Vegetation of Little Brother Island, Cook Strait, in Relation to Spray-bearing Winds, Soil Salinity, and pH

By Mary E. Gillham

on the vegetation, limiting the number of species, modifying life form and accentuating the soil erosion initiated by thousands of burrowing petrels.

The author spent eight days on the island from May 15 to 22, 1957, during which period there was an unusually severe southerly gale in which gusts of 105 m.p.h. were recorded in nearby Wellington. For many hours a continuous rain of salt spray was thrown over the 240ft (74 m) high island from the S.—providing a unique opportunity during subsequent days to observe the reactions of the various plants, all of them halophytes.

The vegetation was not subjected to the depredations of grazing mammals and alien plants were few, 3 of the 5 alien species recorded being confined to a few individuals in the immediate vicinity of the lighthouse.

The principal factor affecting the vegetation apart from wind and spray appeared to be the soil mobility arising from the steep gradients and aggravated by heavy rain squalls and burrowing birds.

I. Position, Topography, Exposure and Climate

Little Brother lies approximately 24 miles N.W. of Wellington on a straight line to Cape Jackson, which is about 10 miles further on. The nearest land is Cape Koamaru, 2½ miles to the W., and there are no large land masses obstructing winds from the N. W. and S. Because of the larger “fetch” in these two directions the highest seas strike the cliffs on the N. W. and S.

The island rises precipitously on the E. side, almost as steeply on the S. and N. and more gradually on the W. This W. slope is the only part of the island other than the E. summit (which is occupied largely by the lighthouse tower), which is exposed to the full force of spray-bearing winds from both N.W. and S. Thus, although lying closer to land than any other of the island coasts, the W. side suffers the greatest exposure to spray. The easier gradient there enables the land vegetation to descend closer to the sea than elsewhere, into positions where it is frequently drenched by sea water. Almost the entire slope, to a height of at least 50ft (15 m)above mean sea level is occupied by a degenerate Salicornia australis sward—a vegetation more characteristic of salt marshes than of cliffs.

A scatter of low stacks off the North landing affords a certain amount of protection to the N. W. coast, and a tall unvegetated tidal islet connected to the S. point, Big Brother and other small islets to the S. and S. E. have a similar effect in the S.

The island is approximately rectangular, with its longest diagonal axis running N. to S. and its shortest dimension from N. W. to S. E. A rough calculation from transects taken across it in two directions suggests that the area is about 5 acres, none of which is flat.

The average annual rainfall of the district is between 40 and 50 inches (1,000 to 1,250 mm), the annual average temperature at Wellington and Nelson, the main centres to W. and E., 54° F. (Dept. Agric., 1957).

II. Geology and Soil

The underlying rock in this part of the Sounds district is Schist (N. Z. Geol. Survey, 1956). Outcropping rocks are numerous except in the scrub communities, and considerable areas bear little or no soil, the principal species, Disphyma australe, being rooted in crevices.

Such soil as is present varies from a moist fibrous peat beneath the low Coprosma repens and Hebe elliptica scrub to a friable non-organic soil on the extensive gravel slides of the N. W. There is a higher proportion of the finer soil particles on the Salicornia-clad slopes of the W., giving a soil similar in appearance to the sodium clays so often associated with this species on salt marshes.

All the mineral soils are subject to erosion during heavy rain, numerous gullies 15–25 cm deep scarring the gravel slides after the night of 15/16 May, during which

0.67″ (c. 17 mm) of rain fell in a few hours. The gullies terminated in spreading gravel fans 15–20 cm deep on the rocks below, and much of this displaced soil was washed into the sea on the subsequent night, when a further 0.35″ (c. 9 mm) of rain fell.

The ground is honeycombed with burrows, the roofs of which collapse underfoot except where major slips have occurred, taking both burrows and vegetation into the sea (Plate 17a). The most efficient soil stabiliser under these conditions is Disphyma australe, which trails long distances from the possible points of attachment and gives a protective surface mat.

III. Birds

The principal burrowing bird is the fairy prion or dove petrel (Pachyptila turtur, Kuhl). The diving petrel (Pelecanoides urinatrix, Gmelin) is also very numerous, but this species often burrows superficially amongst the base of the vegetation rather than in the soil. These above-ground tunnels are numerous in the Poa caespitosa tussock and low Muehlenbeckia complexa thickets of the island summit where the soil consists of little more than powdered leaf remains in hollows of the rocks.

Blue penguins (Eudyptula minor novaehollandiae, Stephens) and numerous tuataras (Sphenodon punctatus) geekos and skinks share the burrows with the petrels.

Red-billed gulls (Larus novaehollandiae scopulinus, Forster) nest on the island, but were not present at the time of the author's visit.

IV. Distribution of Vegetation in Relation to Exposure

1. General Community Structure

Twenty-eight species of plant were collected on the island, and all except 3 of the 5 British aliens identified by the D.S.I.R., Botany Division. The list is as follows, the usual d.a.f.o.r. termmology applying to abundance on the island as a whole. Introduced species are marked with an asterisk.

Hebe elliptica and Tillaea moschata were probably very near the northern limit of their range here. Of the Hebe Cockayne (1907a) said: “In the northern parts and N.E. of the South Island it is quite wanting, nor does it occur at all in the North Island”; of the Tillaea “In the South Island it is common in the S. and S.W., but further N. it is quite local … and is finally found in the neighbourhood of Island Bay, Wellington, and its vicinity, but does not occur further N.” (Island Bay is S.E. of the Little Brother.)

The succulent-leaved, close-growing, yellow-green Lepidium oleraceum was of very different form from the attenuated, 60 cm high plants found in the shelter

“Salt scorching” following a less severe gale had been observed a week previously on the leeward, eastern, petrel-burrowed slopes of Stephen's Island, c. 35 miles to the W. N. W., and information relating to damage suffered by the chief species there is included in Table I. Again the softer-leaved plants had become most limp and blackened, and Rhagodia triandra had survived less well than had the other halophytes present.

Boyce (1954) stresses the importance of the entry of salt into the leaves of plants through wind-induced abrasions as against uptake of salt from the soil, pointing out also that much of the stunting and apparent “wind trimming” of coastal shrubs is actually due to killing of the buds by salt spray rather than desiccation.

V. Soil Salinity

Soil samples were collected at 20-yard (18.45 m) intervals along the two transects, passed through a 2 mm sieve, the chlorides extracted with distilled water and titrated with AgNO₃. Unfortunately it was not possible to seal the samples in the field, so soil salinity cannot be expressed as percentage by weight of soil solution but only as percentage by weight of air-dry soil. This brings the figures for the heavy-weighing mineral soils of Salicornietum, Disphyma-clad rocks and non-organic gravel slides anomalously low as compared with the light, friable soils of the Hebe and Coprosma scrub.

Eight random soil samples taken on the S. and W. sides of the island after the gale gave an average reading of 0.99% chlorides, individual readings ranging from 0.10% beneath dense low Coprosma to 1.70% on part of the Poa-covered summit.

Eight samples from the lower N.W. corner averaged 0.89% and ranged from 0.19% in a rock crevice to 2.58% beneath the seaward extremity of the Disphymetum above the N. landing.

Nine samples from burrow entrances where the protection afforded by the depression below the general level was largely outweighed by the comparative absence of sheltering vegetation, averaged 0.86% chlorides and ranged from 0.17% in a superficial diving petrel burrow among the Muehlenbeckia on the leeward side of the island crest to 2.31% in a burrow among Disphyma on the S. side.

The results of 55 samplings are related to the dominant vegetation in Table II. On the basis of soil salinity the 8 communities concerned fell into the expected order with the exception of the Hebetum which, from its position in the general community structure, would be expected to lie 7th rather than 4th. The results are expressed diagrammatically in the top left hand corner of Fig. 4.

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VI. Soil pH

Two average pH readings were made for each soil sample and an attempt made to relate pH and salinity. Moist soil was tested in the field with two grades of pH papers and air dried soil in the lab with a glass electrode.

Results for the transect soils are included in Fig. 3, and show a general relationship of increasing pH with increasing salmity sufficient to mask the expected decrease associated with high organic content in the peats beneath Coprosma and Hebe. The organic nature of the dry grass remains which composed the bulk of the summit soil may have been responsible for the lowness of the peak in the pH curve over the highest salinity peak, as the saline mineral soils of other rock outcrops showed more marked pH rises.

This relationship of high soil pH and high salmity has been observed adjacent to Devon estuaries and Hebridean sea lochs (Gillham, 1957a, b) and demonstrated experimentally (data in course of publication).

In all instances but one the pH of the sample after air drying lay below that of the moist soil in the field, this being in accordance with the findings of Pearsall (1938) for peaty moorland soils.

In the series of samples from the S. and W. of the island the pH of the moist soil ranged from 4.0–7.0, that of the air dry soil from 3.61–6.58. In the N.W. series readings for moist soil pH ranged from 5.0–7.0, those for air dry pH from 4.33–6.40. Equivalent ranges for the burrow samples were 5.0–7.4 and 4.18–6.62.

The results of 53 samplings in relation to the dommant vegetation are set out in Table III and represented diagrammatically on the left hand side of Fig. 4.

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VII. Comparison With Other Cook Strait Islands

1. General Observations

Situated as it is in “The Funnel”, and using only 74 m above the sea, Little Brother Island is probably one of the most exposed of the Cook Strait islands, and the vegetation is markedly halophytic throughout. Of the other Cook Strait islands visited all but the smallest were sufficiently sheltered in parts to bear a reasonably tall growth of maritime bush floristically much richer than the almost pure stands of Coprosma, Muehlenbeckia and Hebe of the most sheltered parts of Little Brother Island.

All were visited during stormy weather, but only the Little Brother soil samples were taken just after the big gale, so figures for salinity and pH were not directly comparable. The chief feature of interest which emerged, in spite of the small number of samples which it was possible to analyse for the other islands, was the fact that the trends of falling salmity and pH from one community to another bore a fair resemblance to those on Little Brother, the sequence descending further to embrace the mixed bush of the most sheltered habitats.

2. Stephens Island

Soil samples were collected on Stephens Island 35 miles W. N. W. of Little Brother on May 12 and 13, 1957, following a gale during which visible clouds of salt spume had been wafted across the 600ft (192 m) high crest of the island to the leeward eastern slopes. These slopes were grazed by sheep and cattle and heavily burrowed by dove petrels with colomes of sooty shearwaters (Puffinus griseus) in patches of bush.

4. Kapiti Island.

Kapiti Island lies on the opposite side of Cook Strait approximately 28 miles N.E. of the Little Brother. Cockayne (1907b) states that: “Kapiti, together with Mana I. and the Brothers, must be a remnant of the ancient land bridge which connected the N. and S. Islands and their vegetation be the remnant of one formerly more extensive”. Floristic differences are thus likely to be associated principally with the larger size of the former, which occupies 4,990 acres and is rather more than six miles long with the exposed western cliffs rising to 1,725ft (531 m).

Ten days were spent on Kapiti during stormy weather in October-November, 1957, but only the summit was visited on the western side. Data relating to “salt scorching” of coastal species was collected on the eastern side, however, and is summarised in Table V. The Kapiti habitat was much more sheltered than any part of Little Brother, a number of woodland species approaching close to the shore, but parallels can be drawn from the relative degree of damage incurred by species common to both islands.

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Of these only two were seen to have suffered severe salt damage on Kapiti, Muehlenbeckia complexa and Anagallis arvensis, and these were confined to the most sheltered parts of Little Brother in the lee of the island crest and the lighthouse buildings. Sonchus oleraceus, which suffered medium damage on Kapiti, suffered more than any other species on Little Brother, although not approaching very close to the sea. The remaining eight Little Brother species appearing in Table V had suffered negligible damage on Kapiti, though five had been damaged on Little Brother, one of them severely.

VIII. Comparison With Coastal Floras Further Afield

1. Cape Egmont

Cape Egmont juts into the Tasman Sea approximately 130 miles N. of Little Brother and is exposed to the full force of the prevailing westerlies. The point on the coast S. of the lighthouse consists of a boulder beach backed by a low ironrich earth cliff up to 6 m high and much undercut by wave action in parts.

Four days were spent on the cape during rough weather in October, 1957, when waves were breaking over the top of the cliff. Showers of sea water were being deposited on the English grass community beyond the indigenous flora of the coastal strip and balls of sea froth had blown several 100 m inland across the hump-studded volcanic flat.

Few of the coastal halophytes suffered damage, and the only native plant seen to be badly salt scorched was Linum monogynum. Many of the pasture plants had been severely affected, genera showing the worst damage being Anagallis, Bromus, Cerastium, Hypochoeris, Lolium, Lycium, Plantago, Polycarpon, Rumex and Trifolium.

A detailed analysis of species distribution was made on one of the volcanic humps which rose behind the cliff edge intercepting spray blowing inland and causing the coastal belt to broaden at this point. Results are summarised in Table VI.

Of the six Little Brother species present the indigenous Salicornia australis, Disphyma australe and Senecio lautus occurred at the seaward margin of the vegetation, the alien Anagallis arvensis, Hypochoeris radicata and Sonchus oleraceus did not. The southern Tillaea moschata was here replaced by the northern T. sieberiana.

A coastal strip of Disphyma separated the main maritime turf of Samolus repens from the sea. The Samolus zone was about 12 m wide on the knoll but decreased to 2–4 m where backed by level meadow. None of the constituent plants rose more than 1–2 cm from the ground surface except Senecio lautus on the inland margin where stems reached up to 5 cm in the lee of small rocks. The community was quite open and eroding into terraces ½ m or less high in places.

The Samolus turf reappeared on the seaward faces of similar knolls much further inland and had been invaded by few of the aliens from the surrounding sown ley, which were so common on the leeward sides of the knolls.

A tumble of rocks on the crest of the knoll in zone 4 (Table VI) gave sufficient shelter for many of the ley species, but Samolus persisted as one of the most important constituents of the crevice flora.

Zone 5 represented the sheltered inland face of the knoll where many of the halophyles persisted only as scattered individuals and alien grasses and weeds of cultivation shared dominance. On more inland knolls completely surrounded by the ungrazed, ploughed, sown meadow it was evidently only the high incidence of sea spray on the windward slopes which prevented these from being invaded by the introduced plants from below.

Zone 6 represented the pasture community in the lee of the knoll with Lolium perenne dominant and Bromus unioloides frequent. Linum monogynum was the only indigenous angiosperm found in this zone.

Further N. sand occurred on the low earth cliff, and a taller tussocky vegetation dominated by Ammophila arenaria, Leptocarpus simplex and Mariscus ustulatus sheltered scattered penguin burrows. Dense wind-trimmed hedges of Lycium ferocissimum protected the paddocks behind.

Three important differences between the Little Brother and Cape Egmont environments were elevation, topograpy and type of ground. The most seaward margin of vegetation in both instances, however, was subjected to sufficient salt spray to have affinities with salt marsh communities, dominated in the one case by

A typical tree zonation found from the coast inwards on some of the smaller islands off the N. and E. of Stewart Island was from Olearia angustifolia to O. colensoi and O. traillii with an undergrowth of Hebe elliptica to Senecio puffini to Dracophyllum longifolium and Leptospermum sp. and so to the mixed forest dominants. The shiny-leaved Metrosideros lucida commonly overhung the water in sheltered inlets.

4. Bass Strait Islands, S. E. Australia, and Islands Off the W. of Britain

Landings were made on 29 islands and reefs in the Bass Strait, Tasmania, approximately 1,500 miles W.S.W. of the Cook Strait habitats between January and May, 1958, and data collected relating to zonation of vegetation in relation to sea spray. Far fewer differences were observed in these specialised coastal habitats than in the inland flora of the two regions, relatively few species being able to withstand the high incidence of spray and those which could occurring in widely separated areas.

That the same species or genera or ones closely related may also turn up in similar habitats on the opposite side of the world is illustrated in Table VII.

Some of the species common to New Zealand, Australia and Britain are cosmopolitan (e.g., Suaeda maritima) others are introduced in Australasia, but so well established as to have become incorporated in natural communities where few other aliens occur.

Sometimes, as with Asplenium, Salicornia and Spergularia, the Australasian and British species differ only in small characters, in other cases the equivalent ecological niche is occupied by a closely related genus (e.g., the “scurvy grass” of Britain (Cochlearia officinalis, C. anglica and C. danica) is associated with sea spray and sea birds in the same way as are “Cook's scurvy grass” (Lepidium oleraceum) in New Zealand and L. foliosum in Australia.

The zone of needle-leaved xeromorphic grasses (zone 5 in Table VII) between the mixed halophytes of zone 4 and the first woody species (zone 6) is dominated by Poa tussock in the Australasian habitats and fine-leaved Festuca spp. in the British ones.

In addition to those species included in Table VII there is a large number of aliens in New Zealand and Bass Strait which occupy a similar zone to that which they normally inhabit on British cliffs, largely between zones 5 and 6. These include the hardier pasture grasses (Agrostis, Aira, Briza, Bromus, Cynosurus, Dactylis, Holcus, Lolium, Vulpia, etc.), the bramble (Rubus fruticosus agg.) and “weeds” such as Sagina procumbens agg., Cerastium spp., Anagallis arvensis, Hypochoeris glabra and Senecio jacobaea.

The alien Coprosma repens from New Zealand was being spread through the Bass Strait Islands by gulls, starlings and crows which fed on the succulent fruits, and had proved hardier than any of the native woody species. The most salt-resistant indigenous shrub, apart from succulent leaved scramblers such as Tetragonia implexicoma and Rhagodia baccata, appeared to be Leucopogon parviflorus, a hardy sclerophyll which, nevertheless, suffered considerable “salt scorch” or was killed where Coprosma survived with little damage.

Few, if any, woody species survive in zones of equivalent exposure in Britain except as prostrate mat plants.

Acknowledgments

My thanks are due to the University of New Zealand for the provision of a grant towards the expenses of this work, to the Government Marine Department for the provision of boat transport in Cook Strait and Bay of Islands, and to Captains Turner and Traill for putting their boats at my disposal in the Stewart Island waters. Also to Principal Keeper Harris and colleagues for hospitality at the Little