Tuatara: Volume 31, Issue 1, July 1991
New Zealand Botany with a Difference — The Chatham Islands
New Zealand Botany with a Difference — The Chatham Islands
The geology, including plant fossils, and climate of the Chathams are outlined. The presence of schist similar to that of Otago suggests a former land connection with New Zealand. Since isolation extreme fluctuations in the area of the islands with changing sea levels probably led to some plant groups, notably conifers, becoming extinct. The main vegetation types are reviewed — coastal herbs including a number of endemics; coastal forest dominated by Olearia traversit; inland forest on peat-free sites; bog forest on peaty sites dominated by Dracophyllum arboreum; and open bog vegetation. Observations on twenty-one Chatham and Pitt Island reserves or vegetation remnants are presented.
Key words: Chatham Islands, Chatham Islands flora. Chatham Islands vegetation, New Zealand biogeography.
During January 1990 Dr B.V. Sneddon and I visited the Chatham Islands with a guided tour. It was our first visit and it proved to be fascinating, particularly botanically, but also historically and culturally.
The Chathams lie 820 km east of Banks Peninsula towards the end of a raised ridge of sea floor known as the Chatham Rise. The dimensions of the main island approximate those of Stewart Island, but there the similarities largely end. The Chatham Islands are low-lying and bear a surprising amount of peat. In addition to a number of small lakes, a significant part of the area of the main island is occupied by the shallow Te Whanga lagoon separated from the open sea on the east by a sand spit.
Climatically there is a similarity to Stewart Island, even though the Chathams, straddling 44°S, are at about the same latitude as Banks Peninsula. Both have a cool, cloudy, breezy climate with fairly frequent showers, although the winters are relatively mild with few frosts. The explanation for this similarity probably lies with what is known as the subtropical convergence where cold subantarctic ocean water meets warmer subtropical water. The convergence runs just south of Stewart Island then swings northward to intersect with the Chathams. In both places south westerlies, chilled by their passage over subantarctic water, are the predominant winds.
The main Chatham Island has a dumb-bell shape and extends 45 km north to south. The northern peninsulas extend east-west for 50 km; the “waist” narrows to 9 km, although much of this is occupied by the Te Whanga lagoon and the Southern Tableland is 25 km across. Pitt Island, 20 km to the south-east measures 16 × 8 km but the nearby South-east and Mangere Islands are much smaller.
Geologically the Chathams are quite diverse (Hay et al., 1970). The wide northern part of the main island is basically schist, a light coloured, thin layered rock. The schist is exposed at the coast but is buried inland under consolidated sand dunes and peat. The landscape of the north-western peninsula is made more interesting by lines of volcanic hills, some quite conical, which are the solidified necks of volcanic vents. Some are composed of basalt, others in part or whole of agglomerate, comprising angular fragments of basalt embedded in an often paler matrix. These hills are surrounded by aprons of basalt lava and in one case where this reaches the sea there are impressive hexagonal columns.
The central waist of the main island is largely limestone, with hard and soft versions, but again much of it is covered by sand and peat. On the western side page 24 of the lagoon limestone cliffs are exposed, which would have formed when the present site of the lagoon was open sea.
The Southern Tableland comprises more or less horizontal layers of basalt lava and ash in the southern two-thirds and orange-hued tuff in the northern third. The latter derives from volcanic ash that originally settled on the ocean floor.
The Tableland is covered with peat with a cluster of lakes in the southern part. There are also scattered volcanic hills, one of which attains the highest altitude in the islands at 284 m. In the south the Tableland is fringed by impressive cliffs up to 230 m high.
Pitt Island is also low-lying, while the nearby Mangere Islands are flat-topped and impressively cliffed. South-east Island and the southern two-thirds of Pitt Island are basalt, while the Mangeres are mostly agglomerate. In the northern portion of Pitt there is a zone of tuff then a zone of carbonaceous sandstone.
The oldest rocks in the Chathams are the schists dating to the Permian 200 million years ago. They are known to extend at least 130 km west along the Chatham Rise and probably underlie all of the island group. These schists are very similar to those of Otago in the southern South Island of New Zealand and may have been continuous with them when the Chathams and New Zealand were part of the ancient continent of Gondwana.
The next oldest rocks are the sandstones of the north of Pitt Island, dating to the mid Cretaceous, about 100 million years ago.
The basalts of Pitt and the Southern Tableland as well as the tuffs of the latter are probably Eocene in age, 53-37 million years ago, as are the limestones of the “waist” of the main island. The volcanic rocks of the northern main island are more recent, Pliocene to Miocene, up to 26 millions years ago.
During recent geological times, that is over the last few million years, New Zealand has been considerably elevated by mountain forming crustal movements which still continue. The Chatham Islands have been little affected by these events and have remained low-lying. However they have been greatly affected by fluctuations in sea level resulting from extreme climatic fluctuations during this time. For example at the maximum of the last glacial period, about 40,000 years ago, so much water was locked up in ice that sea level was about 130 m lower than at present. Not only were the islands of the Chatham group joined together, but land extended well beyond them. Conversely, in much warmer times in the early Pleistocene, more than a million years ago, sea level was at least 288 m higher than now and the Chathams may have been completely submerged or at best reduced to a few small islands at the present site of the higher parts of the Southern Tableland. Even in the latter case such a reduction in land area would probably have resulted in the extinction of some non-coastal species.
Peat is common almost everywhere and its formation is encouraged by the cool, moist climate and the poorly drained gentle terrain. Peat has been forming for many thousands of years during the geologically recent succession of glacials and interglacials. It is suggested that peat has formed most actively during the cooling phases of interglacials.
There are about 320 native vascular plants in the Chathams and of these 29 species and 8 varieties are endemic. So about 10% of the flora is endemic, forming an element that is more conspicuous than that proportion would suggest, as endemics dominate many of the plant communities. Equally striking is the absence of a number of common New Zealand plants — there are no conifers, no beeches (Nothofagus), no cabbage trees (Cordyline), no manuka (Leptospermum) and none of our larger page 25 flowering trees, for example: tawa (Beilschmiedia), hinau (Elaeocarpus), ratas (Metrosideros) or kamahi (Weinmannia).
The carbonaceous material of Pitt Island sandstone has yielded fossil pollen of ferns, conifers (including probable ancestors of kauri and rimu) as well as a few flowering plants, the age of this material however is Cretaceous, a time when the ancentral Chatham Islands formed part of Gondwana and shared a wider flora. A much more recent pollen sample from peat yielded some conifer and Nothofagus pollen, but the amounts were so small that it was concluded that they had been blown in from New Zealand, as still happens today. So when did conifers disappear from the Chathams region? Was it many millions of years ago, or more recently, when the area of land was drastically reduced or eliminated by rising sea levels?
Ferns and related plants comprise about 19% of the Chathams flora, significantly higher than in New Zealand (8%), but in accord with the prominence of ferns on isolated central Pacific islands.
The vegetation of the Chatham Islands has been greatly altered by human activities, particularly in the past 150 years. The polynesian Moriori was isolated in the Chathams for 800-1000 years. Then came the European whalers, sealers, missionaries and farmers and finally Maori invasions in the 1830's. To reconstruct an earlier vegetation pattern we have to turn to the accounts of observers such as Dieffenback (1841).
The sea shore, both rocky and sandy, was well stocked with species, some shared with New Zealand and mostly still common, while others were very distinctive endemics. Among the latter are the giant forget-me-not, Mysotidium hortensia, an endemic genus, with large rhubarb-like leaves and large white to blue forget-me-not flowers; the giant sow thistle, Embergeria grandifolia, another endemic genus; Leptinella (Cotula) featherstonii and the large, soft textured Aciphylla dieffenbachii. These are palatable to stock and are now difficult to find on the main island, but are more common on some of the smaller islands.
A short distance back from the sea on sand dunes and other sites there was a belt of forest dominated by the tree daisy Olearia traversii, in association with Myrsine chathamica. The former is said to play the role of pohutukawa in the Chathams. It is certainly surprising to see this species as a sizeable tree, as it is grown as a hedge plant in New Zealand. This coastal forest has now largely disappeared for firewood or fenceposts, but there are still scattered trees, unfortunatley often dead or dying.
Further inland on sloping or sandy sites, without peat, was a more diverse forest varying in composition, depending on soil fertility. On more fertile sites karaka (Corynocarpus) dominated, sometimes in association with groves of nikau palms (Rhopalostylis) and ribbonwood (Plagianthus). This type is now reduced to patches, some now fenced, in the northern part of the main island and to the sides of stream-cut valleys in the Southern Tableland and similar sites on Pitt Island.
On peaty sites, on ridges and plateaus that were not too wet, there was a species poor forest type dominated by the tree Dracophyllum arboreum, often with an understorey of Dicksonia tree ferns. Dracophyllum forest has largely disappeared from the north of the main island, but is still well represented on the Southern Tableland and Pitt Island. This forest formed a mosaic with open areas known as “clears” by the European settlers, that were too boggy to support forest. The dominant plant of the clears was the “bamboo-rush” Sporadanthus traversii, a robust rush-like plant that formed a deep fibrous peat. Associated with this were a number of other species including the purple-flowered Olearia semi-dentata and the small Dracophyllum squarrosum. The original “clears” have also been greatly reduced by draining for farming and by frequent fires which have lead to a widespread cover of bracken fern. There are still a few good examples of this type of vegetation in the north of the main island and rather more on the Southern Tableland.
Vegetation of Sites Visited
In the remainder of this article accounts will be given of the localities we visited (see Fig. 1) with observations on the more notable species present.
Main Island — Central and North
(A) Henga Reserve
This is situated immediately behind the Chatham Lodge, where we stayed, on old sand dunes leading to the sea. It was at the fringes of the forest that I had my first close look at Chatham Island plants. I recognised a Myrsine, (M. chathamica), with fairly broad, but short leaves often notched at the tip. This species is also reported to occur on Stewart Island. There was also a Melicytus (M. chatamica) locally called mahoe, but quite distinct from ours. The leaves are fairly thick, flat and stiff in texture with prominent teeth. A common twining vine puzzled me for a while until I realised that it was Muehlenbeckia australia, although it is more robust and has much larger leaves than our version. The karakas seemed no different from ours, but a Coprosma (C. chathamica) was a big surprise. The exposed leaves are quite small and rolled after the fashion of taupata, although unlike those of the latter they are not very shiny., However, the remarkable thing is that this Coprosma is a tree up to 15 m tall with a trunk sometimes more than half a metre in diameter!1 (Fig. 2). It must surely be the biggest species of the genus. The trunk often has a pinkish tinge and has a hammer mark pattern something after the fashion of matai. Another distinctive feature of this species, is that it has broad juvenile leaves that are much larger than those of the adult (Fig. 3). Furthermore, as well as having small pits or cavities (domatia) on the undersides where secondary veins meet the midrib, as is usual in Coprosma, these leaves also have domatia scattered over the entire leaf undersurface, at junctions of higher order veins. I have seen this in some leaves of taupata (C. repens), although scattered domatia are not so frequent in this case.
Going into the bush I observed that karakas, although large, were scattered and that the commonest tree appeared to be Myrsine. The latter has trunks up to about 40 cm diameter, some of which have one or more peculiar collars of protuberant corky tissue contrasting with the general smoothness of the rest of the trunks. Scattered trunks of the Coprosma were also seen. In the undergrowth, kawakawa (Macropiper excelsum) caught the eye, the leaves seeming larger than ours and notable for not having holes eaten into them. The commonest undershrubs however were saplings of the Melicytus up to about head height. I was told that these established after the fences were erected. Looping stems of Muehlenbeckia were common but I saw no supplejack. Unexpected too was the absence of ferns, apart from the epiphytic Pyrrosia. Perhaps the sandy forest floor is not suitable for ferns and instead supports a lush growth of the so-called New Zealand spinach — Tetragonia tetragonioides. Further towards the beach the forest became lower and more windswept with Myrine becoming even more abundant. Emerging from the forest we found the first sand dunes were covered with a luxuriant growth of introduced grasses, but nearer the sea there was more open sand and more native species including Pimelea arenaria, Cyathodes parviflora with white berries, Coprosma acerosa and grey patches of the endemic Geranium traversii in the sand (all that I saw had white flowers in contrast to the pink flowers of the cultivated form). The Coprosma looked rather different from ours, being of a darkish hue and not forming tangled masses, but having its branches pressed closely against the sand. The beach was an impressive page 27 sight, sweeping northwards along Petre Bay and, apart from us, completely deserted. All the beaches we saw in the Chathams had fine white sand and this combined with the pale blue sea and white breakers gave quite a tropical effect, unfortunately belied by the coolness of the air.
(B) Motuhau Point
This is located on the western shore of the southern part of the lagoon where there are good rock carvings on an overhang of the limestone cliffs. The cliffs support a variety of trees and shrubs including the tree coprosma and Myrsine chathamica, as well as the Chatham lancewood, Pseudopanax chathamicus, and kowhai, Sophora microphylla.
The Chatham lancewood has a juvenile form but it is not as distinctive as its namesake on the mainland (Fig. 4). The juvenile is unbranched for some years with leaves that are quite broad but longer than those of the adult and not deflexed. They have a few coarse teeth distally while my observations suggest that the fully adult leaf is broadly rounded at the tip and without teeth. The adult tree has a freely branched, rounded crown (Fig. 5).
The kowhai is restricted to limestone on the Chathams and doesn't have the small-leaved rather chaotically branched juvenile frequently developed by the mainland form. A notable shrub was the bushy, greyish-green Hebe dieffenbachii, one of the three endemic hebes.
Where the beach abuts the cliffs there were a number of good sized trees of Olearia traversii (Fig. 6), in many cases with their shaggy-barked trunks steeply inclined towards the lagoon. The water of the lagoon tasted quite salty and there is salt marsh in places, with familiar mainland species. The beach is whitish with many shells and a prominent plant is the large dark-green nettle Urtica australis (Fig. 7). The species was new to us but it is found elsewhere in the subantarctic islands, Stewart Island and Fiordland. Unlike the shrubby Urtica ferox, on the mainland, this species does not have visible stinging hairs, but they soon revealed their presence when I tried to collect a specimen. My fingers tingled for several days afterwards.
The following day we headed for the north-east point of the island at Point Munning but stopped first at:
(C) Blind Jim's Creek
This is also on the western shore of the lagoon but in its northern part. Dedicated searchers were able to find a few fossil shark's teeth among the shells of the beach. The limestone cliffs supported some of the same species observed on the previous day, but in addition we saw a few plants of Geranium traversii and the New Zealand linen flax Linum monogynum, with pale blue, not white, flowers. There were also several clumps of New Zealand flax, Phormium tenax. This species occurs also along streams and around lakes and was formerly much more common. It is rather different from the common form in New Zealand. It is pale green with very broad, but not particularly long, leaves and the seed capsules are short and tubby.
From here we drove to the north-eastern peninsula via the narrow strip of sand separating the northern edge of the lagoon from the open sea. We called first at the Wiesner farmhouse which is notable for the remains of a Sunderland flying boat which crashed nearby many years ago, but also notable botanically for a private bush reserve.
(D) Wiesner's Bush Reserve
Here we saw a number of nikau palms (Rhopalostylis sapida). The ribbonwood (Plagianthus regius) was abundant along the path through the reserve. Unlike mainland plants, the Chatham ribbonwood does not have a divaricating juvenile with small leaves varying in size and shape. However, the leaves of young plants page 28 are much smaller than the adults, although similar in shape. Some very large karakas were present as well as small Myrsine chathamica and an undergrowth of particularly large kawakawas (Macropiper) and young Melicytus.
We continued on to Point Munning which has also been partly fenced by the Wiesners.
(E) Point Munning Reserve
Here the special interest is a seal colony. The surrounding schist rocks support a diverse flora of fleshy-leaved plants in peaty crevices, including the endemic fern Asplenium chathamense and Senecio radiolatus. Phormium is abundant behind the rocks. The nearby Te Whakaru island, which we did not visit, is connected at low tide and still has vigorous Myosotidium and Embergeria among washed up shells behind small beaches. On the slopes above the point there is an area of shrubs, partly protected by a Phormium “hedge”, which appeared to be a pure association of the endemic Corokia macrocarpa. Our next stop was
(F) Kaingaroa Beach Reserve
This was where the first Europeans, led by Lieutenant Broughton of Vancouver's expedition, landed and claimed the island in 1791 and named it after the Earl of Chatham.
To get to the beach one has to go through the Kaingaroa township's rubbish tip to an area of the beach that was fenced about 1939. Here there is a good colony of the robust sand binding sedge pingao (Desmoschoenus spiralis), its orange-green tufts of leaves looking decidedly handsome against the white sand of the dunes. Associated with it were vigorous clumps of the giant sow thistle Embergeria in flower and seed. The flowers are several cm across and grade from pale purple on the outside to pale yellow at the centre (Fig. 10).
(G) Kaingaroa Point Reserve
This is approached through Kaingaroa township where we noted some robust plants of Embergeria near houses. The reserve includes the former rubbish tip and sandy flat areas behind the coastal rocks supported a few embergerias and myosotidiums, but the most notable feature was a mat of prostrate Hebe chathamica in full flower. This species also grows on coastal cliffs, where it has a draping growth habit. If we had done our background reading more thoroughly, we would have gone a little further on to where a few plants of the endemic Leptinella (Cotula) featherstonii still grow among bird burrows.
We also paid a visit to the seacliffs at Matarakau further east on the norther side of the peninsula.
Even here, on level surfaces, there is a layer of peaty soil and it is probably such soil, on ledges and in crevices of the cliffs, that results in the abundance of herbaceous plants. This is particularly the case with the endemic ice plant, Disphyma papillatum, with its attractive pink flowers, which forms draping massed down the cliffs.
(I) Hapupu Bush Reserve
This was our last stop of a busy but interesting day. Here are found the famous Moriori bark drawings or dendroglyphs on karaka trunks (Fig. 9). Many large karakas dominate this forest, with a few myrsines as well contributing to the canopy. Since the fence was constructed there has been abundant regeneration of young Melicytus in places. No Macropiper and no vines were noted and again ferns were rare. Going towards the sea the karakas diminished and low Myrsine became more common. In open places, Pratia arenaria was abundant on the sandy ground.
Hapupu Bush is located part way down the eastern side of the lagoon near the old airstrip. Passengers were driven in trucks across a shallow part of the lagoon to the Waitangi side!
(J) Nikau Forest Reserve
This, our first stop on the next day, is about a kilometre from the north-western shore of the lagoon. There is quite a large grove of tall nikau palms on one side of the bush (Fig. 10) with draping lichen on some of the trunks as an indication of the moistness of the climate. The main part of the forest is dominated by large karakas with some adult Pseudopanax and Coprosma. Melicytus and Myrsine are uncommon as is the undershrub Macropiper. Unlike the earlier forest visited, the abundantly regenerating tree in the undergrowth is Pseudopanax rather than Melicytus.
Both supplejack (Ripogonum) and Muehlenbeckia are common vines. The former had both flowers and fruits. Phymatodes diversifolius, Asplenium polyodon and an Earina occurred as epiphytes even on a few nikaus (Fig. 11). The Earina was flowering abundantly and the flowers were right for E. mucronata, but the leaves and stems seemed much too short for that species. The plant would match the description for Earina aestivalis, a species which has not been accepted for some time. Some of the earinas in this forest were longer stemmed than the majority and we later saw much longer specimens in southern forests also in full flower. So this poses something of a puzzle.
There were a few Dicksonia squarrosa tree ferns in places, so with the epiphytic species, ferns were somewhat better represented in this forest.
(K) Lake Kaimomi Reserve
This is near the north-eastern shore of the lagoon and is a good example of bog vegetation (Fig. 12). Large hummocks of the bamboo rush Sporadanthus traversii are conspicuous with their long, slender and closely aggregated, apparently leafless stems. This important peat forming species also used to be common in the Waikato bogs, but is now becoming rare there. Other important species are the purple flowered shrub Olearia semi-dentata, the needle-leaved shrubs Dracophyllum squarrosum, Cyathodes parviflora and Coprosma propinqua var. martinii (this and C. acerosa are the only divaricate species on the Chathams). In more open places Gentiana chathamica was in full flower. Phormium was conspicuous around the lake margins as well as the strange sedge Carex sectoides raised above the water on stout “trunks” built up from its own dead leaves and roots over many years. Carex secta on the mainland has a similar form.
(L) Wharekauri coast
This is on the northern side of the north-western peninsula where, contrasting with the white sand of the beach, there is an outcrop of dark basalt. The seaward side of this has a heavy fringe of that most impressive brown seaweed, the bull kelp (Durvillea antarctica), its massive fronds heaving and swirling in endlessly changing patterns in the heavy surf. The bull kelp is a prominent feature of rocky Chatham coasts.
(M) Basalt Columns
These are located on the southern side of the north-western peninsula. The approach is through extensive white sand dunes frequently capped by bushy shrubs of Pimelea arenaria. There are also a number of large Olearia traversii trees, some of which have been blown horizontal but are still growing. The stream flowing through the sand is stained brown by peat and again rock crevices near the sea are well stocked with salt tolerant plants. The more or less vertical, hexagonal columns, that formed as the basalt cooled, are an intriguing sight and comprise a reef and an adjacent shelf and cliff.
Two further forest localities were visited in the centre and north of the main island.
(N) Cape Pattison Reserves
Cape Pattison is towards the end of the north-western peninsula on the northern side. We drove along the western beach, then walked alongside a stream to the first reserve, which was fenced only a few years ago. As a consequence the forest floor is still quite open. The dominant trees are large karaka and Coprosma chathamica and smaller myrsines. Muehlenbeckie and supplejack vines are also quite common. Here we saw a sleeping possum under a hollow log — a signal reminder that this herbivorous pest is also established on the Chathams. Continuing out of the reserve towards the beach on the other side of the peninsula, there is first a grassy area, then a second fenced reserve, enclosing a rather battered example of coastal Olearia traversii forest. It is hoped that this will regenerate to provide a good example of this distinctive forest type.
(O) Plum Tree
This is on the western shore of the southern part of the lagoon. The trees here were the tallest we saw, with a mixture of karakas and coprosmas with massive trunks. In a hollow there was a grove of quite large ribbonwoods. On limestone outcrops near the shore we also found the small herb Colobanthus muelleri, of special interest to Barry Sneddon.
Unfortunately this forest is not yet reserved and as a consequence it is open and grassy underneath.
Main Island — Southern Tableland
We spent two days visiting this part of the island, one on the west side and the other on the east.
(P) Tuku River Reserve
This is on the western side of the tableland near the Tuku camp, where amateur naturalists come each summer to study the tuku petrel once thought to be extinct. The camp is on a level area surrounded by forest dominated by Dracophyllum arboreum, which often forms quite large trees. Its juvenile leaves are very broad, but the adults are needle-like and form a deep litter on the forest floor, which is not unlike that of pine needles. Associated with the Dracophyllum is some Coprosma chathamica, Myrsine chathamica and Pseudopanax chathamicus and a greater abundance of Dicksonia tree ferns in the understorey. Relatively slender wheki (D. squarrosa), its trunks with persistent leaf bases, is the most common, but D. fibrosa is the most striking with its enormous spreading leaf crowns, 3 to 4 times the diameter of its relative, and its massive trunks like elephant legs. The trunks are largely formed of slender, wiry interlaced roots. In Dicksonia fibrosa the old leaves hang downwards and persist as a thick “skirt” around the trunks. This appears to prevent the establishment of epiphytes, with the exception of the fern Rumohra adiantiformis, which often establishes at the vulnerable junction above the old and below the new leaves.
Before descending into the steep-sided Tuku Valley we visited an open very boggy area within the Dracophyllum forest. Here there were scattered rather stunted whekis, a few Pseudopanax and the small Dracophyllum squarrosum. Myrsine coxii, a species that favours very wet sites, was quite frequent. This has a slender trunk, up to 4 cm, ending in a small, dense crown with small leaves notched at the tip.
Crossing the fence into the Tuku Reserve we found many juveniles, broad leaved Dracophyllum arboreum growing through the bracken. These have established since the fence was erected.
From a little below this point we could get a good view into the valley and could see that Dracophyllum forest occupied the crest of the opposite ridge, while a quite different type of forest occupied the steep slopes of the valley (Fig. 13). page 31 Contributing to the latter were scattered large karakas and trees of Pseudopanax, Myrsine and Coprosma. I did not see any Melicytus. However, by far the most abundant plants were tree ferns in great profusion. The dicksonias were the most common, with mamaku (Cyathea medullaris) and ponga (C. dealbata) noted here and there. The gully tree fern, C. cunninghamii was more frequent than either of the last, forming groves in places.
Near the forest edge, as we descended, were a group of Dicksonia fibrosa, which had lost their skirts in this exposed situation and as a result supported several large epiphytes — Pseudopanax (Fig. 14), Coprosma, Myrsine, Macropiper, Corokia and Dracophyllum arboreum. The latter often begins life as an epiphyte, although probably more often on wheki. However, in this cool, moist often misty climate, it is easy for plants to grow as epiphytes, particularly on the fibrous surfaces of Dicksonia fibrosa trunks.
Other small epiphytes that we saw were an Earina in flower with long stems which must be E. mucronata; Tmesipteris elongata on a tree fern, rather short Lycopodium varium, the fern Rumohra adiantiformis, and many filmy ferns including the kidney fern. So, unlike the northern forests, ferns and related plants are conspicuous here.
Among vines Ripogonum (supplejack) and Muehlenbeckia were common and we noted Calystegia tugoriorum in one place. The ferns Phymatosorus scandens and P. diversifolius were frequent and Lycopodium volubile covered the ground in open places.
Hebe barkeri called itself to my attention when I bumped my head on its inclined, slender, but quite tall trunk in quite a dense part of the forest.
(Q) Rangaika Reserve
This is on the eastern side of the tableland more or less south of the township of Owenga. To reach the reserve we walked across boggy country, which has probably been burnt several times in the past. Bracken fern was common at first. There were also many small shrubs of Dracophyllum squarrosum and the endemic Cyathodes robusta with its striking berries — bright red on many bushes, but pink or white on others. Spreading brown-coloured Lycopodium volubile was common in places and here and there we noted Lycopodium varium — robust upright plants, also brown, with strongly recurved strobili. This is the typical form and habitat for the species and I still find it difficult to accept the current view that the epiphyte, formerly known as Lycopodium billardieri, should be included in it.
The endemic Olearia semidentata with its handsome purple flowers became more and more common as we walked south. When we reached the reserve we walked along the access strip beside the western fence. Here Gentiana chathamica was abundantly in flower. Here and there were young Pseudopanax chathamicus and Myrsine chathamica and also bushes of Myrsine coxii and Coprosma propinqua.
We investigated a bush patch, which included Dracophyllum arboreum, Pseudopanax, Coprosma, both dicksonias and one quite tall Hebe barkeri. A fern that we hadn't seen before was Leptopteris hymenophylloides.
In a very boggy place with much sphagnum moss a few plants of the endemic Aciphylla traversii were discovered, some with seed heads. This species is becoming rare so it was good to see a number of seedlings in the vicinity.
Eventually we came to where the fence ended at the southern cliffs and the notable feature here was several bushy small trees of an Olearia with thick strongly toothed leaves. This has been regarded as an endemic species, O. chathamica, but Wilson, in his account of Stewart Island plants includes it in O. oporina of Stewart Island and Foveaux Strait islets. There were only a few flowers, white with purple centres.page 32
Under the olearias were a few large clumps of Astelia chathamica, a handsome plant with relatively broad, silvery, flexible leaves that are often bent downwards. The fruiting heads are striking with their bright orange berries. I was told that this plant is known locally as “Moriori flax”. It evidently used to be quite common in open bogs as well as in forest, but is now relatively rare. Dieffenbach was probably referring to this species when he wrote — “Another plant which has in its leaves some similarity with the lily tribe is very frequent in these open places. Pigs live in hundreds on the tuberous roots of this and also bracken. The leaves are eaten by goats”. The cliff face was densely covered by shrubby versions of many of the forest species.
Finally, for the main Chatham Island, we stopped at a bridge about half way along the road from Owenga to Waitangi.
(R) Makara River
The river cuts down quite deeply here and there is a great deal of tutu (Coriaria arborea) on the banks. Also here and there were shrubs of Coprosma robusta, so common on the mainland. Close inspection however revealed some differences from the mainland form — the stipule tips are not so black and shiny and the stipules become dry and papery a few nodes from the tip. The latter is a characteristic of some large leaved species, but not of typical Coprosma robusta.
On a day with a brisk southerly wind and alternating sunny spells and showers, we flew in a small Cessna to Pitt Island. This provided a few anxious moments. However the views were spectacular: of the lagoon and its narrow exit to the sea; the Southern Tableland with its lakes; the southern cliffs; the white-capped Pitt and South-East Island and, on the return journey the flat topped Mangere Islands with their sheer cliffs.
We landed on an airstrip about the middle of the island and were taken on the back of a truck to Glory Bay for lunch. After this we drove to the southern part of the island where we saw the Saxon wild merino reserve and visited the adjacent Southern Glory Reserve.
(S) Saxon Reserve
The merino reserve has quite an amount of forest, including the endemic tree Brachyglottis (Senecio) huntii, easily seen even at a distance with its bright yellow flowers. Puzzling were groves of dead trunks of the wheki (Dicksonia squarrosa) with younger plants of the same species growing among them. Perhaps the dead plants were decapitated by a storm.
(T) Southern Glory Reserve (Fenced in 1980)
The tree dracophyllum is common here (Fig. 15) and since the fence many broad leaved juveniles have appeared. Several other trees are present including Coprosma, Melicytus, Myrsine, Pseudopanax and the two Dicksonia tree ferns. Muehlenbeckia and supplejack are common vines and the ferns Asplenium lucidum and A. polyodon were seen.
In open places there were masses of the bidi-bid, Acaena anserinifolia, in full fruit, so we had a busy time removing them later.
From here we went to the:
(U) Northern Glory Reserve
Here the nikau palm reaches its southern limit in New Zealand and it is also the furthest point in the world from the equator of any naturally growing palm.page 33
The treeDracophyllum dominates the forest here in association with Dicksonia tree ferns, but there were also several big coprosmas with quite colourful pinkpurple trunks. We were able to get a close view of several Brachyglottis huntii, all in full flower, at the edge of the forest.
A curious sight returning to the airstrip was what appeared at first to be Cordyline indivisa, which doesn't occur on the Chathams. It turned out to be the trunk of a Dicksonia fibrosa with a bigorous clump of Phormium tenax growing from its top!
During our nine days on the Chatham Islands we were able to visit a wide range of vegetation types and saw a considerable number of the indigenous species, although certainly not all. We gained an impression of a flora that is clearly of New Zealand but nevertheless distinctive within that context. It shares some of the features of floras of isolated oceanic islands, but not others. For example, isolated oceanic islands, such as Tahiti and Hawaii, are entirely volcanic in origin, while the Chathams, although having experienced considerable volcanism, are underlain by schist, one of the continental rock types. Like New Zealand and New Caledonia then, the Chathams are a continental fragment that has become isolated. However, New Zealand and New Caledonia have a strong representation of plant groups of limited dispersal ability, notably conifers and Nothofagus, groups that the Chathams and isolated oceanic islands lack. The latter islands and the Chathams also share a relatively high proportion of the readily dispersing ferns and their relatives.
As noted earlier the Chathams, when part of Gondwana, had a good representation of conifers, although whether Nothofagus was present at some time we don't know. New Zealand and New Caledonia retained these groups in isolation, but not the Chathams. Are there any remnants of the Gondwana inheritance in the present Chatham flora or did it completely disappear, possibly as a result of submergence, with later stocking of the re-emerged land by immigrants derived largely or entirely from New Zealand?
Many of the Chatham species or their relatives are wide-ranging in New Zealand, but there are some that are restricted to the far south and the subantarctic and others to the far north and certain lower latitude islands such as the Kermadecs and Rapa. Among the former on the Chathams are the dracophyllums, the shrubby olearias, the myrsines, Brachyglottis (Senecio) huntii and the nettle Urtica australis, and among the latter are the hebes, Corokia macrocarpa and Sporandanthus traversii. Perhaps this pattern results from the proximity of the Chathams to the subtropical convergence. In cooler times the convergence might move further north and the Chathams would be surrounded by cool subantarctic water that would encourage the establishment of higher latitude immigrants. In warmer times the reverse would apply.
Craw (1989) has a different interpretation based on the idea that New Zealand is a fusion of two crustal arcs that may have had quite different origins and have brought with them quite different floras and faunas. One of these arcs is more eastern in New Zealand and characterized by greywacke rock and the other more western and charactertized by schist. Craw's model suggests that the Chathams are also a combination of the two arcs (assuming the greywacke is currently below the sea) and this would account for the northern and southern New Zealand elements in the Chatham flora and fauna, the western arc being associated with southern New Zealand biota while the eastern arc contributed the northern biota.
There are however, as noted earlier, a few distinctive Chatham endemics, including two endemic genera, which have no close relatives elsewhere. These are Myosotidium, Embergeria, Leptinella (Cotula) featherstoniii, and Aciphylla dieffenbachii. It has page 34 been suggested that they may be the remnant of an earlier even more distinctive Chatham flora, that largely disappeared as a result of the almost total submergence of the group during the early Pleistocene.
Descriptions of Chatham species are scattered through accounts of the New Zealand flora as a whole, but I see a need for a separate Flora of the Chatham Islands as a distinctive and isolated part of the New Zealand region. This would require careful comparisons of Chatham species with those elsewhere to clear up some uncertainties. For example, it has recently been suggested that the endemic Olearia chathamica (Wilson, 1982) is in fact the same as O. oporina of Foveaux Strait, although perhaps warranting varietal status, and that the endemic Dracophyllum paludosum is indistinguishable from D. squarrosum of Campbell Island (Wardle, 1987). On the other hand some species considered to be the same as mainland species may warrant at least varietal status, for example. Coprosma robusta and Muehlenbeckia australis. Such a separate flora of the Chatham Islands would be an interesting and worthwhile project.
Buchanan, J. 1875. On the flowering plants and ferns of the Chatham Islands. Transactions of the New Zealand Institute 7: 333-341.
Cockayne, L. 1902. A short account of the plant-covering of Chatham Island. Transactions of the New Zealand Institute 34: 243-325.
Craw, R. 1989. Continuing the synthesis between panbiogeography, phylogenetic systematics and geology as illustrated by empirical studies on the biogeography of New Zealand and the Chatham Islands. Systematic Zoology 37: 291-310.
Dieffenbach, E. 1841. An account of the Chatham Islands. Journal of the Royal Geographic Society 11: 195-215.
Findlay, J.F. 1956. Chatham Island. Wellington Botanical Society Bulletin 28: 1-7.
Given, D.R. and Williams, P.A. 1984. Conservation of Chatham Island Flora and Vegetation. Botany Division, D.S.I.R.
Hay, R.F., Mutch, A.R., Walters, W.A. 1970. Geology of the Chatham Islands. N.Z. Geological Survey Bulletin M.W. Series 83.
Kelly, G.C. 1983. Distribution and ranking of remaining areas of indigenous vegetation in the Chatham Islands, with site notes and introductory test. Department of Lands and Survey Land Resources Inventory map sheet with extended legend.
Lloyd, D.G. 1982. Variation and evolution of plant species on the outlying islands of New Zealand with particular reference to Cotula featherstonii. Taxon 31: 478-487.
Mueller, F. 1984. The Vegetation of the Chatham Islands. Melbourne.
Richards, E.C. 1952. The Chatham Islands. Christchurch.
Ritchie, I.M. 1970. A preliminary report on a recent botanical survey of the Chatham Islands. Proceedings of the New Zealand Ecological Society 17: 52-56.
Taylor, Richard. 1868. The Past and Present of New Zealand. London.
Travers, H.H. 1869. On the Chatham Islands. Transactions of the New Zealand Institute 1: 173-180.
Wardle, P. 1987. Dracophyllum (Epacridaceae) in the Chatham and subantarctic islands of New Zealand. New Zealand Journal of Botany 25: 107-114.
Wilson, H.D. 1982. Stewart Island Plants. Field Guide Publications, Christchurch.
Wright, A.C.S. 1959. Soils of Chatham Island. New Zealand Soil Bureau Bulletin 19.page 35
Fig. 2. Trunk of Coprosma chathamica in Henga Reserve surrounded by regenerating Melicytus chathamicus.
Fig. 3.Coprosma chathamica. The large leaves left and centre are the juvenile form, the small adult leaves are on the right.
Fig. 9. Moriori carving on a karaka trunk, Hapupu Reserve. Note the regenerating Melicytus chathamicus behind the trunk.
Fig. 10. A grove of Nikau palms on one side of the Nikau Bush Reserve with a volcanic hill in the background.
Fig. 11. Nikau palms at Nikau Bush Reserve with clusters of berries and epiphytic Earina mucronata, Asplenium polyodon and lichens.
Fig. 12. Bog vegetation near Lake Kaimomi with conspicuous Sporadanthus traversii and Phormium tenax.
Fig. 13. Forest in the Tuku River Reserve. Across the river the large tree fern crowns are sheki-ponga (Dicksonia fibrosa) and the smaller ones wheki (Dicksonia squarrosa).
1 Taylor (1868) recorded even larger specimens — “several kinds of trees, which in New Zealand are little more than shrubs, here attain the size of timber trees. The karamu, a species of coprosma. is often more than three feet in diameter, and is sawn into planks”.