The Tableland Forest.
Greater moisture and a more peaty soil with a great capacity for holding water seem to be the chief öcological factors which have separated this formation from the lowland
forest as a distinct plant-formation. As a proof of this we find the two trees which give the distinctive character to this forest, Senecio huntii and Dracophyllum arboreum, occurring in lowland swamps and in swampy forests, but they are almost absent from the drier ground. It is true that other Chatham Island trees also grow in swamps (see “Swamp Formation”), but they most likely are merely the plants of a line of tension between forest and swamp, and would be readily eradicated by trees better adapted for wet surroundings. Thus, Olearia traversii, Corynocarpus Lævigata, and Rhapalostylis baueri are absent from the tableland forest; Hymenanthera chathamica is rare; and, with the exception of Veronica giganiea, the other trees of the lowland forest are much inferior in numbers to the two above-mentioned dominant species. The largest forest on Chatham Island belongs to this formation. It occupies the whole of the south-west corner of the island, extending north - wards to a line connecting Tuku and Pipitarawai. All the gullies of the tableland are also filled with this class of forest; and in some cases, these gullies becoming deep as they approach the sea, their forests are of considerable size. In most other cases the trees of the tableland gullies form merely patches or long lines. The most abundant tree of the formation under consideration is Dracophyllum arboreum; next comes Senecio huntii, which, besides occurring in the interior of the forest as a regular constituent, forms the outermost row of trees, almost to the exclusion of all others, near the line of tension between forest and bog. Such a line of Seneciohuntii is a magnificent spectacle when the trees are covered with their masses of yellow blossoms. Veronica gigantea occurs also very abundantly, while the remaining forest trees, Coprosma chathamica, Myrsine chathamica, Pseudopanax chathamica, and Corokia macrocarpa, are found in smaller but still very considerable numbers. These seven trees do not by any means always occur in the proportion just stated; indeed, almost any one of them may become of more importance in the small patches of forest. The central portion of the large forest mentioned above has only quite recently been disturbed, a sheep-track having been cut through it about a year prior to my visit. This had been very little used, so, with the exception of some trifling damage by wild pigs, that part of the forest was fairly unmodified. Its trees are low, varying in height from 6m. to 9 m., and their foliage forms a flat upper surface, as does that of the lowland forest before described, except that here and there Dracophyllum arboreum raises its needle-like leaves slightly above the general level. The tree-trunks are rarely straight, but lean at various angles. The soil consists of black peat with a layer of brownish humus
on its surface. Everywhere the floor of the forest is very uneven, and is covered with many dead and decaying stems of trees or tree ferns and mounds of humus. Every tree-trunk, tree-fern stem, and dead tree is covered with multitudes of filmy ferns. The lowest layer of vegetation consists of young tree ferns, especially of Dicksonia squarrosa, and other ferns such as were enumerated before for the lowland forest. Here and there at certain seasons of the year the small red fruit-body of a fungus is abundant. The next layer of vegetation is composed of the fully grown tree ferns Dicksonia squarrosa and Dicksoma antarctica. Many of these latter are more than 4.5 m. in height, and their huge spreading fronds serve to intensify the shade of the forest and to conserve the moisture of the atmosphere. This undergrowth of tree ferns is often so dense that their stems almost touch each other. Epiphytic on the tree-fern stems are Trichomanes venosum, Hymenophyllum multifidum, H. dilatatum, Trichomanes remforme, Aspidium capense, Asplenium lucidum, and various seedling trees. The filmy ferns are often so thick that they completely hide the trunk of tree or fern on which they grow. In many places the ground also is covered with a thick carpet of these delicate plants. In deep forest-clad gullies, where a stream at the bottom and the wet ground constantly discharge water-vapour into the air of the forest, where it is confined by the double shade of tree-tops and fern-fronds, it can readily be seen that such a gully is a station of the most intensely hygrophytic character, especially if the climate of Chatham Island, with its many morning mists and light showers, be borne in mind. In such a place, too, the wind, that factor nearly always to be reckoned with when considering New Zealand plant forms or distribution, can have but little drying influence. In such a station Trichomanes remforme, the kidney fern, often grows with extreme luxuriance, the ground, fallen trees, tree-trunks, and tree-fern stems being covered with its great almost round green leaves, the younger ones of which are much brighter green and so thin as to be almost transparent. In many places this fern receives a cousiderable amount of sunlight; but this can do no damage, since the air which surrounds the fronds is always sufficiently moist.
It has already been pointed out what an excellent station for the welfare of seedling plants tree-fern stems offer—a very much better one, indeed, than the crowded and sometimes not too well-drained forest floor. Such seedling trees very frequently reach a large size, especially Dracophyllum arboreum, the roots of which plant, penetrating deeply into the soft mat of aerial roots of the fern, finally reach the ground. The plant then grows with redoubled vigour, and in the
end the tree fern, enclosed between these now very thick roots, slowly dies, and Dracophyllum arboreum remains in its place as a forest tree, its former roots now playing the part of a stem. Sometimes a large number of roots growing so closely together as to apparently coalesce make up such a “root-stem,” to use a term from a letter of Mr. H. Carse. In one instance where a tree of D. arboreum had been felled while making the forest track, thus giving an opportunity for examining its structure, I counted thirty roots, which varied in thickness from 16 cm. by 7 cm. to 5 cm. by 3 cm., or even smaller, and, as seen from the above measurements, longer in one direction than the other, owing to the pressure, the whole making a “root-stem” 45 cm. in diameter. The similar behaviour of certain trees, notably of Panax arborum, in the New Zealand forest is well known, and the Rev. W. Colenso has gone into the matter at considerable length so far as the Seventy-mile Bush, in Hawke's Bay, is concerned (13, p. 252, et seq.). Mr. H. Carse, who very kindly sent me some notes on this subject, taken in the forest near Mauku, Auckland, has written an account of this interesting matter, which I anticipate will appear in this volume, so there is no need to go into the subject at greater length here.
Dracophyllum arboreum, the dominant tree of the tableland forest, is especially interesting because of the changes which it exhibits during its life-history. In its final form it is a low tree, attaining at times a height of some 9 m. It has a short thick trunk below and, above, spreading branches bearing masses of needle-shaped leaves, resembling much those of Dracophyllum paludosum before described. Besides occurring in the tableland forest, the tree is found in lowland swamps and in the drier portions of the bogs, where it marks either a retreat or advance of the present forest. The early seedling leaves are very much broader than the adult, and resemble the seedling leaves of Dracophyllum paludosum, which, however, are rather narrower. The early seedling form persists for a long time in the ontogeny of the individual, and young plants are quite common 1.5 m. in height possessing only the broad leaves, as shown in the photograph I took in the neighbourhood of Lake Te Kua Taupo (Plate XIX.). Usually when a plant has attained to this size it suddenly puts forth leaves of the adult type, and both leaf-forms exist upon the, same individual at the same time. As the tree continues to develope all trace of seedling leaves may vanish, and finally there will be a plant merely with the needle-like leaves, looking like a very large spreading specimen of Dracophyllum paludosum. Usually, however, many “reversion shoots” of the most extreme
juvenile type make their appearance from any part of the tree—indeed, it is uncommon to see a tree without these “reversion shoots.”
Senecio huntii, second only in importance to Dracophyllum arboreum as a constituent of the tableland forest, grows to about the same height as the latter. It has a stout erect trunk, varying in thickness according to the size of the tree. The trunk at a distance of 1 m., more or less, from the ground divides, giving off two or more branches, which at first spread out laterally, but finally bend upwards. These branches divide again and again into smaller ones, which always at first spread out laterally, the whole branch system from beneath looking not unlike the ribs of a huge umbrella. At the extremities of the branches are rosettes of leaves. These consist usually of from twenty to twenty-four leaves inserted so closely together that the whole occupy 2 cm. or rather more of the end of the branchlet on an average. In some cases, however, the internodes are more greatly developed; in the most extreme case measured twenty-three leaves occupied a space of 10 cm., while an opposite case gives twenty-seven leaves for 2 cm. These ultimate branches are suffruticose, and 2 cm. in diameter. The leaves themselves are lanceolate and sessile, 12 cm. long by 3.5 cm. broad in the broadest part. Their upper surface is pale, bright shining-green in colour, except where covered by a thin pellicle of whitish cobwebby tomentum. The under-surface is of a paler colour, being greyish-green, and provided with numerous short glandular hairs. The margins and adjacent portions of the leaves are often more or less recurved, thus rendering the under-surface of the leaf concave. So flexible is the leaf that it can be rolled up into a spiral from apex to base without tearing or breaking it in any way. On the under-surface of the leaf is a strong keeled midrib, which is of great importance, since it serves to maintain the rather flaccid blade in the best position with regard to the light. Each ultimate leaf-bearing or flower-bearing branchlet increases in length until it has brought its leafy portion side by side with the neighbouring rosettes of leaves; and in order to get its leaves into a suitable position with regard to the light such a branchlet is often arched first downwards and then upwards. Thus all the rosettes are brought side by side, touching but not getting in the way of each other, the whole leafy head of the tree having the form of a half-globe. Seen at a distance, the foliage of S. huntii forms a dense bluish hemispherical mass, which when in full bloom exhibits leaves and bright-yellow flowers in an equal proportion. The branches are extremely brittle, a sudden snap quickly breaking them; but yet they are not easily broken by the wind, their great weight of foliage notwithstanding,
since they are at the same time more or less elastic, while the suffruticose extremities of the ultimate branchlets can yield very considerably to the wind-pressure. The branches are marked with many old leaf-scars and covered with a pale bark, which is somewhat papery and readily peels off. The flower-heads are in subcorymbose panicles, pyramidal in shape, 10 cm. to 14 cm. long by 10 cm. broad through the thickest portion. Flower-stalks and involucres are densely covered with glandular hairs, which, together with those of the leaves, give out a peculiar aromatic odour to the atmosphere.