Forest Vines to Snow Tussocks: The Story of New Zealand Plants
Macquarie Island at 54°S is the furthest south and the chilliest of the outlying islands. It is long and narrow, about 35 km by 3-6 km, and although the highest point is only 433 m above sea level, it rises like a wall from the sea to a narrow plateau dotted with ice-formed lakes. In general form it is not unlike some of the narrow, flat-topped mountains of the Otago region. Geological evidence suggests that it was completely under ice during the last glaciation and it is generally believed that the present flora derives from long distance dispersal since that time. During the last glaciation however, sea level was 100 m or more lower than now so it is possible that some plants survived on land which is now below the sea.
On the raised beach terraces and lower cliff slopes the vegetation is mostly dominated by large tussocks of the grass Poa litorosa which develop trunk-like bases of sometimes a metre in height (Fig. 115). Stilbocarpa polaris, which belongs to a distinctive genus restricted to the southern extremity of New Zealand and its subantarctic islands, is associated with the tussocks particularly in places enriched by animals. Although Stilbocarpa belongs to the mostly woody Araliaceae (including Pseudopanax) it is completely herbaceous with large sort leaves, after the style of rhubarb, with blades sometimes as much as half a metre across. The flower heads are also large with masses of small, yellowish green flowers. The upper cliff slopes are mostly occupied by a short tussock grassland in which Pleurophyllum hookeri may be a conspicuous component. Pleurophyllum is a genus restricted to the subantarctic islands, but related to both Celmisia and Olearia. The three species of Pleurophyllum are notable for rosettes up to a metre or more across of broad, longitudinally ribbed leaves, and for their colourful flower heads. In P. hookeri the flower heads, coloured reddish purple, are relatively small as their 'petals' are greatly reduced.
There is a sudden change of vegetation type on the windswept summit plateau; there is much exposed rock debris and the conspicuous plants are of cushion form. Some of these are mosses of the genus Racomitrium, but the most common cushion plant is Azorella selago of the Umbelliferae (carrot family). This is the only occurrence of the genus in the New Zealand region, but it and related genera are prominent in the Andes of South America where some species form huge cushions page 206larger than our 'vegetable sheep'. Azorella selago also occurs at the southern tip of South America and on other islands scattered around the Antarctic Ocean. The Azorella cushions form a distinctive pattern well described by Gillham:156 'On leeward slopes with downhill winds Azorella plants coalesce laterally to give terraces as much as a mile long, the ribbon development arising from the prevention of both upward and downward growth by the wind. As many as 100 terraces, each about 1½ yards high, may run parallel to the contours to form a gigantic staircase.' The cushions grow directly on rock or glacial till and form little or no peat, suggesting an affinity with the high alpine cushion moorland of the flat-topped Otago mountains. Dwarfed versions of the Pleurophyllum and Stilbocarpa grow in the cushions as well as, surprisingly, the fern Pyrrosia serpens. The latter is often common as a sun epiphyte on trees in New Zealand and other species of the genus occupy similar habitats in the Asian tropics.
In all there is a modest total of 40 native species of vascular plants on Macquarie. Two grasses among these, Deschampsia penicillata and Poa hamiltonii,157 are considered to be endemic although related to other subantarctic species, and three others are not found elsewhere in the New Zealand region, but are shared with several other higher latitude islands as well as the southern tip of South America — Azorella selago, already discussed, Acaena adscendens and Ranunculus biternatus.158
About six of the remaining species are shared only with the New Zealand mainland and the rest are scattered around the subantarctic zone including the New Zealand subantarctic islands, and in some cases the New Zealand mainland or both New Zealand and Australia.
The low endemism of the Macquarie flora and the wide distributions of most of the species indicate recent derivation. Nothofagus logs, determined anatomically as belonging to South American species, have been washed up on Macquarie, but after such a journey it is unlikely that any seeds would have survived in bark crevices.159 Fern spores could have arrived by wind and seeds could have travelled internally or externally with either migratory sea birds or land bird stragglers.