(b.) The Tall Scrub Formation.
Even at a distance it can plainly be seen that much of the surface of both islands is occupied by a thick growth of low trees. A closer view shows that some of these stand out distinctly above the others. This at first led me to think that the former might be the rare Meryta sinclairii, which has for a long time been reputed as occurring on the Poor Knights.* But, as shown further on, these plants are merely Cordyline australis, so that the presence of Meryta on these islands still remains a moot point.
The formation under consideration occupies the gullies, together with that flatter ground forming the surface of the islands above the precipices. It seems to be of greatest extent on the southern island, where alone I had an opportunity of penetrating into it. Had the time not been so short it would have been quite easy to have gone right through the scrub to the summit of the island, but as it was I was only able to examine the part at no great distance from the sea.
Unlike the cliff formation, which is identical with that of the neighbouring mainland, the scrub is quite distinct from any allied formation with which I am acquainted in the New Zealand biological region, not because it contains any peculiar or rare species, but from the special combination of its members.
Seen from without, the scrub presents a dense mass of foliage, greyish or green in colour. Between the scrub proper and the open ground bordering on the sea is a broad, thick belt of Phormium tenax, while in places within this again is a good deal of low-growing Metrosideros tomentosa, the representative here of the characteristic belt of that tree along most of the rocky shores in northern New Zealand. Here, too, outside the scrub, is Myoporum lœtum—not an erect tree as usual, but semi-prostrate. This unusual habit did not surprise me, for on the Moko Hinou Islands and on Cuvier I had already observed numerous absolutely prostrate plants, looking on this account altogether different from the normal tree. How far this prostrate habit is hereditary and the plant an elementary species, or whether it is merely a case of fluctuating variation, the result of constant winds on plants which would otherwise be upright, has yet to be ascertained—an easy enough matter to determine by means of culture experiments.
The two dominant plants of the scrub are Suttonia divaricata (Myrsinaceœ) and Macropiper excelsum (Piperaceœ). Melicytus ramiflorus (Violaceœ) appears to come next in abundance.
[Footnote] * See T. Kirk, “An Account of the Puka (Meryta sinclairii, seem.),” Trans N.Z. Inst., vol. ii, p. 100, 1870.
Associated with these, but in much smaller proportion, are Hymenanthera latifolia (Violaceœ), Myoporum lœtum (Myaporaceœ), Entelea arborescens (Tiliaceœ), Geniostoma ligustrifolia (Loganiaceœ), Corynocarpus lœvigata (Anarcardiaceœ), and Sideroxylon costatum (Sapotaceœ).
The scrub is about 3 m. tall. The low trees or tall shrubs—, call them as you please—have usually rather slender naked trunks and dense heads of foliage. The ground is bare for the most part, but here and there are seedlings of the different species, together with Veronica macroura* and a few ferns. It was pleasant to note that the bell-bird (Anthornis melanura), now all but extinct in many places, was plentiful. Further from the sea the scrub probably changes its character considerably, for Cordyline australis (Liliaceœ) becomes one of the most abundant members, its much-branched heads raised above the other foliage and rendered conspicuous at a distance through this and their yellowish-green colour.
With the exception of Suttonia divaricata, the presence of which was most unexpected, and which separates this scrub most distinctly from any other formation, its other members are what might be expected in a northern coastal forest. But S. divaricata is by no means a common plant in the north of New Zealand, so Mr. T. F. Cheeseman informs me. It, however, is much commoner as we go further south, until on the Auckland and Campbell Islands it becomes one of the characteristic forest or scrub plants.† From Mr. R. H. Matthews, of Kaitaia, to whom I wish to express my obligation for botanical assistance, I learn, however, of a still more anomalous station for this shrub—viz., on mangrove islands in the Rangaumu Estuary.
The most striking ecological fact about this scrub of the Poor Knights is that, notwithstanding the small size and consequent exposure to fierce winds of the islands, the foliage of many of the plants is abnormally luxuriant. Macropiper excelsum is probably that large-leaved variety originally discovered by Cheeseman on the Kermadec‡ and Three Kings Islands.§ The leaf-blades of my specimens measure ±16 cm. by ±16·6 cm.
[Footnote] * If the identification be accurate this extends the range of this plant considerably to the northward, at the same time affording evidence that the Whangarei habitat of Colenso is correct.
[Footnote] † Cockayne, L., “A Botanical Excursion during Midwinter to the Southern Islands of New Zealand” (Trans. N.Z. Inst., vol. xxxvi, 1904, p.251).
[Footnote] ‡ “On the Flora of the Kermadec Islands” (Trans. N.Z. Inst., vol. xx, 1888, p. 154).
[Footnote] § “Notes on the Three Kings Islands” (Trans. N.Z. Inst., vol. xxiii, 1891, p. 412; see also p. 415 as to the large-leaved Geniostoma).
Those of Myoporum lœtum measure ±14·5 cm. by ±6·2 cm., whereas Kirk gives from 2·5 cm. to 10 cm. long by 1·3 cm. to 3·8 cm. broad.* The leaves of the Melicytus and Geniostoma, too, are considerably above the average. But most surprising of all are the leaves of Suttonia divaricata. These on specimens from the Southern Islands measure 11 mm. by 10 mm.,† but those of Poor Knights plants are 33 mm. by 22 mm.; moreover, they are thin, and not “somewhat coriaceous.”
Such luxuriance of foliage on wind-swept small islands, far out in the open ocean, where the contrary might be expected, is not easy of explanation. There is far more shelter than might be thought at first glance, for usually the formation will only get the wind from one quarter, while the dense growth of the whole also protects the individual members. The air, too—although no statistics are available—may be assumed to be always highly charged with moisture, and so will check transpiration and encourage leaf-development. Finally, the volcanic soil of the islands is probably extremely fertile. Mr. T. Kirk long ago called attention to a similar condition of affairs on the lava-field of Rangitoto, the well-known landmark in the Hauraki Gulf, the richness of whose vegetation in conjunction with the apparent absence of soil and water must strike even the most careless observer. My above explanation, so far as it goes, adds little to that originally put forth by Kirk.‡ I must confess, however, that it seems to me at best but a partial solution of this puzzling question.
[Footnote] * “Forest Flora,” p. 253.
[Footnote] † Loc. cit, p. 251.
[Footnote] ‡ “Notes on the Botany of Waiheke, Rangitoto, and other Islands in the Hauraki Gulf” (Trans. N.Z. Inst., vol. xi, 1879, pp. 451, 452).