Plant Communities of the Mokohinau Islands, Northern N.Z.

By Mary E. Gillham Department of Botany, University of Melbourne

The group consists of about 12 islands and a scatter of reefs with Fanal Island (not visited) lying approximately 6 miles S. E. of the main group. The largest, Burgess Island, is between ⅝ and ¾ ml. long, about ¼ ml. wide and covers approximately 400 acres. The relative size of the others in the group can be seen in Fig. 1.

II. Geology And Soil

The rocks are igneous with the exception of a small patch of raised beach conglomerate at the landing place. The group is part of a chain of volcanic islands and rises to 366ft (112 m.) above sea level at the Burgess Island lighthouse. Burgess

Figure 1
Mokohinau Islands. Map Showing Location of Seabird Colonies, Transects and Soil Sample Areas

Island consists of an extrusive cumulo-dome of glassy rhyolite of late Pliocene age surmounted by pyroclastic rocks and intruded by rhyolite dykes and the andesite plug on which the lighthouse stands (Fleming, 1950).

Non-organic soil exposed and eroding on the cliffs is of sandy or gravelly texture but most of the island soils have a fair amount of incorporated humus. Of 15 samples tested all were on the acid side of neutral, the pH ranging from 5.2–6.6 (average, 6.1). Readings are inserted in the appropriate localities on the map in fig. 1.

III. Rainfall And Exposure

The annual average rainfall is from 32–33in but only 9in had fallen in 1957 up to the middle of August and little more was expected, as the summers from August onwards are generally dry. In 1956, however, 60in had fallen, the highest total since records were first kept by the lighthouse personnel 20 years previously.

Heavy seas from the Pacific Ocean strike the northern and eastern coasts of the island, but a certain amount of protection is afforded in the W. and S. by Northland and Great Barrier Island. The exposure suffered is much greater than on the forested island of Little Barrier, 20 miles to the S., but Fanal Island supports a covering of bush. Nevertheless, the small patches of relict bush on Burgess Island.

are confined to sheltered, W. facing slopes, suggesting that, once cleared, bush species may not readily regenerate in the most exposed situations. Even the most salttolerant species, pohutukawa (Metrosideros excelsa), taupata (Coprosma repens) and ngaio (Myoporum laetum) showed signs of severe salt scorching more than 200ft (61 m.) above sea level and a considerable distance from the coast.

In a recent severe N.W. gale the sea had broken across the low-lying part of the island between the Blowhole and the landing beach and transformed the freshwater marsh there into a brackish one. The water level rose and all existing foliage was killed, regeneration not taking place until several months had elapsed (Smith, in lit.).

IV.—Burning And Grazing History

The islands were previously inhabited by Maoris, whose descendants return annually for the mutton bird harvest. It seems likely that either regenerated or original forest remained after Maori occupation; this was probably destroyed subsequently by fire rather than by grazing, although both would have contributed towards preventing its regeneration (Tohana in lit.).

There had been no burning for many years when the lighthouse was erected in the early 1880s, but the vegetation was quite open (Anderson in lit.).

An early lighthouse keeper, Sandager, writing in 1889, listed 7 woody species as important constituents of the vegetation (Carmichaelia, Coprosma, Metrosideros, Myoporum, Olearia, Pittosporum and Veronica), but stated that these were more or less scattered and stunted owing to the lack of shelter. In 1957, Olearia and Pittosporum were not recorded, Veronica (Hebe) was seen only on the smaller ungrazed islands and only 1 tree of Carmichaelia was found on Burgess Island (more on the western isles) suggesting a further reduction in woody species during the past 70 years.

From at least as early as 1920 the lighthouse keepers grazed stock on Burgess Island and burned the vegetation at intervals of approximately three years to keep the sedges (Mariscus and Scirpus) in check. In 1932 Trig Island and Maori Bay Island in the W. were burned by fishermen (Anderson in lit.) the vegetation having regenerated in the subsequent 25 years to a flax (Phormium) community with scattered shrubs.

It is fairly certain that grass seeds (Paspalum and Stenotaphrum and probably also Lolium and Dactylis) were broadcast on Burgess Island after burning (Anderson in lit.) and these now occupy a considerable area.

V.—Vegetation

the bush and the buffalo-grass of the flat across which came winds from the quarter with the smallest distance of fetch. Lighthouse Hill gave shelter from winds from the Pacific.

The bush occupied a low steep cliff and the boulders at its foot and was accessible to grazing animals which had severely pruned the Macropiper

Its preservation as the only inland community of native bush which had escaped the depredations of fire and grazing livestock on Burgess Island, suggested that the constituent species might be more resistant to these two suppressive factors when not having to suffer the added rigours of exposure to the worst salt-bearing winds. It is of interest in this connection that the only two stands of Metrosideros surviving on grazed parts of Burgess Island were on sheltered W. facing cliffs near the Cauldron in the N. and Pohutukawa Gully in the N.W., whereas on the ungrazed islands the species occured also on exposed cliffs.

The presence of the garden species Viola odorata and Zantedeschia aethiopica and the proximity of the old tapu Maori grave in a walled, overgrown garden a few hundred yards away, suggested that this sheltered locality, near the only beach where it was practicable to make a landing, might have been occupied by a former island community.

With the exception of Blechnum and Pyrrosia, both of which were rare on the islands, the following species were recorded only in this mixed bush:—

3. Flax Communities

All but the lowest of the smaller, ungrazed islands were dominated by Phormium colensoi which formed thickets 4–6 ft. (1–2 m.) high and was scarcely penetrable in places. Metrosideros excelsa was abundant with it, Cassinia retorta, Carmichaelia aligera and Myoporum laetum frequent and Hebe salicifolia and Muehlenbeckia complexa occasional. Important constituents of the ground flora were Arundo conspicua, Pimelea urvilleana, Pteridium esculentum and Scirpus nodosus.

These outer islands had suffered less modification from human occupation than had Burgess Island and the proportion of aliens in the flora was smaller—33% as opposed to 48% on Burgess Island. Less time was spent investigating the outer islands but this would not wholly account for the fact that less than half as many species were recorded there as in the more heterogeneous flora of Burgess Island (54 and 121 spp. respectively).

Isolated flax plants occurred on the more sheltered of the inaccessible cliffs of Burgess Island where they found a refuge from grazing animals rather than because of their affinity for sea spray. Where not pushed back to this marginal habitat by grazing, Phormium preferred slightly more inland situations, giving way to other spp. on exposed coasts (see Fig. 2)

4. Sedge Communities

The two most important Cyperaceous plants on the islands were Scirpus nodosus and Mariscus ustulatus, but an unidentified species of Carex was locally dominant on exposed, ungrazed cliffs.

a. Scirpus nodosus

Scirpus nodosus formed dense swards on the northern, western and eastern peninsulas of Burgess Island most remote from the centres of human occupation. As the latter were approached the Scirpus communities became progressively more

open, with European pasture species growing between the tussocks until they eventually merged into grass pasture with Scirpus occasional or absent.

In the extreme W. Pteridium esculentum was co-dominant with the Scirpus but this became rare on slopes with an exposed aspect. On E. facing coasts the Scirpus became patchier and merged with exposure-tolerant grasses such as Holcus lanatus, finally giving way to belts of halophytic species.

High above sea level on the W. of Trig Island the Scirpus was co-dominant with Phormium and it persisted as an important subordinate species in the main flax communities of the western isles.

b. Mariscus ustulatus

Mariscus ustulatus covered much less ground than Scirpus and occurred mainly on fairly wet ground and cliffs. It was sometimes associated with Scirpus or pasture plants or formed a belt between these and the Leptocarpus simplex or Disphyma australe of the lower cliffs.

The species was apparently fairly plastic in its water requirements, growing on both water-logged flats and well drained slopes. With Leptocarpus simplex and Stenotaphrum secundatum it co-dominated the marsh behind the landing bay, but gave way to Leptocarpus, Cotula coronopifolia, Isolepis cernuus and grasses in the wetter parts of the Cauldron flat which it dominated.

Very little was seen on the western isles where it may have been unable to compete effectively with the ungrazed Phormium of similar but more robust life form.

c. Leptocarpus simplex

Leptocarpus simplex (Restionaceae) dominated not only areas with a high water table but also a number of well drained cliff areas subjected to heavy spray. Its distribution was essentially coastal and the water around its roots was often brackish.

5. Grass Communities

Grass communities were of two main types, almost pure swards of Stenotaphrum secundatum and mixed swards of European grasses and legumes, often associated with Scirpus nodusus. They occurred around the centres of human activity viz:— the landing jetty, the lighthouse, the keepers' dwellings and the S. E. block of the island which was occupied by an R.N.Z.A.F. camp during the World War II.

a. Buffalo grass

The strongly-growing alien Stenotaphrum secundatum occupied the sheltered central valley of Burgess Island, and varied from a lawn-like turf a few cm. high where stock congregated around water troughs, to dense mats 1 m. thick. This last growth habit was that most commonly seen, the top of the mattress-like layer being fairly level where subjected to little wind action but undulating, from 15 to 100 cm. deep where at all windswept, giving local turbulence as in the hummocky Holcus lanatus swards of exposed British clifftops (Goodman and Gillham, 1954). Mariscus ustulatus often replaced the Stenotaphrum locally where there was much falling spray or where the water table rose above the surface seasonally. The Stenotaphrum seemed to approach exposed coasts only in moist areas where falling spray was diluted. The dense growth habit of the plant, however, rendered it fairly immune to spray damage because there were always young shoots pushing up through the protective mat of old ones which might suffer damage in salt storms.

The aggressive growth of the dominant excluded most subordinates and the purity of the swards is illustrated in the accompanying table where Stenotaphrum is seen to form an average ground cover of 94%.

Floristic Analysis of Two Buffalo Grass Communities.

VI. Plant Zonation in Relation to Exposure

Fig. 2 shows the prevalence of coastal belts of halophytes on cliffs with a N.E. aspect. On more sheltered S. W. cliffs these belts are too narrow to be included on the vegetation map but the western cliffs of Trig Island were not explored, so the omission of coastal belts there may be of no significance.

Zonation from the sea upwards on the E. coast of Trig Island where the situation was uncomplicated by grazing was as follows:—

1. Disphyma australe. A. narrow discontinuous strip descending to 3–12 m above sea level, depending on the local topography and exposure.

2. Carex sp. Drooping clumps occupying crevices in the Disphyma zone or on vertical faces higher up where there was roothold for little else.

3. Myoporum laetum. Dense thickets seldom more than 1 m high on steep slopes; sometimes descending to the lower limit of land vegetation, but apparently less spray-tolerant than Disphyma or Carex. Mostly pure but with patches of these 2 spp., Coprosma repens and Phormium colensoi.

4 (a). Phormium colensoi. Dominant of the main slopes with patches of Myoporum in the lower part, Metrosideros excelsa and other spp. in the upper part.

(b). Metrosideros excelsa. Dominant on the steeper more broken cliffs at this level.

5. Muehlenbeckia complexa and Scripus nodosus co-dominant with the Phormium on windswept crests and Pteridium esculentum co-dominant in local shelter. Arunda conspicua and Myoporum quite common.

Leptocarpus simplex frequently replaced some of the Disphyma zone near sea level on the other S. W. islands or on cliff edges 50–80 m high in the N. and E. Other salt marsh plants found 70 m or more above sea level on the N. and E. coasts were Salicornia australis, Samolus repens, Spergularia media, Cotula coronopifolia and Isolepis cernuus, the roots of the last two penetrating the dung pellets of sheep in the rockier situations.

Coprosma repens seemed to be the most exposure-tolerant of the shrubs, as on islands of Cook Strait and Bass Strait (Gillham, 1960b). The leaves of Myoporum laetum suffered severe salt-scorching in almost all localities, but the plants as a whole, though stunted, survived to cover considerable areas. Where the Myoporum was damaged so also were mesophytes such as Plantago lanceolata and some of the native and alien pasture grasses.

Soon after the mat of Stenotaphrum secundatum had been cut away from around the Metrosideros excelsa saplings on Lighthouse Hill the leaves of the young trees suffered severe salt damage in a bad storm, although growing much further from the coast than was normal (Smith, in lit.) A less xeromorphic leaf type than usual, formed in the shelter of the Stenotaphrum, may have contributed to the plants' susceptibility.

Fig. 4 shows zonation of plants in three coastal areas; the boulder beach near the Burgess Island landing, the cliffs of Pohutukawa Gully in the N.W. of Burgess Island and a lower-lying Metrosideros community in the N.E. of Maori Bay Island.

Fig. 5 shows cliff zonation in four other localities: transect VII from Disphyma through the alien Dactylis, Lotus and Holcus to pasture containing native Muehlenbeckia, Oxalis and Dichondra: transect VIII from Disphyma through the alien Lotus, Lolium and Holcus to the native Scirpus: transect IX from Disphyma through Lotus, Holcus and the native Leptocarpus and Pimelea to cassinia scrub: transect X. from Myoporum, Disphyma and Arundo through Pimelea and Muehlenbeckia to Phormium, Scirpus and Carmichaelia with aliens unimportant (see Fig. 1 for localities).

VII. Relationship of Vegetation and Grazing Animals

1. Grazing

In August, 1957, Burgess Island supported a small herd of about 12 dairy cattle, a herd of feral goats numbering not more than about 30, and a few pigs, the sheep having been removed the previous March.

The goats lived mainly on the N.E. segment of the island, which was accessible only by a narrow isthmus and not visited by the cattle, but they made occasional sorties along the northern cliffs to the W. Some of the cattle were pastured on the European grasses between here and the lighthouse, one of the bulls shared the central of the five northern peninsulas with the pigs and the milking cows had the run of the rest of the island except for part of the inland face of Lighthouse Hill.

During the winter, the dairy, cows fed mainly at night, resting on the Stenotaphrum of the “home paddock” during the day but grazing very little. After the evening milking they drank at the troughs in the Stenotaphrum of the landing flat, then dispersed W., N.W., and S.E. to the areas occupied by European pasture spp. and Scirpus nodosus. They grazed their way outwards, eating the grasses and legumes and reaching towards the edge of the more peripheral zones of pure Scirpus

Where livestock converged on a gateway across the narrow neck of land N. N.E. of the lighthouse the ground was 75% bare, even during the moist winter season. The three natives, Dichondra repens, Disphyma australe and Senecio lautus were the most abundant species in this more maritime environment and only 61% of the 18 spp. recorded were aliens.

Of the 18 spp. growing among the short trampled Stenotaphrum around the water troughs all were aliens.

Species surviving trampling were all of low growth habit and many were hemicryptophytes, those normally having tall inflorescences (e.g., Cirsium, Erigeron and Rumex) producing either stunted ones or none at all.

3. Manuring

Species stimulated to robust growth on the heavily dunged soil around the stockyard were Amaranthus lividus, Chenopodium murale, Malva rotundifolia?, Modiola caroliniana and Phytolacca octandra—all aliens.

A characteristic seedling flora occured on each of the many patches of cow dung along the top of the boulder beach. It had nothing in common with the Muehlenbeckietum of the community as a whole and the seeds had probably arrived with the dung and germinated more or less simultaneously to give an even stand a few cm high Ninety-one per cent were aliens (unless Geranium dissectum be regarded as cosmopolitan) although only 40% of the beach spp. were alien:

VIII. Gull Colonies

Mokohinau was described by Fleming (1946) as the chief breeding place of the red-billed gull (Larus novae-hollandiae) in the North Auckland area apart from the Three Kings Islands. He estimated the 1944 population as consisting of approximately 8,000 birds; Buddle (1947a) estimated the 1945 population as 18,000–20,000 birds, of which 13,000 were breeding.

The gulls were resident on the island from the end of August until March. Thus for about seven months each year certain areas of vegetation were subjected to trampling and disturbance and their floristic composition modified accordingly. During the intervening five months, which coincided with the moister winter growing season, there was considerable regeneration of an annual flora which was largely destroyed when the birds returned in early spring. There was, in addition, more permanent regeneration in vacated areas due to local changes of nesting site from year to year.

For the years 1944 and 1957 nesting colonies are plotted in Fig. 1, and a S. W. coast site occupied in 1936 (Buddle, 1947b) but not necessarily representing

the entire population, is also included. Buddle reported considerable variations in nesting sites between 1944 and 1945, with some of the northern colonies enlarging and coalescing.

The vegetation able to perennate in the gull colonies throughout the breeding season was largely halophytic and Disphyma australe was particularly characteristic, covering large areas up to and above 60 m above sea level on the northern cliffs where the gulls were numerous. This species was equally characteristic of other types of seabird colonies in New Zealand and S. E. Australia (Gillham, 1960a). In many colonies Disphyma formed a continuous mat broken only by circular patches where nests had been built on top of it. The nest material had dispersed by the time the birds returned in the subsequent year and the patches were still largely bare, but tentative colonisation by the following species had occurred:–

Mariscus ustulatus survived well in the gull colonies but Scirpus nodosus was much less conspicuous, although often dominant of the surrounding terrain.

Floristic composition of the gull vegetation was analysed by means of valence squares and transects, and the results are summarised in Fig. 6. The analyses were carried out when the cyclic succession had reached its most advanced state—at the end of the growth period when the birds were returning in the latter part of August to initiate the destructive phase.

The native halophytes, Disphyma australe and Senecio lautus were the most constant species, showing 76% and 74% frequency in 130 valence squares and 86% and 82% frequency in the 50 squares of the two belt transects. Bromus catharticus (44% and 48% frequency) was the most characteristic of the aliens (cf. tern and gannet colonies of New Zealand and Australia, Gillham, 1960a).

Many of the European pasture species appeared to benefit from the enhanced fertility of the gull colonies in the less exposed areas (4–8 in Fig. 6) but the American Buffalo grass seldom occurred in nesting colonies. A lush growth of Parietaria debilis was typical of well manured crevices but was rare away from the gulls on Mokohinau (although growing as a boulder beach species on Little Barrier and a woodland species on Bass Strait islands).

A vacated black-backed gull's nest (Larus dominicanus) in a Metrosideros-Phormium community was occupied by lush Tetragonia trigyna with Asplenium flaccidum, Bromus catharticus, Chenopodium allani, Stellaria media and Trifolium repens.

IX. Petrel Colonies

The grey-faced petrel (Pterodroma macroptera) is the most abundant burrowing bird of the Mokohinau group. No attempt has been made to estimate its numbers, but Buddle (1947a) states that it is well distributed over all the islands, particularly the three western ones. This is the northern mutton bird, and 3,000–3,500 chicks are taken annually by Maoris during the early part of December (Sandager, 1889, and Buddle, 1947a); a large proportion of the catch at present comes from Fanal Island. Burgess Island colonies are mapped in Fig. 1, but no survey was made on the western islands, only those burrows seen at the head of Maori Bay being indicated on the map.

As long as the soil was of sufficient depth and suitable texture for burrowing, the petrels seemed little concerned as to what plants they burrowed among, and the 11 areas investigated were occupied by 11 different plant communities, ranging