Art. XXXIV.—Plant-acclimatisation in New Zealand
[Read before the Otago Institute, 10th July, 1900.]
A European botanist landing at any one of the principal ports of New Zealand would be at once struck with the distinctively British facies of the vegetation. Perhaps this impression would be most vividly produced at Lyttelton and Christchurch, and least of all at Dunedin, for here the comparatively good state of preservation of the native bush in and about our. Town Belt, and the proximity of so much high uncultivated land, have left more of the indigenous vegetation than is to be seen in the neighbourhood of any other of our large towns. The causes which have led to the displacement of so much of the native flora as has disappeared, and to the naturalisation of so many foreign species of plants, are numerous and in many cases not easily traceable; the facts themselves are conspicuous enough, even though the explanations may not be forthcoming.
In bringing this subject before you I am not going to attempt at all to catalogue our introduced, plants. That, I hope, will be done in the completest manner by Mr. Cheeseman in the Flora which he has now in preparation. What I desire to do is to suggest reasons for the great increase and aggressive character of some forms, and also for the failure of others to establish themselves under, what seem at first sight equally favourable conditions. Why, for instance, should gorse and broom, cooksfoot grass and meadow poa (Poa pratensis), establish themselves so strongly as to become serious pests in many parts, while attempts to introduce many
favourite flowers and to get them to grow as wild flowers always end in failure? This aspect of the subject has received but little attention hitherto, yet it is an extremely interesting point of view from which to consider it.
When we speak of plant acclimatisation, or, more correctly, naturalisation, in New Zealand we mean within historic times. Probably the ancestral forms of all existing species of plants in the country were introduced at one time or another from adjacent lands, as the relationship of our indigenous flora with that of Australia, of the antarctic region, of Polynesia, of South America, and of Eur-Asia is more or less distinctly traceable. But though we may in some cases almost infer the comparative age of some of our species from finding identical or slightly different forms in adjacent countries, we cannot arrive at any definite information on the subject. Very little is yet known about the fossil. flora of these Islands, and until this has been studied very little light can be thrown on the origin of our flora. When, however, we limit our investigations to the species introduced within the epoch comprised within such times as the Islands have been known to Europeans we have in many cases perfectly definite information, and in regard to all have a certain amount of sure knowledge. A few plants were certainly introduced by the Maoris from the South Sea Islands—e.g., the kumara (Ipomæa chrysorrhiza), the taro (Calocasia antiquorum), and the hue or gourd (Cucurbita sp.)—but these occur only on the site of former native settlements, and show no tendency to spread, and barely even to hold their ground.
The Rev. Richard Taylor, in “Te Ika a Maui,” speaking of the occurrence of Clianthus puniceus in the neighbourhood of native pas, tells a story which was related to him about the reputed introduction of this plant. A French vessel was captured in the Bay of Islands, and many of the boxes were emptied on a small island in the Kerikeri River, and were found to contain nothing but seeds. A few years later the whole island was covered with Clianthus, the beauty of its flowers attracted attention, and the seeds were carried about, to all parts by the natives. Taylor, who was an unscientific man, thinks there is some probability in the story. But species do not arise in this sudden manner from nowhere in particular. There are only two species of Clianthus— an Australian species (C. dampierii), known as Sturt's “desert pea,” and the New Zealand C. puniceus. Their unique form and difference from all other genera of Leguminosæ point to considerable antiquity for the genus, and their diversity of detail from one another to a considerable age also for the species. The probability is that our species would either
have become very rare or have altogether, disappeared, as some other large conspicuous plants have done, had it not been that the natives sowed the seeds in the vicinity of their pas for the sake of the flowers, which they used to stick in their hair or their ears for ornament. Why I suggest the disappearance is that the plant is probably edible and nutritious, and there would seem to have been some form or forms of animal life introduced since the development of the species which was tending to exterminate it.
A case somewhat corresponding to this is actually furnished by the large forget-me-not, misnamed the Chatham Island lily (Myosotidium nobile). This handsome plant is much more nearly allied to Australian species of Cynoglossum than to any existing New Zealand genus. I think, therefore, that it must formerly have been spread over these Islands, but that it has died out, or, more probably, been eaten out by some more recent form of animal life, perhaps by moas, which were formerly enormously abundant, and were vegetablefeeders.
The real history of plant-naturalisation here dates from Cook's first visit to the Islands.
The modes by which the naturalised plants of any country are introduced may be considered under four heads—(1) The deliberate scattering of seeds in the open ground or in selected localities; (2) as escapes from cultivation in gardens, orchards, or fields; (3) accidental, along with other seeds, or with hay, straw, packing, &c, with, introduced varieties of plants or animals, or in ballast of ships; and (4) by wind, birds, &c.
(1.) In regard to the first of these modes, the remarkable thing is not the number of species of plants, which have been introduced and naturalised in this manner, but rather the smallness of the number of those which have so established themselves as compared with the number of those which it has been sought to introduce. Hooker records in the “Handbook of the New Zealand Flora,” page 757, that “the late Mr. Bidwill habitually scattered Australian seeds during his extensive travels in New Zealand. If this be true, it is remarkable how few Australian plants have naturalised themselves in the Islands, considering both this circumstance and the extensive commerce between these countries.” The same experience has accompanied very numerous attempts since Bidwill's time to introduce what were considered desirable species. I do not know that it is possible to point with certainty to a single species which has thus become established in this part of the colony. It is just possible that the watercress (Nasturtium officinale) was so introduced by the early settlers into the Canterbury Plains, though I have no information to that effect, while it is equally possible that
its introduction was accidental. Similarly, it would appear* that the Cape water-lily (Aponogetono distachyon) was planted in the streams about Waimate by the early missionaries. Of course, the scattering of grass-and clover-seeds on burnt lands, on farms, and sheep-runs comes under this category, but, with these exceptions, I cannot suggest any other examples.
It must very frequently have been the case during the past fifty or sixty years that persons, from motives of sentiment, have sown broadcast the seeds of flowers which they admired or were familiar with in the neighbourhood of their early homes in the Old Country, and which they thought would thrive here on account of the similarity of soil and climate. This is particularly true of such flowers as violets, primroses, cowslips, bluebells, heaths, &c, and of fruits like the bilberry (or blaeberry) and cranberry. But these plants do not, as a rule, belong to what may be called aggressive species. They cannot always succeed even in growing in open competition against the indigenous vegetation, and they never make the slightest headway against many of the vigorous introduced forms. Even where individual plants become established, they nearly always fail to produce seed, and this is the chief reason why such species, do not become naturalised. In their native countries their flowers are visited and fertilised by certain species of insects, and these are totally wanting here. Our indigenous insects are unable to fertilise them, and so they do not produce seed. There are no doubt other differences which affect their success in the struggle for existence. The rapidity of germination of their seeds, the subsequent rapidity of growth of the young plants, and many other factors, which have not been sufficiently looked into in this connection, all bear on this question. I have in past years sown quantities of the seeds of many flowering-plants of Great Britain along the wayside in one of the suburban roads leading through our Town Belt, but from none of them have plants appeared except from those of foxglove, whose strong coarse foliage enables it to hold its own against most of its neighbours. If the others have germinated they have nearly always been smothered by cocksfoot or other coarse grass. In gardens many of our European flowers seed now on account of the general prevalence of humble-bees, but many others remain unfertilised.
(2.) Escapes from cultivation are much more common, and a few of these have succeeded in establishing themselves. Those who have travelled to any extent in the colony will, no doubt, recall numerous instances of this. Jungles of scarlet
[Footnote] * Trans. N.Z. Inst., vol. ii., p. 143.
geranium, fennel, parsnip, and other garden plants are common in waste ground about Napier, Auckland, and other towns, particularly in the North Island. Such plants are either fertilised by hive-bees or by various kinds of flies, or are perfectly self-fertile. In the Bay of Islands and elsewhere in the North I have noticed the American aloe (Agave americana) and the common flag-lily (Iris germanica) growing on the sites of abandoned gardens. At Kerikeri I saw, in 1884, groves of wattles (probably Acacia decurrens, var. dealbata), and these apparently were spreading in all directions; and the late Mr. Kirk recorded in 1872* that Robinia pseudacacia was similarly establishing itself in a grove on the Auckland-Drury Railway.
In our own southern portion of the colony such garden escapes occur freely, though not to the same extent as in the warmer climate of the North Island; yet of late years they have increased from a new and recently introduced cause. Foxgloves, musk, monkey-flowers (Mimulus luteus, var.), mullein, and a few other flowers have run wild in many localities, but they do not show any tendency to become generally distributed. On the other hand, the great increase in number of our imported fruit-eating birds (blackbirds and thrushes) during the last twenty years has led to a remarkable increase and a much wider distribution of plants bearing succulent fruits. Thus, the elderberry has become a nuisance in our Town Belt, while to a less extent wild roses, brambles, raspberries, and gooseberries are spreading, and this always in the neighbourhood of bush-or scrub-covered land. Similarly. I find such plants as Cape fuchsia (Leycesteria formosa), holly, barberry, and mountain-ash (and the list might easily be extended) appearing not only in various parts of my garden, but also in places at some distance from it. In all the cases mentioned above it is noteworthy that the flowers are visited, and almost certainly fertilised, by bees and flies, and are not dependent on any specialised form of insect for their seeding. It may be accepted as a general rule that cultivated plants are not well fitted to compete unaided in the hard and complex struggle for existence against wild species. Hence we do not find them holding their own in the open, except in a few cases.
(3.) In regard to the introduction of naturalised plants by accidental means—viz., along with seeds, hay, straw, &c., in the soil surrounding other introduced plants, by animals, in ballast of ships, or in any other chance manner—this is certainly the source or mode in which by far the greatest number of such aliens have come to these Islands. Agricul-
[Footnote] * Trans. N.Z. Inst., vol. ii., p. 137.
tural seeds are especially responsible for the majority of our most common weeds, and I have given an example of this in the first paper read by me before this Institute.*
It is evidently the case that weeds of cultivation, such as chickweed, shepherd's purse, groundsel, &c., must have developed their special characteristics within comparatively recent times—that is to say, they have developed them pari passu with the development of cultivation of the land by the human race. This, from the naturalist's point of view, does not point to any great antiquity.
An examination of any list of the naturalised plants of a district—for example, that of the plants of Port Nicholson by the late Mr. Kirk,† or that of our own immediate neighbourhood issued a couple of years ago by the Dunedin Field Club—reveals certain interesting general facts. Thus all, or nearly all, are capable of self-fertilisation, if they are not habitually self-fertilised. If one looks at the weeds in any unkept bit of garden-ground at the present midwinter season (July), they will probably find some or all of the following species producing seed in abundance from flowers which never open and which are more or less imperfect in structure: Shepherd's purse (Capsella bursa-pastoris), winter-cress (Barbarea vulgaris), bitter-cress (Cardamine hirsuta), hedge-mustard (Sisymbrium officinale), wart-cress (Senebiera didyma), chickweed (Stellaria media), mouse-ear chickweed (Cerastium glomeratum and C. triviale), groundsel (Senecio vulgaris), sowthistle (Sonchus oleraceus), spurge (Euphorbia peplus), and perhaps various others. This faculty of producing more or less imperfect self-fertilised flowers is almost an essential feature in all such plants, many of which are thus enabled to produce fruit at all seasons of the year, and almost independent of the weather. It is perhaps the most characteristic feature about them. Another point is that most of them produce very small and very numerous seeds; and still another, that a large proportion of them come to maturity very rapidly, and that their seeds germinate quickly. These characters are all retrogressive from one point of view—that is to say, the plants exhibiting them have tended to become less instead of more specialised in their development; but by this degradation of their reproductive organs they have really become better adapted for the peculiar conditions which are imposed upon them in their struggle with the gardener and agriculturist.
(4.) I am not aware that any plants naturalised within historic times have been introduced by means of either wind,
[Footnote] * Trans. N.Z. Inst., vol. ix., p. 538
[Footnote] † Trans. N.Z. Inst., vol., x., p. 362.
birds, or insects, though there is no inherent improbability in this mode of introduction. Many of us can recall the sultry day a few years ago when the sky was darkened and the sun became lurid from the dense smoke of Australian bush-fires, and it is quite-clear that light seeds and spores might very easily be carried along at considerable elevations by a similar westerly wind. Many of our native species of flowering-plants are either identical with or closely allied to Australian forms, and this is particularly the case with such plants as the pappus-bearing composites—e.g., Celmisia longifolia, Craspedia fimbriata and C. alpina—several species of Erechtites, &c., and the various species of Epilobium (E. confertifolium, E. glabellum, E. junceum, &c.), which have a tuft of hairs on the seeds.
Again, on the west coast of the North Island, and particularly on the peninsula north of Auckland, Australian birds have been met with which had managed to survive the long flight across the Tasman Sea. Such birds are probably weak from exhaustion on reaching the shore, and are killed by gulls or other enemies, those which survive dying in the course of time without reproducing their kind. Such birds may bring seeds in their crops or stomachs (though this is not very likely after such a long flight), or they may have seeds attached to their feet or their feathers. Both the cuckoos, too, come to us from across the seas—one from Australia and the other from the South Sea Islands; while migratory birds like the sandpipers, &c., travel vast distances from shore to shore. Yet, with all these possible modes of conveyance, we cannot point to a single instance of plant-naturalisation due to these agencies.
A large proportion of the plants which have succeeded in establishing themselves in this country belong to what Sir Joseph Hooker has called the “Scandinavian flora,” the aggressive and colonising power of which has been dwelt upon by him, by Charles Darwin, and by A. R. Wallace. Darwin's explanation, it may be remembered, is that this power of colonising is due to the development of these plants in the most extensive land-area of the globe, where competition has been most severe and long continued. A discussion on this subject will be found in the chapters dealing with the flora of New Zealand in Wallace's “Island Life,” and much additional information is also given in his “Darwinism.”
There are one or two aspects of this question of plant-naturalisation which have not yet received much investigation, though they are very interesting.
Often when a species is first introduced into a country or district it exhibits most extraordinary vitality for a time, and then appears gradually to lose its exuberance of growth,
and to assume a more normal rate of individual development. It is as if the restrictions which formerly kept it within certain limits had been removed, and it sprang with a bound to a height of vigour which it was not able to maintain, and then had gradually fallen back to a level at which it could maintain itself. Anglers recall the marvellous rapidity with which trout grew when they were first put into our streams. The food-supply was practically unlimited, and they increased most remarkably in size and weight. But succeeding generations have not been able to keep up the same phenomenal rate of growth, for not only is the food-supply diminished, but the young fish have to run the gauntlet of the old ones, which are their worst enemies. This factor does not, of course, enter to anything like the same extent into the relations of plants to one another, but it is to be taken into account in considering that well-established plants are most formidable rivals to the seedlings of their own kind round about them. A somewhat analogous case seems to be that of the humble-bees. During the first few years after their liberation in Canterbury these insects increased enormously in numbers, and bee-keepers frequently expressed the opinion that they would soon crowd the hive-bee out of existence. But, as far as I can make out, this rate of increase has not been maintained, and these insects are now by no means troublesome on account of their numbers.
The same phenomenon has been witnessed in the case of some plants. The marvellous growth of watercress in the streams of the Canterbury Plains, producing as it did stems of 12 ft. in length and ¾in. in diameter, has often been adduced. But I do not think that this huge type of growth has been kept up to any extent, though I am open to correction on this point. As far as I have seen it about Christ-church, the plant seems to grow larger than the parent plants in Britain, but it does not attain its former recorded dimensions.
When the Oamaru district was first ploughed the common thistle (Carduus lanceolatus) took absolute possession of the soil. I remember walking in 1872 through many hundreds of acres on the Balruddery and Elderslie properties, between the Waiareka and Kakanui Rivers, and the only available track was on the dray-ruts, and even there the thistles were waist-deep, while on both sides they formed a wall 6 ft. or 7 ft. high-During the first year or two of their occupation not a blade of grass or other plant could show itself, but afterwards the ground seemed to become somewhat sick of thistles. Meanwhile the soil was enriched by the plentiful supply of vegetable matter which was produced on it and ultimately
worked in again, the subsoil was penetrated by the roots, which decayed in it and thus helped to decompose and break it up, and on such soil 50 and 60 bushels of wheat were taken off per acre immediately after the thistle-crop.
No doubt numerous other cases of the same kind could be adduced, and it would be interesting to find out whether such individual development anywhere tends to be maintained, and, if so, whether it tends to the production of any permanent variety. I think it possible that the development in the past of such large species of plants as Myosotidium nobile, Aciphylla squarrosa and A. colensoi, Ranunculus lyallii, Ligusticum latifolium, and Pleurophyllum criniferum, all of which are giants as compared with their nearest relations, is due in great part to their isolation in these Islands, and the comparative absence of severe competition.
Another curious change which has been noticed as taking place is the adaptability of some of our indigenous plants to the changed conditions brought about by settlement. Some of the native species appear to be able to hold their own, and even to benefit by these altered circumstances. I have already recorded the fact that with the increase of blackbirds and thrushes, many succulent-fruited plants have become widely dispersed. This is true of native as well as of introduced species. Fuchsias are increasing, not diminishing, in numbers in the Town Belt, and in my garden I find a species of Coprosma (C. robusta) and the common cabbage-tree (Cordyline australis) coming up where none were sown, and where I do not remember any growing naturally for some twenty-five years. The fruit of the latter is often eaten by starlings, and thus distributed. Again, some creeping plants furnished with rooting stems or underground stolons are able to spread in cultivated ground and in pasture. I have noticed three species particularly aggressive—viz., Epilobium nummularifolium, Hydrocotyle asiatica, and H. muscosa.*
Note.—In “Darwinism,” p. 29, 2nd edition, 1889, Wallace quotes W. T. L. Travers, and, apparently on his authority, states that “the most noxious weed in New Zealand appears, however, to be the Hypochæris radicata, a coarse yellow-flowered composite not uncommon in our meadows and waste places. This has been introduced with grass-seeds from England, and is very destructive. It is stated that excellent pasture was in three years destroyed by this weed, which absolutely displaced every other plant on the ground. It grows in every kind of soil, and is said even to drive out the white-clover, which is
[Footnote] * The Rev. A. Don, Chinese missionary, states that Raoulia australis is greatly on the increase in the interior of Otago. It is one of the few plants which is not being eaten out by rabbits.
usually so powerful in taking possession of the soil.” This is a rather overstated case. The Hypochæris, or cat's-ear, usually misnamed Cape-weed with us, is only troublesome where sheep cannot get at it. But on sheep-runs and farms on which sheep are fed the plant disappears, as these animals eat it down to the ground, and so completely eradicate it.
The following notes by Mr. D. Petrie are supplementary to the above paper:—
(1.) A leading fact which might have been emphasized more is that the spread of weeds is mainly due to useful plants—their competitors—being regularly checked and eaten down, while the weeds are mostly allowed to grow without check of any important nature. Almost all weeds found in our northern pastures owe their spread to this—e.g., several buttercups, numerous docks, pennyroyal, Holcus mollis and H. lanatus, and many weedy grasses, various spurges, mallows, mulleins, and so forth. In many cases their spread is facilitated by the ready germination of their seeds, by the long time that the seeds retain their vitality in the soil, and by the readiness with which their earliest roots strike deep down into the soil, which allows the plants to establish themselves in hot, dry weather. Black medick (Medicago lupulina), meadow plantain (Plantago lanceolata), the docks and spurges, all start and thrive in hot, dry weather, when more superficial-rooting seedlings die off. The introduced speedwells and poor man's weather-glass (Anagallis arvensis) are much in the same case.
(2.) The decline of plants that have taken possession of a district for some years is no doubt due to temporary exhaustion of some element of plant-food needful for their vigorous growth. This principle lies at the base of the theory of the rotation of crops. In Central Otago, when I first knew it, Carduus lanceolatus was the prevailing weed on open downs and dry hill-slopes. Some years after C. pauciflorus completely replaced it, and this will, no doubt, be now giving way to something else. The doctrine that the Scandinavian plants possess extraordinary vigour, which is the cause of their aggressive character, seems to me very doubtful. In each single species particular advantages can generally be assigned that will readily explain their rapid spread. In the peninsula north of Auckland there are very large areas of land on which European weeds have but slightly established themselves, though the ground is frequently cleared of all native vegetation by fires. In these areas native plants mostly grow up with great readiness, especially species of Leptospermum and Pomaderris, Haloragis tetragyna and H. minuta, besides various cyperaceous plants. The pre-eminence in aggressive
characters of North European plants is decided enough, but many non-European plants are now widely spread here, and are, indeed, very aggressive. I may instance Modiola multiflora, a North American malvaceous plant, an Australian Plantago, two species of Erigeron, and Kyllinga. The rat-tail grass, too, is no doubt introduced, and has been most aggressive, while the South African Cyperus (minimus?) is nearly as ubiquitous as sorrel.
(3.) The agency of birds in scattering seeds is most noticeable here (Auckland). The Cape gooseberry, the blackberry, and the inkweed (Phytolacca) are now spread over vast areas entirely through their agency.
(4.) Here, as in the South, a few native plants are spreading—e.g., Haloragis tetragyna, H. minuta, Aristotelia racemosa, Fuchsia excorticata, Pomaderris phylicifolia, Erechtites, &c.; but the most aggressive plant of all is Pteris aquilina, which is rapidly overrunning much of the land that has been cleared of bush, and which permanently establishes itself before roots are sufficiently decayed to admit of ploughing.
(5.) I suppose our most abundant and most widespread introduced weed is Hypochæris radicata. This furnishes a good example of the mode in which an aggressive plant spreads. Its seeds germinate easily, the roots strike down to the moist layer promptly, the rosulate leaves keep neighbouring plants from encroaching on it when established, and the seeds when mature are wafted afar by the wind. Add to this that cattle and horses will not touch it, and its rapid and universal diffusion calls for no special constitutional vigour. The specific advantages thus assigned sufficiently account for its spread. I do not know of any reason for thinking its seeds possess long vitality, but in spite of this drawback it has advantages enough to fully explain its predominance. We found it on the topmost rock of Mount Hikurangi (5,500 ft.), on the east coast, where the wind must have brought it from many miles’ distance. It was the only weed that we noticed on that mountain, a region which has never been reached by cattle, sheep, or horses, and has never been overrun by fire. Again, take the case of sorrel. It is widely spread by seeds, which are eaten but not injured or digested by grazing animals, and it spreads by underground runners with great quickness. It forms large tufts of foliage that keep off or smother competitors in grazed land. These facts seem to me enough to account for its spread without postulating any special constitutional vigour. Most other cases of aggressive plants are, I think, to be accounted for by special advantages of habit and growth, and these are matters that will well reward studious inquiry.