Sand-dunes of considerable extent and varying height occupy a large proportion of the ground adjacent to the sea. They extend along the whole east coast from Te Whakaru in the north to Ouenga in the south, along most of the north coast from Kaingaroa to Waitangi West, and along a very large part of the coast of Petre Bay in the west. Before the introduction of herbivorous animals these dunes were covered in many places with a dense forest, consisting chiefly of Olearia traversii and Myrsine chathamica, and reaching in many places almost to the water's edge. At that time moving sand-dunes may have been unknown. But now there is a very different state of affairs. True, the forest still fringes the coast-line in many places, but here and there it is broken through by great hills of drifting sand, which have buried wholly or in part the former plant-covering Such moving hills have in some instances passed beyond the limits of the former wooded area, and are encroaching rapidly on the inland meadow land. A striking example of this encroaching sand burying the forest may be observed between Waitangi and Te One. There in places tree-tops project from the summit of the highest dunes. In one spot on the landward side on the flat is a grove of Olearia traversii where every stage of burial can be observed, from the tree-tops almost covered, to their bases just covered by the sand. This advance landwards of the sand is very serious from the economic point of view; but happily the settlers have found a remedy within recent years in the planting of marram-grass on the moving dunes. This, as might be expected from the results of planting this grass in other countries, has proved a
very great success, and now, where but three or four years ago was a desert of sand, tall grass may be seen waving in the breeze, each clump so close to the next that no sand is visible. Nor need a grass little relished by stock be alone made use of. Elymus arenarius has also been planted, and thrives equally well, and Mr. J. Barker tells me that at Kaingaroa stock eat it with avidity.* Although the sand-dune vegetation has been much changed by the advent of domestic animals, it is possible to get a fairly good idea of what it was like in its original state by examining, in as many localities as possible, those portions which have been the least changed. The dunes abutting on that part of the Wharekauri Beach the plant-covering of which has been described above are well adapted for the purpose in view, insomuch as they are still covered with vegetation almost to high-water mark, the forest forming a wide belt, separated from the sea-shore by a narrow zone, only a few inches in width, of stable dunes covered with certain characteristic sand-dune plants of more lowly growth. And this locality also affords a striking example of the different stages in the evolution of the vegetation of a sandy coast, commencing with the more or less open vegetation of the strand, and passing by way of low dunes fixed by various sand-binding plants to the final higher dunes covered with forest.
Commencing at the junction of shore and dune, the sand at first forms merely low mounds or ridges. The vegetation, though fairly abundant for a medium such as sand, is open, many places being quite bare. On the ridges grows the common New Zealand grass Festuca littoralis. This grass casts its “seeds”† in large masses alongside the parent plant, where, being soon buried by the drifting sand, they readily germinate. Behind the Festuca are higher mounds clothed with Pimelea arenaria, the long cord-like underground stems of which put forth adventitious roots near their extremities, which latter, bending upwards, raise themselves above the encroaching sand. The leaves are closely imbricating near the extremities of the branches, but below are a little more open. They are all most densely silky on the under-surface, a most efficient protection against excessive transpiration. Owing to the leafy extremities of the stems being erect,
[Footnote] * Other plants used for binding sand with success in Europe are Ammophila baltica, Calamagrostis epigea, Carex arenaria, and with these are used various species of Pinus, Picea, Betula, and Alnus. (See “Handbuch des deutschen Dunenbaues,” P. Gerhardt, Berlin, 1900.)
[Footnote] † For the sake of convenience, the term “seed” is used throughout this paper in its popular acceptation, and includes, of course, various kinds of fruits.
the semi-rosettes of leaves can receive the incident light to the best advantage. Below, the stems are marked faintly with the old leaf-scars, thus defining those portions of the plant which have formerly been terminal. It is probable that the oldest portions of the plant—i.e., the most deeply buried portions—die, while the plant continues to increase by the rooting of its terminal shoots. Such a plant might, then, attain to a very great age, so long as it was able to hold its own against advancing sand or denuding wind. The Pimelea mounds are from 1 m. to 1.2 m. in height. Sometimes the Pimelea grows unmixed with other vegetation, but more often its protection is taken advantage of by other plants, especially by Deyeuxia billardieri, Isolepis nodosa, and Acœna novœ-zelandiœ. Such a plant of Pimelea arenaria as described above averages about 4 m. in length. 3 m. in breadth, and 47 cm. in height. Another abundant plant of this zone is Carex pumila, which here grows in association with Convolvulus soldanella. This Carex has also the power of growing upwards as the sand covers it, and, with its stout, long, creeping rhizomes, assists the dune very materially in resisting the wind and the advancing sand. Ranunculus acaulis is also abundant here, playing its part as a sand-binder, and in habit much the same as described before when treating of the strand vegetation. Less abundant than any of the foregoing is Euphorbia glauca, which forms small colonies extending sometimes into the more open parts of the adjacent forest. Its seedlings are fairly plentiful on the sand near the parent plants. Although not very abundant on that part of the dunes here treated of, Scirpus frondosus is by far the most characteristic sand-dune plant of the island, and, indeed, of the whole New Zealand area. It can form settlements and hold its own in positions where no other New Zealand flowering-plant can exist, and only the most constant and furious winds can destroy a dune where it is properly established. Indeed, for sand-binding power it is probably not equalled either by Ammophila arenaria or by Elymus arenarius.
The sand-dunes bearing the forest zone are higher than those just described, and extend inland for a distance of 300 m., more or less. The plant-covering consists near the sea entirely of Olearia traversii and Myrsine chathamica; further inland other trees put in an appearance. O. traversii always maintains its character as a low tree, but M. chathamica loses altogether its tree-like habit as it nears high-water mark, and under the influence of the numerous and sometimes violent sea-breezes becomes a leafy shrub with dense close branches. The difference in appearance between the two forms of this plant is so great that one might very easily mistake them for two distinct plants.
Originally such dune forests* must have been very dense; even now the trees are quite close in many places. On the dunes facing Petre Bay in more than one locality the liane Muhlenbeckia adpressa may be seen climbing over the Olearias, with its numerous interlacing, bare, rope-like stems.
Compared with the sand-dunes of most parts of New Zealand† a marked difference lies in the extreme closeness with which the arborescent vegetation of the Chatham Island dunes approaches the shore. This has been brought about, I should imagine, by the general moisture of the atmosphere, the extremely equable climate, and the freedom from periods of drought. In such a climate as this, as long as the dunes are stable, there is little hindrance to trees, especially those of xerophytic habit, establishing themselves and driving the original sand-fixing vegetation towards the sea into an ever-decreasing area, until finally such plants, with their special adaptations against drought and salt in the soil on the one hand and instability of the substratum on the other, would be confined to the narrow zone where in the least changed portions of the sand-dune formation they are now to be found. Moreover, the sand usually, or perhaps always, overlies a layer of peat, and this will be of great benefit for tree-growth. As an example of how a sand-dune forest might originate, my notes furnish the following clue: “On the sand-dunes between Waitangi and Te One Pimelea arenaria occurs in large quantities, forming fixed dunes. Where this plant has quite conquered the drifting sand”—this is, of course, recent drifting sand caused by the destruction of the original vegetation by cattle and sheep—“it encourages the growth of other plants—e.g., Acœna, Gnaphalium luteo-album, and even young seedling plants of Olearia traversii.” With no enemy to trample it down or feed upon it, and under the shade of the Pimelea, O. traversii, thanks to its rapid growth, would soon be well established, and with its enormous number of “seeds” and their extreme suitability for wind dissemination, to say nothing of its xerophytic structure, young plants would soon be established in all favourable localities. Olearia traversii, as it occurs on the dunes, is a low tree, with a rather dense head of foliage and a bare trunk covered with rough bark. The leaves are 5.5 cm. long by 2.5 cm. broad, or shorter and narrower They are rather thick but soft, of a bright shining-green on the upper surface and with the under-surface clothed with extremely dense
[Footnote] * The more inland part of the dune forests are treated of under the heading “Lowland Forest.”
[Footnote] † For an account of certain sand-dunes in the North Island of New Zealand, see Cheeseman (Trans. N.Z. Inst., vol. xxix., p. 364).
white tomentum, as are also the short petioles and ultimate branchlets. The epidermis is two-layered, and the vascular bundles are surrounded by a sheath of stereome.
Probably Sonchus grandifolius originally grew in some abundance on the fixed dunes, but at present it is to be found only in a few places. Tetragonia trigyna also must at one time have been more or less common. In one place on the inland side of the dunes, near Lake Te Roto, I noticed a few plants of Dodonœa viscosa.