(b.) The Olearia lyallii Forest.
This is a forest-formation of remarkably local distribution. It is not found at all in New Zealand proper, but occurs only on the Snares, on Ewing Island, and perhaps to a very limited extent in one or two places on Auckland Island itself And yet where it does occur it grows with great luxuriance; seedlings of all sizes are abundant. Nor does there seem any reason why it should not be the dominant forest of the Southern Islands. We have here possibly one of those cases where a very slight difference in adaptation to environment, not to be estimated in the present state of knowledge, has led to one formation becoming dominant and another remaining stationary in a limited area; or, what is perhaps likely here, a former primeval forest adapted to slightly different conditions has been ousted by a newer formation as the conditions changed.
“Viewed from the sea,” to quote from my note-book, “Ewing Island has a decidedly attractive appearance.” This is owing to the fine natural shrubbery of Olearia lyalli which fringes the shore, its huge leaves exposing their silvery under-surface as blown by the frequent wind, and contrasting finely with the green of the upper surface. Alternating with the Olearia are pale-green clumps of Veronica elliptica, while in front of it are in places dense tall tussocks of some species of grass, more than 1.5m. in height, with “trunks” similar to those of the “niggerhead.” In front of the tussock again is a rather dense growth of the bright-green grass Poa foliosa. Near the depot the beach is stony, consisting in part of boulders strewn with seaweeds and in part of solid rock worn down to sea-level, backed up by low rocky faces dripping with water, and their summits crowned with Veronica or Olearia.
The individual trees of the formation are some 6.1m. to 9.1m. tall, some being erect, while others have the lower part of the trunk prostrate. Such trunks are ± 50 cm. in diameter. From very near the base the main trunk branches into three or four secondary trunks covered with a rough
bark. The ultimate branches divide into several short branches radiating upwards and outwards after the manner of Olearia chathamica of Chatham Island (23, p. 298), O. operina of the west coast sounds, or Rhododendron ponticum, so common in gardens, finally giving off from near their extremities numerous large leaves, which form a dense mass. These leaves are thick and coriaceous, their laminæ ± 20.5 cm. × ± 13.5 cm., and provided with a short stout petiole expanded and sheathing at the base. The upper surface is dark-green, varnished and quite glabrous except at the margins and on the broad midrib. The under-surface is densely clothed with flannelly tomentum quite white in colour. This tomentum is rather more than half the thickness of the leaf-substance proper. The midrib on the under-surface of the leaf is excessively stout, measuring at the base 9 mm. × 4 mm. They young leaf, just when it is unfolded from the bud, is white and soft like a piece of flannel, being extremely tomentose on both surfaces. The ultimate shoot-axes are 1.5 cm. in diameter, and densely covered with white tomentum similar to that of the leaf. Shade-leaves are rather larger than sun-leaves, and have the veins less prominent. A cross-section of the leaf shows a large-celled epidermis on the upper surface, having the outer wall but little thicker than the inner walls. There is a dense palisade parenchyma with water tissue extending in places into it from the epidermis. Round the vascular bundle is a stereome sheath which extends to the upper epidermis, while on its under-surface is a large-celled water tissue. On the under-surface are a number of glandular hairs.
In the forest of Ewing Island trees with prostrate trunks are very much more common than those whose trunks are erect. As a rule, indeed, more than half the trunk is prostrate upon the ground. Kirk thus describes Olearia lyallii as it grows on the Snares (56, p. 215), “When growing in level situations it is erect, with open spreading branches, but when growing on slopes exposed to the wind it is often inclined or with a prostrate trunk, the roots partly torn out of the soil; and the branches, rooting at the tips, give rise to new trunks, which in their turn are brought to the ground and repeat the process.” As for the plants on Ewing Island, Kirk writes (56, p. 219), “Most of them are erect and well grown, but a few exhibit the inclined position so frequent on the Snares.” Chapman thus describes the behaviour of O. lyallii on the Snares: “When this” [O. lyallii] “grows a certain height it falls down with the weight of the leaves and the pressure of the wind and takes root where it touches ground; then it grows upwards again, and after a while it falls again, tearing its oldest roots up and rooting itself a third time: thus the
trunk is almost gifted with the power of locomotion” (16, p. 494). I have given the above quotations partly because they differ somewhat from my account and partly because this prostrate habit seems to me a matter requiring further investigation as to its causes. That it may at the present time be frequently caused by the wind no one can deny, but there is no reason on that account why the prostrate habit should not be, in pare at any rate, hereditary. The behaviour of seedlings may furnish a clue. In the interior of the forest beneath the dense leafy canopy the air is comparatively still, when without a gale is blowing. Of the quite young seedlings growing in the ground which I examined, and of which only a few leaves were developed, most had their stems prostrate on the ground for more than half their length. In one instance, in an older plant, 30 cm. was erect and 15 cm. was prostrate. On the other hand, a number of young seedlings growing closely together on the trunk of a tussock were orthotropous, but in this case density of growth or their relation to gravity may have caused the difference. At any rate, it is an interesting matter requiring further investigation, for it bears directly on the inheritance of acquired characters.
The floor of the Olearia lyallii forest consists of coarse peat covered with numerous fallen leaves in various stages of decay. For the most part it is quite bare, owing probably to the incursions of the numerous sea-lions. There is no undergrowth of shrubs, consequently the forest is much more open than is the rata forest. Growing on the forest-floor are many rather distant patches of young Olearia plants, while in other places are Asplenium obtusatum and Lomaria dura. The young plants of O. lyallii early on develop quite large leaves; e.g, a plant 30 cm. tall had leaves measuring 13 cm. × 8 cm.
Ecologically and floristically the formation under consideration is related to the O. chathamica formation of Chatham Island, the O. colensoi formation of Stewart Island, and the O. operina + Senecio rotundifolius formation of the west coast sounds, though all three must be designated scrub, and not forest. Like the above formations, and situated like them in a rain-forest climate, the O. lyallii formation is distinctly xerophytic; but the mild moist atmosphere has caused production of excessive leaf-surface, just as in the rata forest there is great lateral growth of the Metrosideros. Occupying only a narrow zone on the sheltered side of the island, the O. lyallii gives place to the rata forest, which certainly seems on that account the better able of the two to withstand the wind, while in its turn on Enderby Island rata gives place on the windward side to Cassinia scrub, and this in its turn to tussock.