Art. XLIII.—On the Spread of Cassinia leptophylla.
[Read before the Wellington Philosophical Society, 22nd September, 1873.]
Most persons who have been in the habit of passing over the hills on the eastern side of Wellington Harbour, or of visiting the Miramar Peninsula, occupied by Mr. Coutts Crawford as a sheep farm, will have noticed the increase, during the last few years, of an indigenous shrub commonly known by its native name as the Taiwhenu, or sea mat-cord, but known to botanists as Cassinia leptophylla. They will, probably, also have observed how much this shrub is already interfering with the use of the land referred to for pastoral purposes, occupying, as it does in many places, patches of several acres in extent, and everywhere preventing, by the rapidity of its growth, the attempts of the sheep to make their way through it, or to reach the scanty grass growing about it.
Whilst passing through Queen Charlotte Sound during recent visits to Nelson, I was struck with the very rapid extension of the same plant over all the open parts of the hills, and was led to make enquiries in reference to it, as
it affected the pastoral occupiers, the results of which I will mention in the sequel. I have, moreover, heard that this plant is already interfering greatly with the use of the natural pastures on the east coast of the North Island, more particularly within the Wellington Province; and that sheep-farmers there are looking upon its presence as a very serious and not easily preventible evil. But before mentioning the results of my enquiries in regard to it, I propose to notice a few interesting facts, most of which have come under my own observation, in connection with what Dr. Hooker has termed the “replacement of species,” a term used by him to designate the permanent changes which take place in the flora of a new country, as the result of the introduction of competing foreign organisms. The facts which I purpose mentioning, however, though properly associated with such permanent effects, relate only to certain temporary changes in the character of the introduced vegetation, not brought about intentionally or by the direct application of labour.
In former papers, read before this Society, I pointed out that acclimatization (a term, by the way, which I object to as involving a fallacy, and which I think ought to be replaced by ‘naturalization’), or the introduction of foreign organisms into a country presenting suitable conditions for their growth and subsistence, is the result of both intentional and unintentional action on the part of man. His intentional action is usually directed to the attainment of beneficial ends, but is constantly accompanied (unavoidably to a certain extent) by mischievous results. Of these latter I may instance the introduction of various forms of Coccus and Aphis; and of the larvæ of various species of destructive Coleoptera and Diptera, most of which have been brought in Wardian cases; and of the seeds of innumerable weeds, such as those of Rumex, Stellaria, Hypochæris, Euphorbia, etc. I also pointed out the results which are usually produced upon the indigenous vegetation in countries previously unoccupied by civilized man, by the introduction of animals and of competing foreign organisms.
It must not, however, be supposed that the struggle thus brought about between the introduced and indigenous forms of life is a mere battle between these two forces, for, in reality, each individual species concerned, whether local or foreign, is fighting “on its own hook,” striving against all as against a common enemy, and seeking to secure for itself the greatest share in ultimate occupation. As may be understood, many of the combatants, if not altogether disabled, are seriously enfeebled, whilst the fortunes even of those which are able to show the strongest front are somewhat various. Now, we know that in the hill districts of both islands of New Zealand the chief object of the European settler is to replace the native vegetation by grasses suitable for the maintenance of cattle and sheep, and various processes, more or less direct in their action, are employed in order to attain this end.
Many tracts of such country, as in the Wellington Province for example, are exclusively covered with forest, having a more or less dense undergrowth. Other such tracts are chiefly covered with fern, patches of forest and scrub occupying the gullies and valleys. Others again, chiefly on the eastern sides of both islands, are occupied by native grasses of more or less value for feeding purposes. But whether the indigenous growth consists of forest, scrub, fern, or grass, the great aim of the European is to remove it, and to replace it with a vegetation which experience has taught him to look upon as the most enduring for pasture purposes, even though it may, in some respects, possess a less feeding value than the native growth.
Amongst the ruder processes resorted to is that of periodically burning the indigenous growth, and scattering grass seed upon the bared soil, without any attempt to turn over the surface. This process has produced fair pastures in naturally loose soils, especially in the Provinces of the North Island. In the Wellington Province, however, the young grass is speedily overrun with a growth of Carduus lanceolatus, which maintains its position with more or less luxuriance for several years. But it has been found that the temporary inconvenience resulting from the presence of this thistle is more than compensated by the improvement it effects in the soil, and the greater luxuriance of growth in the grass after its disappearance. The grass, however, after having survived this first attack, is usually invaded by a much more powerful foe, the HypochŒris radicata, a plant common enough in England, but confined to waste places, and never attaining there the development which it has attained all over New Zealand. I have seen hundreds of acres of land which had been carefully laid down in English grass so completely overrun by this plant that it would have been difficult to discover a blade of grass amongst it, especially in dry weather. In the end, however, the strength of this new enemy becomes exhausted, and the grass again becomes master of the field.
But new foes have lately appeared in Nelson and Canterbury, in the shape of the Malva sylvestris, which is most largely developed in Nelson, and the Achillea millefolium, which has chosen the pastures of Canterbury for its habitat. I observed lately in Nelson whole paddocks overrun with the mallow, which, unlike its predecessor the hawk-weed, is not eaten by any animal, and threatens seriously to interfere with the production of grass; whilst in Canterbury I noticed the yarrow spreading in the grass fields, and completely displacing the grass wherever it grew.
I might quote many other cases illustrative of the struggle which is going on between various species of introduced organisms for the ultimate occupation of the soil from which the indigenous vegetation has been removed by the more complete systems of culture; but, however interesting these may be, they are far less so than the attempt which Cassinia leptophylla is making to
vindicate the title of the native vegetation to the exclusive possession of the open lands which are used as natural pastures, or which have only been subjected to the ruder forms of cultivation. Amongst the causes which are probably leading to the present extension of this plant (independently of the facility which it enjoys, in common with other members of the Compositæ, for the dispersion of its seeds) are the increased production of fertile seeds, owing to the abundance of European bees all over the country; the disturbance of the surface soil by the treading of animals; and possibly the destruction, or at all events a great diminution in numbers, of some form of insect life which formerly fed upon its flower-heads. In connection with the latter suggestion I may mention that it is extremely difficult to obtain mature seeds of many of the indigenous Compositæ. The flower-heads of the varies species of Celmisia, which are so abundant in the mountain districts of the South Island, for example, are usually attacked by a small Hemipterous insect, several individuals of which are generally to be found in each head. On one occasion I collected one hundred heads of Celmisia coriacea, the seeds of which, to outward appearance, were in good condition, but scarcely one of them contained a single sound seed, out of some eighty or a hundred which had been produced upon it, nearly the whole having been destroyed by the insects in question. But whatever the causes which are leading to the spread of the Cassinia may be, the fact and its evil consequences are certain, and it only remains to be considered what is best to be done under the circumstances.
The first and most natural idea which occurs to the mind of the occupier of land thus invaded is, to endeavour to destroy the plant by fire, as is done in the case of other indigenous growths; but this has been found to be impossible, except where its growth is very dense, and not even then until it has attained several years of age. On small holdings, or where grass is specially valuable, it may pay to employ labour to eradicate it with the adze-hoe; but upon large or inferior runs the expense of such a process puts it out of the question. As the result of the enquiries which I made in Queen Charlotte Sound, I am led to believe that in the latter case it is best to let the plant take its course until it attains a growth sufficiently dense to admit of its being burnt. I was informed that it is usually ripe for this operation in about five years, and that after such a burning it does not re-appear, whilst the soil which it occupied has become better fitted for the growth of grass in consequence of its being opened out through the decomposition of the roots. We know that the thistle and the hawk-weed have both died out over immense tracts of country, owing, in all probability, to the exhaustion of some material necessary for their subsistence; and I think it probable that the same cause will operate in the case of the Cassinia, even if its destruction be not precipitated by either of the processes above referred to.