Many of the associated

species are also fast disappearing, and on these dried-out soils manuka (Leptospermum scoparium) is now commonly present to complete the destruction of the former cover. Extensive fires have recently swept much of the area.

As already indicated by Crosby-Smith, the distribution of the various associations is consequent very largely on the water content and degree of aeration of the peat. Much of the area south of the railway line to Bluff is occupied by fresh water and saline swamps and marsh rather than by bogs, and here Phormium and Carices are abundant but replaced by Leptocarpus simplex in the saline marshes. The only cryptograms noted in the swampy areas were Sphagnum falcatulum and S. australe and an occasional patch of Polytrichum commune.

To the north of the railway and road Donatia bog was formerly extensive, with the drier, more elevated areas occupied by tussocks of Danthonia rigida var. rubra or by Leptospermum. However, large areas were dominated by Gleichenia circinata and by Hypolaena lateriflora. On the driest ridges bracken (Pteridium esculentum) was quite common.

On the margins of the peat area the Danthonia tussocks were more massive, and the ground between them often sheltered epacrids and heaths, as well as the three species of Cladonia constituting the sub-genus Clathrina.