The south-west of Chatham Island encompasses dramatic coastal scenery, a fertile farming belt and forested catchments rising to a moorish tableland containing lakes and low peaks. The south-west supports the most extensive forest on Chatham Island, expanses of upland bamboo rush & Chatham aster and important populations of threatened plants and animals. It is the only breeding sites in the world for taiko and the Chatham Island mudfish. The largest protected area occurs here with the Tuku Nature Reserve and adjoining South Chatham covenant at about 2,500 hectares.
The tableland is highly distinctive. The flat to undulating topography belies the fact it is the highest region of Chatham Island at 250-280m altitude. The island’s tallest peaks occur on the tableland’s northern edge. The south-west climate is damp compared to the north of the island. Cloud cover is more persistent inland resulting in higher rainfall and lower evapo-transpiration. The gentle topography slows drainage producing wet ground conditions that favour moisture loving species such as sphagnum moss and the endemic bamboo rush (native only to Chatham Island). The latter grows with swamp heath and the endemic shrub swamp aster which produces abundant purple daisies. Dead plant material only partially decomposes in the wet conditions and accumulates as peat beds. Under the bamboo rush/heath community the peat may be 9m in depth it overlies ash from the Taupo eruption of 20,000 years ago while water can be found about 50cm below the surface.
The extensive tableland has a mosaic of vegetation types that reflect the degree of wetness and disturbance. Peats may form domes thereby creating better drained portions that allow woody plants like tarahinau trees to establish (the largest tree in this NZ genus). Wetter zones may develop at the edges and encroach on forest that established in drier conditions. People have burnt the rush/shrubland thus the vegetation pattern also reflects the time since burning and the original plant cover. “Clears” originally referred to the bamboo rush aster community but more recently also refers to burnt sites with ferns and woody regeneration. The true clears are considered one of the most distinctive plant communities of the Chathams.
Pre-historic fires probably gave rise to some of the tableland lakes. The well drained banks around the lakes support more diverse forest than that of the tableland; the mixed broadleaved-forest contains hoho valuable to wildlife such as parea. The water side conditions and open banks are ideal for the critically threatened Chatham Island toetoe and Chatham speargrass. The threatened endemic Chatham Island mudfish is known only from Lakes Rakeinui, Te Rangatapu and Tuku-a-Taupo.
The south west contains the forested headwaters of several catchments draining north towards Waitangi, including the Mangahau and Nairn. Further south this forest continues into the large Tuku-a-tamatea and Waiparua valleys. The majority of forest on Chatham Island occurs in this sector. At least three quarters of the island’s forest has been lost to the combined pressures of animals & storm damage and some clearance. The tarahinau broadleaved forest is dominated by endemic trees most prominently tarahinau but also hoho, CI matipo & CI karamu and hokataka. Tree ferns, supplejack and ground ferns often fill the understorey. Orchids may be prominent on the ground especially two endemics a greenhood and spider orchid. Regeneration failure, through browsing of seedlings, results in loss of the understorey and the filling of gaps in the canopy by tree ferns. None the less this is the most resilient forest with tarahinau being the toughest tree of all. Once freed of stock pressure regeneration can be spectacular even where bracken has replaced the forest. Most trees can also establish by perching on and taking over tree ferns.
Rare trees of the forest include the endemics rautini (CI Christmas tree which has showy yellow flowers) and Barker’s koromiko (the largest Hebe in the country). At the turn of the 20th century rautini was common contrasting with today; both possums & feral animals have had a devastating impact. Possum-free Pitt Island provides an interesting comparison with its abundance of rautini. CI nikau have largely disappeared along with the coastal forest however a good population occurs in the shelter valley flats of the Waiparua River (just 2 other good sites occur on Chatham Island). Nearer the sea the forest in the valleys develops a lowland character with species such as kawakawa appearing. Little of the lowland forest remains so that species like nikau and ribbonwood are largely missing; the better soils have given rise to fertile farmland home to non-native sheep and cattle.
The forests are a key habitat for several endangered birds including taiko, a large gadfly petrel, rediscovered in the Tuku-a-tamatea catchment by David Crocket in 1978. Taiko number about 150 with up to 15 pairs breeding each year in burrows on the forest floor. Parea, the endemic Chatham pigeon, declined from an island wide distribution to just 40 birds in 1990. Habitat protection and predator control has seen recovery to more than 500 birds concentrated in this part of the island. Other native birds include CI red crowned parakeet and CI warbler which have a wider distribution but which are well represented in the south-west. In the last few years the Sweetwater covenant has been partially predator fenced and used to relocate taiko from the Tuku and for the expansion of the Chatham petrel population, until recently only remaining on South-East and Pitt Island.
At the coast the banks and bluffs feel the impact of the prevailing south-westerly winds and driven salt spray; the steepest bluffs in the south are 200m tall and those in the west around half that. Much of the original vegetation has been lost but pockets survive where there is protection from grazing. Remnants include; coastal scrub & flax, coastal grass & herb-field and salt tolerant turfs. Coastal scrub combines tightly woven trees and shrubs often made up of akeake, Dieffenbach’s koromiko, kawakawa, mahoe and hokataka. The south-west is the second most important area for the threatened kakaha or Moriori flax which grows in this scrub and flax.
Next to the sea shore and on exposed promontories salt tolerant succulents form a dense carpet. The most noticeable element is the endemic CI iceplant with its vibrant display of pink flowers in spring. These mats and adjacent rockier sites are also home to small populations of the poo-loving Cooks scurvy grass; this plant was eaten with other coastal species in the early exploration days to ward off scurvy.
The third community combines herbaceous species and native grasses like Cox’s tussock and Poa chathamica. Fencing of parts of th
e coast has seen the expansion of Coxella and CI sowthistle and to a lesser extent forget-me-not from ledges onto gentler slopes, these threatened mega herbs are highly admired members of the Chatham flora.
The South west corner of main Chatham is one of the least visited regions of main Chatham but nonetheless an extremely important habitat for our threatened plants and birds.