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Forest Vines to Snow Tussocks: The Story of New Zealand Plants

Cushion Bogs

page 183

Cushion Bogs142

Bogs develop in the subalpine and low alpine zones where drainage is poor.143 Suitable sites are hollows formed by glaciers and flat areas under high rainfall conditions on glacial terraces, mountain passes and flat topped mountain ridges. Where rainfall is high, rain may provide most of the water in the bogs, but in drier eastern areas drainage from the surrounding slopes makes the major contribution. When the water table is near the surface, the dominant plants in the bogs are of cushion form. Cushion plants are freely but closely branched; and the ultimate twigs, with their upper living and lower dead leaves, are so closely pressed together lengthwise that the exposed tips of the living leaves form a firm, continuous, often unyielding surface. Many cushion plants can be stood on without suffering any visible effects. The leaf tips of each ultimate branchlet may form a circular pattern or be compressed into an hexagonal shape. Depending on the species the cushions range from a few centimetres to a metre or more in diameter. They are broadly convex to almost hemispherical in form and are separated by shallow to quite deep hollows.

A number of sedges and related plants occur in mountain bogs throughout the country. The Centrolepis and Gaimardia species form soft moss-like cushions; Oreobolus pectinatus with its leaves distinctively arranged in two rows in one plane forms a firmer cushion while other species of the genus form flat mats. Of other cushion plants, Donatia novae-zelandiae forms much larger, broadly convex, extremely dense cushions whose dark green leaf tips contrast strongly with the numerous, small white flowers in season (Fig. 104). It occurs from the southern North Island southwards and also in the Tasmanian mountains.

As well as the cushion plants there may also be prostrate but less compact dwarf shrubs including Dracophyllum muscoides, D. politum and the pigmy pine (Lepidothamnus laxifolius). In the hollows between the cushions and prostrate shrubs are a variety of small sedges and mosses and small species of several large genera including Astelia linearis with its red jelly bean-like fruits, Celmisia glandulosa, Gentiana lineata (in the far south) and dense mats of Coprosma perpusilla (formerly C. pumila) Several species of the insect catching sundews (Drosera) with their glistening leaf glands are also frequently present.

In the highest parts and margins of bogs, red tussock (Chionochloa page 184 rubra) may often be found. In the lowest, wettest parts, soft masses of the yellow-green bog moss (Sphagnum) predominate.