Forest Vines to Snow Tussocks: The Story of New Zealand Plants
These islands lie about 820 km east of Banks Peninsula at 44°S. The main island is about 45 km north to south with a narrow waist 8 km across, widening to about 50 km in the north and to about 25 km in the south. The island is generally low lying with the highest point in the south only 284 m. About one quarter of the island's area is taken up by the Te Whanga lagoon and there are a number of other smaller lakes. Pitt Island, 16 km by 8 km, a little to the south, is the only other island of any size.
The Chatham Islands are a little greater in area than the Aucklands and lie in milder latitudes, so they have a larger vascular flora of about 320 species with a higher proportion of woody plants. At least 29 of the species and 8 susbspecific taxa are endemic and the great majority of the remainder are shared with New Zealand.
Rainfall on the main island is moderate, ranging from a minimum of 600 mm in the lowlands to probably over 1200 mm on the southern table land. Cool south-west winds predominate, atmospheric humidity is high, skies are often cloudy and temperatures are relatively cool for the latitude. Frosts, however, are rare and a few droughts exceeding 15 days can be expected each year. It is not a climate that one would expect to lead to the extensive development of peat, but in fact 60 per cent of the land surface is covered by peat and peat soils. Most of the peat is in raised bogs maintained by rainwater and hence is of low fertility. Wright,168 who surveyed the soils of Chatham Island, suggests that a high level of salt deposition from strong winds may inhibit decay of plant litter and so encourage peat formation on the many flat or gently sloping sites.
Again there are some difficulties in reconstructing the original vegetation as a long period of Maori occupation was followed by European settlers with their attendant sheep and cattle. Early botanical accounts, especially that of Cockayne169 early this century, are helpful in determining the nature of the original vegetation cover. At these earlier times, although there were still extensive forests, a number of the hillslopes and ridges at lower elevations and some ridges at higher elevations were occupied by a low plant cover dominated by bracken fern. As on the mainland this community was probably induced by Maori fire.page 213
The forests of the Chatham Islands are relatively low as none of the taller trees of the New Zealand mainland are present. There are no conifers at all, no beeches (Nothofagus), nor any of the other canopy dominants such as tawa (Beilschmiedia tawa), kamahi (Weinmannia racemosa), hinau (Elaeocarpus dentatus), or northern or southern rata (Metrosideros robusta, M. umbellata). In more open sites a notable absentee is the cabbage tree (Cordyline australis).
On steeper slopes, too well drained to permit the development of peat, the forest is relatively rich in species. A close canopy at 6 to 13m is dominated by endemic species: Coprosma chathamica (the largest species of the genus), Pseudopanax chathamicus, Melicytus chathamicus, Myrsine chathamica, the striking yellow-flowered Brachyglottis (Senecio) huntii, and on more fertile soil the nikau palm, Rhopalostylis sapida.
Among the few understory species are kawakawa (Macropiper excelsum) and tree ferns, particularly Dicksonia squarrosa and D. fibrosa. The common vines are supplejack (Ripogonum scandens) and Muehlenbeckia australis. A number of epiphytes are present including the fern Pyrrosia serpens and the orchids Earina mucronata and E. autumnalis, but there are no asteliad nests.172 This type of forest is now restricted to gullies in the south of the main island (Fig. 118) and the southern part of Pitt Island.
On more or less level sites in the lowlands, karaka (Corynocarpus laevigatus) dominated where drainage was good, giving way to Plagianthus regius var. chathamicus on the most fertile soils and to Sophora microphylla on calcareous sediments. On coastal cliffs and some dune country Olearia traversii played a similar role to that of pohutukawa (Metrosideros excelsa) in the northern North Island. In swampy situations there was a distinctive forest of this species in combination with Coprosma chathamica. These forest types have been almost entirely cleared for farming.
Plant communities associated with bogs (Fig. 119) are restricted to the main island and these are mostly found on gentle slopes at higher elevations where rainfall is high and mists frequent.page 214
Bog Forest is dominated by Dracophyllum arboreum and was once very extensive. It is now largely restricted to the southern tableland. The Dracophyllum has quite broad leaves when young, but the adult leaves are needle-like. This species often begins life as an epiphyte on a tree fern and eventually becomes free standing on its own coalesced roots. Of Dracophyllum arboreum Wright168 says: 'In many respects a parallel can be drawn between the kauri tree (Agathis australis) of Northland … and Dracophyllum arboreum in the Chathams. Both trees accumulate a large mound of organic residues, both condition strongly the soil leaching processes and bring about podsolisation, and both become the dominant species when the soils are sufficiently conditioned. The litter in the case of each species consists of leaf, twig and bark residues of high resinous composition.' The tree ferns Dicksonia squarrosa and D. fibrosa are prominent in the understory and filmy ferns on the forest floor and, as epiphytes, are a prominent feature.
In wetter places areas of this type of vegetation formed a mosaic with bog forest and were known as 'clears' by the settlers. The dominant species is the bamboo-rush, Sporadanthus traversii, which forms a deep and fibrous peat. This genus with its single species is restricted to New Zealand and despite the inroads of farming is still to be found locally in the Waikato Basin of the North Island as well as the Chathams.
In the wettest hollows of the bogs Sphagnum moss predominates and shrubs of Dracophyllum scoparium and the purple-flowered Olearia semi-dentata are scattered throughout. Most of the 'clears' have been completely altered by repeated burning.
A low shrubbery dominated by the endemic Styphelia robusta occupies dry ridges in the peatlands of the southern tableland.
A full range of coastal habitats, from sand dunes to rocky cliffs, can be found in the Chatham Islands. A number of the species are the familiar plants of such sites on the New Zealand mainland, but there are also some notable endemics. Among these, certain large herbs once formed a band of lush vegetation at the top of many sandy and stony storm page 216beaches: the nettle Urtica australis, Embergeria (Sonchus) grandifolius and Myosotidium hortensia. M. hortensia, now widely cultivated, could be called a giant forget-me-not. The leaves, like those of Stilbocarpa in the su-bantarctic, are rhubarb-like and almost fleshy and the flowers in large heads are blue in the centre grading outwards to white. It is a plant stock find very palatable and is now quite rare. Today's sand dunes are dominated by the introduced marram grass.
Endemics of sea cliffs include Hebe chathamica, H. dieffenbachii and the grasses Poa chathamica and Festuca coxii. Myosotidium hortensia may still be found in inaccessible places. Dense shrubberies of Olearia chathamica may be prominent along cliff tops. A large, grey-green, remarkably soft-leaved Aciphylla, A. dieffenbachii, was formerly common along the southern cliffs of the main island and is still frequent on Pitt Island. A smaller species, A. traversii, is a plant of tableland bogs, but it, too, has been reduced by stock.
In highly fertile peaty places riddled with muttonbird burrows, grow luxuriant masses of Carex trifida and the endemics Leptinella featherstonii and Senecio radiolatus. Such sites are now confined to the smaller islands.
With their absence of groups of poor dispersal ability, particularly conifers, the Chathams have an isolated oceanic rather than a continental flora. New Zealand seems to be the principal source area but, as with the subantarctic islands, the presence of a significant element of distinctive endemics indicates quite a long period in isolation.