Art. V.—Remarks on the Sand Dunes of the West Coast of the Provincial District of Wellington
[Read before the Wellington Philosophical Society, 20th August, 1881.]
Every person who has travelled from Wellington to Wanganui by the present coach road, must have been struck by the large extent of the dunes which lie inside the shore line from Paikakariki northward. These dunes, as will have been observed, consist of sand washed up by the waves, and then heaped up above the tide line by the action of the prevailing westerly winds. The depressions which occur amongst them are often of considerable extent, and where these lower areas continue moist throughout the year, they support a comparatively dense vegetation, whilst such of them as usually remain dry are mere arid wastes of shifting sand, without any vestige of plant life. The sand of which the dunes are usually composed is not exclusively silicious matter, but contains a proportion of calcareous and other mineral substances, and of animal and vegetable remains, which help to give it a capacity under certain conditions for sustaining vegetable growth, and accordingly we find that where the surface remains undis-
turbed, and the sand is so placed as to be capable of retaining moisture, it is generally covered with vegetation (more or less luxuriant according to the degree of moisture present), of the special character which affects this description of habitat. I give at the foot of this paper a list of the most conspicuous dune plants indigenous to New Zealand, many of which would be found valuable in other countries.
Now this vegetation confines the sand, and would, if undisturbed by man, or by grazing or burrowing animals, entirely prevent its motion under the influence of the wind, whilst, wherever the surface is not confined by plant growth or by a crust of vegetable matter, the sand is constantly rolled forward in the direction of the prevailing winds. Instances, indeed, are abundant in other countries, of populous and fertile districts having by this means been converted into barren wastes. A recent example of this was observed in connection with the dunes which lie between the Adour and the estuary of the Gironde, on the west coast of France, the sands of which were found, where not fixed by vegetable growth, to advance eastward at a mean rate of about sixteen and a half feet a year, the result being that a large extent of fertile land was destroyed before effectual measures could be taken to arrest the evil. Other instances of the mischief which results from disturbing the vegetation upon the surfaces of sand dunes will be given in the sequel, whilst, to bring the matter home, I may mention that Mr. Hadfield (who occupies a tract of land between the rivers Otaki and Ohau, on the west coast of this Provincial District) informs me that the sands of the dunes between those rivers are advancing inland at a rapid rate and threaten great injury, unless effectual steps be taken to prevent it. I have observed the same thing occurring on the shores of Pegasus Bay, but in less degree owing to the fact that the strong westerly winds which are frequent there, blow off shore, and prevent any rapid inland extension of the sand under the influence of the easterly winds which prevail on that coast.
It has been a question of interest in Europe, whether, and to what extent, the generally bare condition of coast dunes is to be attributed to the improvidence and indiscretion of man, and recent investigations seem to have shown that, in almost every case, the inland advance of dune sands may be traced to man's interference with natural operations. A patent instance of this is given in connection with the dunes of the Frische Nebrung, on the coast of Prussia. It is related by Willibald Alexis (as quoted by Mr. Marsh, in his interesting and valuable work on Physical Geography), “that the dunes of the Nebrung were formerly covered with a great pine forest, which extended to the water's edge, and bound, with its roots, the dune sand and the heath uninterruptedly from Dantzig to Pillau.
King Frederick William the First, however, wanted money, and a certain Herr Von Korff promised to provide it for him without loan or taxes, if he could be allowed to remove something quite useless. He thinned out the forests of Prussia, which then, indeed, possessed little pecuniary value, but he felled the entire woods of the Frische Nebrung, so far as they lay within the Prussian territory. The financial operation was a success. The king had money, but, in the elementary operation which resulted from it, the State received irreparable injury. The sea winds rush over the bared hills; the Frische Haff is half choked with sand; the channel between Elbing, the sea, and Königsborg is endangered, and the fisheries in the Haff injured. The operation of Herr von Korff brought the king 200,000 thalers. The State would now willingly expend millions to restore the forests again.”
It has been proved, however, that where man and cattle and burrowing animals have been excluded from the surfaces of dunes, these have gradually become clothed with various species of plants and finally covered with trees, leading to the assumption, that wherever dunes are found in a bare condition, it is to be attributed to man's interference, either direct or indirect, with the natural operations under which they would become and remain covered. It has been found, moreover, that dunes begin to protect themselves very soon after human trespassers and grazing animals have been excluded from them, herbaceous and arborescent plants (of which upwards of three hundred species are known to flourish in such habitats) speedily fixing themselves in the depressions and thence extending to the surfaces of the sandhills. To quote the words of an author on this subject: “Every seed that sprouts binds a little of the sand, and gives shelter and food for the growth of others, and a few favourable seasons suffice to cover the greater portion of the surface with a net-work of vegetation which almost effectually prevents the motion of the sand.” Those who have observed the rapid spread of the toi (Arundo conspicua), amongst the sand dunes on our West Coast (especially where they are not occupied for depasturing purposes), will have seen an example of this natural operation, and one, too, which points to a ready and simple means for preventing the further inland motion of these sands. This plant by the large amount of shade which it makes, and the protection it affords to the surface from the drying action of the wind, would materially assist in promoting the growth of more useful plants whenever it may be deemed advisable to adopt any system of artificial reclamation.
In the latter part of the last century, simultaneous active steps were taken in Denmark, in Prussia, in the Netherlands, and on the west coast of France, for the protection of the surfaces of the dunes in those countries, and for rendering them in some degree valuable, and most satisfactory
results have followed these efforts in each instance. In France, especially, operations were carried on upon a large scale, under the direction of Bremontier, the system which he used being very much the same as one independently adopted in Denmark at about the same time. Bremontier's efforts were crowned with special success, owing in some measure to the nature of the climate, but chiefly to the liberal assistance which he received from Government, which placed large sums at his command in aid of the work. The area of dunes which has been secured from drifting and converted into valuable plantations by his method, exceeds 100,000 acres, now yielding a large annual revenue in turpentine and resin, independently of the value of the timber from which these are produced, whilst, as a further and more important result of his labours, the fixture of these sands has saved a much larger area of valuable country from the destruction with which it was threatened.
In the neighbourhood of Cape Breton, another process is successfully employed, both for preventing the drifting of the sand and for rendering the surface directly productive. The method there adopted consists of planting vineyards upon the dunes, the vines being protected by hedges of Erica scoparia, so disposed as to divide the vineyard into rectangular spaces of forty or fifty feet square. The same heath would grow luxuriantly on our West Coast dunes, and there are extensive areas amongst them, especially to the north of the Rangitikei River, which appear to me to be admirably adapted for the cultivation of the vine in the manner used at Cape Breton. The vines there are said to thrive admirably, and the grapes to be amongst the best grown in France. Dunes are, it must be remembered, favourable for the growth of vines, fresh sea-sand being regularly employed, in the west of France, as a manure for the vine, alternately with ordinary manure, with the advantage that, as the surface of the vineyard is by this means constantiy raised, the vines as constantly throw out fresh roots and thus promote a vigorous upper growth.
Coming back to our West Coast dunes, it seems clear that if the observations made by Mr. Hadfield be accurate, as applied to the district between the Otaki and the Ohau, there can be little doubt that similar results are taking place further to the northward, where nearly the whole of the coast dunes are included in sheep and cattle runs. The revenue derived from the occupation of such tracts of country by pastoral tenants, cannot possibly compensate for the injury which will be done by the inland advance of the sand, and although it may not be expedient that Government should as yet engage in such operations as those which have been carried on in France, it is in the highest degree important that it should put a stop to further interference with the surface of the dunes, and thus allow them a
chance of again becoming clothed with a protective growth. The subject is not one to be treated lightly, seeing that the area of dunes on the West Coast of this Provincial District alone cannot be less than 150,000 acres, and that the prevalent winds are generally westerly, and, therefore, exactly those which are likely to do serious mischief.
As enquiries are frequently made on the subject, I think it well to add a few words as to the mode in which forest trees are cultivated on dunes. The principal tree so cultivated on the French dunes is the Pinus maritima, which, besides being valuable for timber, yields a considerable annual revenue from turpentine and resin. It is always grown from seed on the spot which it is intended to occupy, the young shoots being protected for several seasons by the branches of other trees either planted in rows, or formed into wattled hedges, or staked down over the surface of the sand. The sand grasses too are used for the purpose of shelter, and as the pine does not thrive well close to the sea, these grasses (especially Ammophila arundinacea and Elymus arenarius) are planted along the beach and for some distance inland, and these when grown effectually prevent the sand from overwhelming the young trees.
It is found that under the shade of the pine, while still young, deciduous trees and a great variety of herbaceous and shrubby plants thrive well, and contribute to the rapid formation of a coating of vegetable mould. In fact, so soon as the pine has become well established, the reclamation of the sand waste may be looked upon as an accomplished fact. Turpentine is extracted from these trees for several years before they are cut for timber, and although this has a tendency to check the growth of the tree, it is found to improve the quality of the timber. The trees commence yielding turpentine at the age of about eighteen or twenty years, and have been found to yield from that age, up to the age of eighty or a hundred years, an annual return, independently of the value of the timber itself, of about £1 an acre. It may interest you to know that Ammophila arundinacea and Elymus arenarius, as well as other foreign sand grasses, have been introduced and successfully cultivated by Mr. Coutts Crawford at Miramar Peninsula, where they have already been of great service in preventing the spread of the sand over valuable pasture ground.
The following is a list of the principal plants found upon the sand dunes of New Zealand:—
Of primary value for fixing the sands:
Coprosma acerosa, A. Cunn.
Convolvulus soldanella, Linn.
Pimelea arenaria, A. Cunn.
Leptocarpus simplex, A. Rich.
Carex pumila, Thumb.
Hierochloe redolens, Labill.
Spinifex hirsutus, Labill.
Arundo conspicua, Forst.
Scedonorus littoralis, Palisot.
Gahnia arenaria, Hook, fil.
Of secondary value:
Hymenanthera crassifolia, Hook. fil.
Plagianthus divaricatus, Forst.
Haloragis alata, Jacq.
Tetragonia expansa, Murray.
Aciphylla squarrosa, Forst.
Coprosma baueriana, Endl.
Cyathodes acerosa, Br.
Chenopodium glaucum, Linn. var. ambiguum.
Atriplex cincea, Poiret.
Salicornia indica, Willd.
Muhlenbeckia adpressa, Lab.
" complexa, Meisn.
Phormium tenax, Forst.
" colensoi, Hook. fil.
Juncus maritimus, Lam.
Cyperus ustulatus, A. Rich.
Scirpus maritimus, Linn.
Carex virgata, Sol.
Zoysia pungens, Willd.
Dichelachne stipoides, Hook. fil.
Agrostis pilosa, A. Rich.